{ "mode": "man", "parameter": "dash", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/dash/1/json", "generated": "2026-09-24T12:52:00Z", "synopsis": "dash [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\n[commandfile [argument ...]]\ndash -c [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\ncommandstring [commandname [argument ...]]\ndash -s [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\n[argument ...]", "sections": { "NAME": { "content": "dash — command interpreter (shell)\n", "subsections": [] }, "SYNOPSIS": { "content": "dash [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\n[commandfile [argument ...]]\ndash -c [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\ncommandstring [commandname [argument ...]]\ndash -s [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o optionname] [+o optionname]\n[argument ...]\n", "subsections": [] }, "DESCRIPTION": { "content": "dash is the standard command interpreter for the system. The current version of dash is in\nthe process of being changed to conform with the POSIX 1003.2 and 1003.2a specifications for\nthe shell. This version has many features which make it appear similar in some respects to\nthe Korn shell, but it is not a Korn shell clone (see ksh(1)). Only features designated by\nPOSIX, plus a few Berkeley extensions, are being incorporated into this shell. This man page\nis not intended to be a tutorial or a complete specification of the shell.\n", "subsections": [ { "name": "Overview", "content": "The shell is a command that reads lines from either a file or the terminal, interprets them,\nand generally executes other commands. It is the program that is running when a user logs\ninto the system (although a user can select a different shell with the chsh(1) command). The\nshell implements a language that has flow control constructs, a macro facility that provides\na variety of features in addition to data storage, along with built in history and line edit‐\ning capabilities. It incorporates many features to aid interactive use and has the advantage\nthat the interpretative language is common to both interactive and non-interactive use (shell\nscripts). That is, commands can be typed directly to the running shell or can be put into a\nfile and the file can be executed directly by the shell.\n" }, { "name": "Invocation", "content": "If no args are present and if the standard input of the shell is connected to a terminal (or\nif the -i flag is set), and the -c option is not present, the shell is considered an interac‐\ntive shell. An interactive shell generally prompts before each command and handles program‐\nming and command errors differently (as described below). When first starting, the shell in‐\nspects argument 0, and if it begins with a dash ‘-’, the shell is also considered a login\nshell. This is normally done automatically by the system when the user first logs in. A lo‐\ngin shell first reads commands from the files /etc/profile and .profile if they exist. If\nthe environment variable ENV is set on entry to an interactive shell, or is set in the\n.profile of a login shell, the shell next reads commands from the file named in ENV. There‐\nfore, a user should place commands that are to be executed only at login time in the .profile\nfile, and commands that are executed for every interactive shell inside the ENV file. To set\nthe ENV variable to some file, place the following line in your .profile of your home direc‐\ntory\n\nENV=$HOME/.shinit; export ENV\n\nsubstituting for “.shinit” any filename you wish.\n\nIf command line arguments besides the options have been specified, then the shell treats the\nfirst argument as the name of a file from which to read commands (a shell script), and the\nremaining arguments are set as the positional parameters of the shell ($1, $2, etc). Other‐\nwise, the shell reads commands from its standard input.\n" }, { "name": "Argument List Processing", "content": "All of the single letter options that have a corresponding name can be used as an argument to\nthe -o option. The set -o name is provided next to the single letter option in the descrip‐\ntion below. Specifying a dash “-” turns the option on, while using a plus “+” disables the\noption. The following options can be set from the command line or with the set builtin (de‐\nscribed later).\n\n-a allexport Export all variables assigned to.\n\n-c Read commands from the commandstring operand instead of from the\nstandard input. Special parameter 0 will be set from the commandname\noperand and the positional parameters ($1, $2, etc.) set from the re‐\nmaining argument operands.\n\n-C noclobber Don't overwrite existing files with “>”.\n\n-e errexit If not interactive, exit immediately if any untested command fails.\nThe exit status of a command is considered to be explicitly tested if\nthe command is used to control an if, elif, while, or until; or if the\ncommand is the left hand operand of an “&&” or “||” operator.\n\n-f noglob Disable pathname expansion.\n\n-n noexec If not interactive, read commands but do not execute them. This is\nuseful for checking the syntax of shell scripts.\n\n-u nounset Write a message to standard error when attempting to expand a variable\nthat is not set, and if the shell is not interactive, exit immedi‐\nately.\n\n-v verbose The shell writes its input to standard error as it is read. Useful\nfor debugging.\n\n-x xtrace Write each command to standard error (preceded by a ‘+ ’) before it is\nexecuted. Useful for debugging.\n\n-I ignoreeof Ignore EOF's from input when interactive.\n\n-i interactive Force the shell to behave interactively.\n\n-l Make dash act as if it had been invoked as a login shell.\n\n-m monitor Turn on job control (set automatically when interactive).\n\n-s stdin Read commands from standard input (set automatically if no file argu‐\nments are present). This option has no effect when set after the\nshell has already started running (i.e. with set).\n\n-V vi Enable the built-in vi(1) command line editor (disables -E if it has\nbeen set).\n\n-E emacs Enable the built-in emacs(1) command line editor (disables -V if it\nhas been set).\n\n-b notify Enable asynchronous notification of background job completion. (UNIM‐\nPLEMENTED for 4.4alpha)\n\n-p priviliged Do not attempt to reset effective uid if it does not match uid. This\nis not set by default to help avoid incorrect usage by setuid root\nprograms via system(3) or popen(3).\n" }, { "name": "Lexical Structure", "content": "The shell reads input in terms of lines from a file and breaks it up into words at whitespace\n(blanks and tabs), and at certain sequences of characters that are special to the shell\ncalled “operators”. There are two types of operators: control operators and redirection op‐\nerators (their meaning is discussed later). Following is a list of operators:\n\nControl operators:\n& && ( ) ; ;; | || \n\nRedirection operators:\n< > >| << >> <& >& <<- <>\n" }, { "name": "Quoting", "content": "Quoting is used to remove the special meaning of certain characters or words to the shell,\nsuch as operators, whitespace, or keywords. There are three types of quoting: matched single\nquotes, matched double quotes, and backslash.\n" }, { "name": "Backslash", "content": "A backslash preserves the literal meaning of the following character, with the exception of\n⟨newline⟩. A backslash preceding a ⟨newline⟩ is treated as a line continuation.\n" }, { "name": "Single Quotes", "content": "Enclosing characters in single quotes preserves the literal meaning of all the characters\n(except single quotes, making it impossible to put single-quotes in a single-quoted string).\n" }, { "name": "Double Quotes", "content": "Enclosing characters within double quotes preserves the literal meaning of all characters ex‐\ncept dollarsign ($), backquote (`), and backslash (\\). The backslash inside double quotes is\nhistorically weird, and serves to quote only the following characters:\n$ ` \" \\ .\nOtherwise it remains literal.\n" }, { "name": "Reserved Words", "content": "Reserved words are words that have special meaning to the shell and are recognized at the be‐\nginning of a line and after a control operator. The following are reserved words:\n\n! elif fi while case\nelse for then { }\ndo done until if esac\n\nTheir meaning is discussed later.\n" }, { "name": "Aliases", "content": "An alias is a name and corresponding value set using the alias(1) builtin command. Whenever\na reserved word may occur (see above), and after checking for reserved words, the shell\nchecks the word to see if it matches an alias. If it does, it replaces it in the input\nstream with its value. For example, if there is an alias called “lf” with the value “ls -F”,\nthen the input:\n\nlf foobar ⟨return⟩\n\nwould become\n\nls -F foobar ⟨return⟩\n\nAliases provide a convenient way for naive users to create shorthands for commands without\nhaving to learn how to create functions with arguments. They can also be used to create lex‐\nically obscure code. This use is discouraged.\n" }, { "name": "Commands", "content": "The shell interprets the words it reads according to a language, the specification of which\nis outside the scope of this man page (refer to the BNF in the POSIX 1003.2 document). Es‐\nsentially though, a line is read and if the first word of the line (or after a control opera‐\ntor) is not a reserved word, then the shell has recognized a simple command. Otherwise, a\ncomplex command or some other special construct may have been recognized.\n" }, { "name": "Simple Commands", "content": "If a simple command has been recognized, the shell performs the following actions:\n\n1. Leading words of the form “name=value” are stripped off and assigned to the envi‐\nronment of the simple command. Redirection operators and their arguments (as de‐\nscribed below) are stripped off and saved for processing.\n\n2. The remaining words are expanded as described in the section called “Expansions”,\nand the first remaining word is considered the command name and the command is lo‐\ncated. The remaining words are considered the arguments of the command. If no\ncommand name resulted, then the “name=value” variable assignments recognized in\nitem 1 affect the current shell.\n\n3. Redirections are performed as described in the next section.\n" }, { "name": "Redirections", "content": "Redirections are used to change where a command reads its input or sends its output. In gen‐\neral, redirections open, close, or duplicate an existing reference to a file. The overall\nformat used for redirection is:\n\n[n] redir-op file\n\nwhere redir-op is one of the redirection operators mentioned previously. Following is a list\nof the possible redirections. The [n] is an optional number between 0 and 9, as in ‘3’ (not\n‘[3]’), that refers to a file descriptor.\n\n[n]> file Redirect standard output (or n) to file.\n\n[n]>| file Same, but override the -C option.\n\n[n]>> file Append standard output (or n) to file.\n\n[n]< file Redirect standard input (or n) from file.\n\n[n1]<&n2 Copy file descriptor n2 as stdout (or fd n1). fd n2.\n\n[n]<&- Close standard input (or n).\n\n[n1]>&n2 Copy file descriptor n2 as stdin (or fd n1). fd n2.\n\n[n]>&- Close standard output (or n).\n\n[n]<> file Open file for reading and writing on standard input (or n).\n\nThe following redirection is often called a “here-document”.\n\n[n]<< delimiter\nhere-doc-text ...\ndelimiter\n\nAll the text on successive lines up to the delimiter is saved away and made available to the\ncommand on standard input, or file descriptor n if it is specified. If the delimiter as\nspecified on the initial line is quoted, then the here-doc-text is treated literally, other‐\nwise the text is subjected to parameter expansion, command substitution, and arithmetic ex‐\npansion (as described in the section on “Expansions”). If the operator is “<<-” instead of\n“<<”, then leading tabs in the here-doc-text are stripped.\n" }, { "name": "Search and Execution", "content": "There are three types of commands: shell functions, builtin commands, and normal programs –\nand the command is searched for (by name) in that order. They each are executed in a differ‐\nent way.\n\nWhen a shell function is executed, all of the shell positional parameters (except $0, which\nremains unchanged) are set to the arguments of the shell function. The variables which are\nexplicitly placed in the environment of the command (by placing assignments to them before\nthe function name) are made local to the function and are set to the values given. Then the\ncommand given in the function definition is executed. The positional parameters are restored\nto their original values when the command completes. This all occurs within the current\nshell.\n\nShell builtins are executed internally to the shell, without spawning a new process.\n\nOtherwise, if the command name doesn't match a function or builtin, the command is searched\nfor as a normal program in the file system (as described in the next section). When a normal\nprogram is executed, the shell runs the program, passing the arguments and the environment to\nthe program. If the program is not a normal executable file (i.e., if it does not begin with\nthe \"magic number\" whose ASCII representation is \"#!\", so execve(2) returns ENOEXEC then) the\nshell will interpret the program in a subshell. The child shell will reinitialize itself in\nthis case, so that the effect will be as if a new shell had been invoked to handle the ad-hoc\nshell script, except that the location of hashed commands located in the parent shell will be\nremembered by the child.\n\nNote that previous versions of this document and the source code itself misleadingly and spo‐\nradically refer to a shell script without a magic number as a \"shell procedure\".\n" }, { "name": "Path Search", "content": "When locating a command, the shell first looks to see if it has a shell function by that\nname. Then it looks for a builtin command by that name. If a builtin command is not found,\none of two things happen:\n\n1. Command names containing a slash are simply executed without performing any searches.\n\n2. The shell searches each entry in PATH in turn for the command. The value of the PATH\nvariable should be a series of entries separated by colons. Each entry consists of a\ndirectory name. The current directory may be indicated implicitly by an empty directory\nname, or explicitly by a single period.\n" }, { "name": "Command Exit Status", "content": "Each command has an exit status that can influence the behaviour of other shell commands.\nThe paradigm is that a command exits with zero for normal or success, and non-zero for fail‐\nure, error, or a false indication. The man page for each command should indicate the various\nexit codes and what they mean. Additionally, the builtin commands return exit codes, as does\nan executed shell function.\n\nIf a command consists entirely of variable assignments then the exit status of the command is\nthat of the last command substitution if any, otherwise 0.\n" }, { "name": "Complex Commands", "content": "Complex commands are combinations of simple commands with control operators or reserved\nwords, together creating a larger complex command. More generally, a command is one of the\nfollowing:\n\n• simple command\n\n• pipeline\n\n• list or compound-list\n\n• compound command\n\n• function definition\n\nUnless otherwise stated, the exit status of a command is that of the last simple command exe‐\ncuted by the command.\n" }, { "name": "Pipelines", "content": "A pipeline is a sequence of one or more commands separated by the control operator |. The\nstandard output of all but the last command is connected to the standard input of the next\ncommand. The standard output of the last command is inherited from the shell, as usual.\n\nThe format for a pipeline is:\n\n[!] command1 [| command2 ...]\n\nThe standard output of command1 is connected to the standard input of command2. The standard\ninput, standard output, or both of a command is considered to be assigned by the pipeline be‐\nfore any redirection specified by redirection operators that are part of the command.\n\nIf the pipeline is not in the background (discussed later), the shell waits for all commands\nto complete.\n\nIf the reserved word ! does not precede the pipeline, the exit status is the exit status of\nthe last command specified in the pipeline. Otherwise, the exit status is the logical NOT of\nthe exit status of the last command. That is, if the last command returns zero, the exit\nstatus is 1; if the last command returns greater than zero, the exit status is zero.\n\nBecause pipeline assignment of standard input or standard output or both takes place before\nredirection, it can be modified by redirection. For example:\n\n$ command1 2>&1 | command2\n\nsends both the standard output and standard error of command1 to the standard input of com‐\nmand2.\n\nA ; or ⟨newline⟩ terminator causes the preceding AND-OR-list (described next) to be executed\nsequentially; a & causes asynchronous execution of the preceding AND-OR-list.\n\nNote that unlike some other shells, each process in the pipeline is a child of the invoking\nshell (unless it is a shell builtin, in which case it executes in the current shell – but any\neffect it has on the environment is wiped).\n" }, { "name": "Background Commands – &", "content": "If a command is terminated by the control operator ampersand (&), the shell executes the com‐\nmand asynchronously – that is, the shell does not wait for the command to finish before exe‐\ncuting the next command.\n\nThe format for running a command in background is:\n\ncommand1 & [command2 & ...]\n\nIf the shell is not interactive, the standard input of an asynchronous command is set to\n/dev/null.\n" }, { "name": "Lists – Generally Speaking", "content": "A list is a sequence of zero or more commands separated by newlines, semicolons, or amper‐\nsands, and optionally terminated by one of these three characters. The commands in a list\nare executed in the order they are written. If command is followed by an ampersand, the\nshell starts the command and immediately proceeds onto the next command; otherwise it waits\nfor the command to terminate before proceeding to the next one.\n" }, { "name": "Short-Circuit List Operators", "content": "“&&” and “||” are AND-OR list operators. “&&” executes the first command, and then executes\nthe second command if and only if the exit status of the first command is zero. “||” is sim‐\nilar, but executes the second command if and only if the exit status of the first command is\nnonzero. “&&” and “||” both have the same priority.\n" }, { "name": "Flow-Control Constructs – if, while, for, case", "content": "The syntax of the if command is\n\nif list\nthen list\n[ elif list\nthen list ] ...\n[ else list ]\nfi\n\nThe syntax of the while command is\n\nwhile list\ndo list\ndone\n\nThe two lists are executed repeatedly while the exit status of the first list is zero. The\nuntil command is similar, but has the word until in place of while, which causes it to repeat\nuntil the exit status of the first list is zero.\n\nThe syntax of the for command is\n\nfor variable [ in [ word ... ] ]\ndo list\ndone\n\nThe words following in are expanded, and then the list is executed repeatedly with the vari‐\nable set to each word in turn. Omitting in word ... is equivalent to in \"$@\".\n\nThe syntax of the break and continue command is\n\nbreak [ num ]\ncontinue [ num ]\n\nBreak terminates the num innermost for or while loops. Continue continues with the next it‐\neration of the innermost loop. These are implemented as builtin commands.\n\nThe syntax of the case command is\n\ncase word in\n[(]pattern) list ;;\n...\nesac\n\nThe pattern can actually be one or more patterns (see “Shell Patterns” described later), sep‐\narated by “|” characters. The “(” character before the pattern is optional.\n" }, { "name": "Grouping Commands Together", "content": "Commands may be grouped by writing either\n\n(list)\n\nor\n\n{ list; }\n\nThe first of these executes the commands in a subshell. Builtin commands grouped into a\n(list) will not affect the current shell. The second form does not fork another shell so is\nslightly more efficient. Grouping commands together this way allows you to redirect their\noutput as though they were one program:\n\n{ printf \" hello \" ; printf \" world\\n\" ; } > greeting\n\nNote that “}” must follow a control operator (here, “;”) so that it is recognized as a re‐\nserved word and not as another command argument.\n" }, { "name": "Functions", "content": "The syntax of a function definition is\n\nname () command\n\nA function definition is an executable statement; when executed it installs a function named\nname and returns an exit status of zero. The command is normally a list enclosed between “{”\nand “}”.\n\nVariables may be declared to be local to a function by using a local command. This should\nappear as the first statement of a function, and the syntax is\n\nlocal [variable | -] ...\n\nLocal is implemented as a builtin command.\n\nWhen a variable is made local, it inherits the initial value and exported and readonly flags\nfrom the variable with the same name in the surrounding scope, if there is one. Otherwise,\nthe variable is initially unset. The shell uses dynamic scoping, so that if you make the\nvariable x local to function f, which then calls function g, references to the variable x\nmade inside g will refer to the variable x declared inside f, not to the global variable\nnamed x.\n\nThe only special parameter that can be made local is “-”. Making “-” local any shell options\nthat are changed via the set command inside the function to be restored to their original\nvalues when the function returns.\n\nThe syntax of the return command is\n\nreturn [exitstatus]\n\nIt terminates the currently executing function. Return is implemented as a builtin command.\n" }, { "name": "Variables and Parameters", "content": "The shell maintains a set of parameters. A parameter denoted by a name is called a variable.\nWhen starting up, the shell turns all the environment variables into shell variables. New\nvariables can be set using the form\n\nname=value\n\nVariables set by the user must have a name consisting solely of alphabetics, numerics, and\nunderscores - the first of which must not be numeric. A parameter can also be denoted by a\nnumber or a special character as explained below.\n" }, { "name": "Positional Parameters", "content": "A positional parameter is a parameter denoted by a number (n > 0). The shell sets these ini‐\ntially to the values of its command line arguments that follow the name of the shell script.\nThe set builtin can also be used to set or reset them.\n" }, { "name": "Special Parameters", "content": "A special parameter is a parameter denoted by one of the following special characters. The\nvalue of the parameter is listed next to its character.\n\n* Expands to the positional parameters, starting from one. When the expansion oc‐\ncurs within a double-quoted string it expands to a single field with the value\nof each parameter separated by the first character of the IFS variable, or by a\n⟨space⟩ if IFS is unset.\n\n@ Expands to the positional parameters, starting from one. When the expansion oc‐\ncurs within double-quotes, each positional parameter expands as a separate argu‐\nment. If there are no positional parameters, the expansion of @ generates zero\narguments, even when @ is double-quoted. What this basically means, for exam‐\nple, is if $1 is “abc” and $2 is “def ghi”, then \"$@\" expands to the two argu‐\nments:\n\n\"abc\" \"def ghi\"\n\n# Expands to the number of positional parameters.\n\n? Expands to the exit status of the most recent pipeline.\n\n- (Hyphen.) Expands to the current option flags (the single-letter option names concatenated\ninto a string) as specified on invocation, by the set builtin command, or im‐\nplicitly by the shell.\n\n$ Expands to the process ID of the invoked shell. A subshell retains the same\nvalue of $ as its parent.\n\n! Expands to the process ID of the most recent background command executed from\nthe current shell. For a pipeline, the process ID is that of the last command\nin the pipeline.\n\n0 (Zero.) Expands to the name of the shell or shell script.\n" }, { "name": "Word Expansions", "content": "This clause describes the various expansions that are performed on words. Not all expansions\nare performed on every word, as explained later.\n\nTilde expansions, parameter expansions, command substitutions, arithmetic expansions, and\nquote removals that occur within a single word expand to a single field. It is only field\nsplitting or pathname expansion that can create multiple fields from a single word. The sin‐\ngle exception to this rule is the expansion of the special parameter @ within double-quotes,\nas was described above.\n\nThe order of word expansion is:\n\n1. Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic Expansion (these\nall occur at the same time).\n\n2. Field Splitting is performed on fields generated by step (1) unless the IFS variable is\nnull.\n\n3. Pathname Expansion (unless set -f is in effect).\n\n4. Quote Removal.\n\nThe $ character is used to introduce parameter expansion, command substitution, or arithmetic\nevaluation.\n" }, { "name": "Tilde Expansion (substituting a user's home directory)", "content": "A word beginning with an unquoted tilde character (~) is subjected to tilde expansion. All\nthe characters up to a slash (/) or the end of the word are treated as a username and are re‐\nplaced with the user's home directory. If the username is missing (as in ~/foobar), the\ntilde is replaced with the value of the HOME variable (the current user's home directory).\n" }, { "name": "Parameter Expansion", "content": "The format for parameter expansion is as follows:\n\n${expression}\n\nwhere expression consists of all characters until the matching “}”. Any “}” escaped by a\nbackslash or within a quoted string, and characters in embedded arithmetic expansions, com‐\nmand substitutions, and variable expansions, are not examined in determining the matching\n“}”.\n\nThe simplest form for parameter expansion is:\n\n${parameter}\n\nThe value, if any, of parameter is substituted.\n\nThe parameter name or symbol can be enclosed in braces, which are optional except for posi‐\ntional parameters with more than one digit or when parameter is followed by a character that\ncould be interpreted as part of the name. If a parameter expansion occurs inside double-\nquotes:\n\n1. Pathname expansion is not performed on the results of the expansion.\n\n2. Field splitting is not performed on the results of the expansion, with the exception of\n@.\n\nIn addition, a parameter expansion can be modified by using one of the following formats.\n\n${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of\nword is substituted; otherwise, the value of parameter is substituted.\n\n${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of\nword is assigned to parameter. In all cases, the final value of para‐\nmeter is substituted. Only variables, not positional parameters or\nspecial parameters, can be assigned in this way.\n\n${parameter:?[word]} Indicate Error if Null or Unset. If parameter is unset or null, the\nexpansion of word (or a message indicating it is unset if word is omit‐\nted) is written to standard error and the shell exits with a nonzero\nexit status. Otherwise, the value of parameter is substituted. An in‐\nteractive shell need not exit.\n\n${parameter:+word} Use Alternative Value. If parameter is unset or null, null is substi‐\ntuted; otherwise, the expansion of word is substituted.\n\nIn the parameter expansions shown previously, use of the colon in the format results in a\ntest for a parameter that is unset or null; omission of the colon results in a test for a pa‐\nrameter that is only unset.\n\n${#parameter} String Length. The length in characters of the value of parameter.\n\nThe following four varieties of parameter expansion provide for substring processing. In\neach case, pattern matching notation (see “Shell Patterns”), rather than regular expression\nnotation, is used to evaluate the patterns. If parameter is * or @, the result of the expan‐\nsion is unspecified. Enclosing the full parameter expansion string in double-quotes does not\ncause the following four varieties of pattern characters to be quoted, whereas quoting char‐\nacters within the braces has this effect.\n\n${parameter%word} Remove Smallest Suffix Pattern. The word is expanded to produce a pat‐\ntern. The parameter expansion then results in parameter, with the\nsmallest portion of the suffix matched by the pattern deleted.\n\n${parameter%%word} Remove Largest Suffix Pattern. The word is expanded to produce a pat‐\ntern. The parameter expansion then results in parameter, with the\nlargest portion of the suffix matched by the pattern deleted.\n\n${parameter#word} Remove Smallest Prefix Pattern. The word is expanded to produce a pat‐\ntern. The parameter expansion then results in parameter, with the\nsmallest portion of the prefix matched by the pattern deleted.\n\n${parameter##word} Remove Largest Prefix Pattern. The word is expanded to produce a pat‐\ntern. The parameter expansion then results in parameter, with the\nlargest portion of the prefix matched by the pattern deleted.\n" }, { "name": "Command Substitution", "content": "Command substitution allows the output of a command to be substituted in place of the command\nname itself. Command substitution occurs when the command is enclosed as follows:\n\n$(command)\n\nor (“backquoted” version):\n\n`command`\n\nThe shell expands the command substitution by executing command in a subshell environment and\nreplacing the command substitution with the standard output of the command, removing se‐\nquences of one or more ⟨newline⟩s at the end of the substitution. (Embedded ⟨newline⟩s be‐\nfore the end of the output are not removed; however, during field splitting, they may be\ntranslated into ⟨space⟩s, depending on the value of IFS and quoting that is in effect.)\n" }, { "name": "Arithmetic Expansion", "content": "Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substi‐\ntuting its value. The format for arithmetic expansion is as follows:\n\n$((expression))\n\nThe expression is treated as if it were in double-quotes, except that a double-quote inside\nthe expression is not treated specially. The shell expands all tokens in the expression for\nparameter expansion, command substitution, and quote removal.\n\nNext, the shell treats this as an arithmetic expression and substitutes the value of the ex‐\npression.\n" }, { "name": "White Space Splitting (Field Splitting)", "content": "After parameter expansion, command substitution, and arithmetic expansion the shell scans the\nresults of expansions and substitutions that did not occur in double-quotes for field split‐\nting and multiple fields can result.\n\nThe shell treats each character of the IFS as a delimiter and uses the delimiters to split\nthe results of parameter expansion and command substitution into fields.\n" }, { "name": "Pathname Expansion (File Name Generation)", "content": "Unless the -f flag is set, file name generation is performed after word splitting is com‐\nplete. Each word is viewed as a series of patterns, separated by slashes. The process of\nexpansion replaces the word with the names of all existing files whose names can be formed by\nreplacing each pattern with a string that matches the specified pattern. There are two re‐\nstrictions on this: first, a pattern cannot match a string containing a slash, and second, a\npattern cannot match a string starting with a period unless the first character of the pat‐\ntern is a period. The next section describes the patterns used for both Pathname Expansion\nand the case command.\n" }, { "name": "Shell Patterns", "content": "A pattern consists of normal characters, which match themselves, and meta-characters. The\nmeta-characters are “!”, “*”, “?”, and “[”. These characters lose their special meanings if\nthey are quoted. When command or variable substitution is performed and the dollar sign or\nback quotes are not double quoted, the value of the variable or the output of the command is\nscanned for these characters and they are turned into meta-characters.\n\nAn asterisk (“*”) matches any string of characters. A question mark matches any single char‐\nacter. A left bracket (“[”) introduces a character class. The end of the character class is\nindicated by a (“]”); if the “]” is missing then the “[” matches a “[” rather than introduc‐\ning a character class. A character class matches any of the characters between the square\nbrackets. A range of characters may be specified using a minus sign. The character class\nmay be complemented by making an exclamation point the first character of the character\nclass.\n\nTo include a “]” in a character class, make it the first character listed (after the “!”, if\nany). To include a minus sign, make it the first or last character listed.\n" }, { "name": "Builtins", "content": "This section lists the builtin commands which are builtin because they need to perform some\noperation that can't be performed by a separate process. In addition to these, there are\nseveral other commands that may be builtin for efficiency (e.g. printf(1), echo(1), test(1),\netc).\n\n:\n\ntrue A null command that returns a 0 (true) exit value.\n\nfalse A null command that returns a 1 (false) exit value.\n\n. file\nThe commands in the specified file are read and executed by the shell.\n\nalias [name[=string ...]]\nIf name=string is specified, the shell defines the alias name with value string. If\njust name is specified, the value of the alias name is printed. With no arguments,\nthe alias builtin prints the names and values of all defined aliases (see unalias).\n\nbg [job] ...\nContinue the specified jobs (or the current job if no jobs are given) in the back‐\nground.\n\ncommand [-p] [-v] [-V] command [arg ...]\nExecute the specified command but ignore shell functions when searching for it. (This\nis useful when you have a shell function with the same name as a builtin command.)\n\n-p search for command using a PATH that guarantees to find all the standard utili‐\nties.\n\n-V Do not execute the command but search for the command and print the resolution\nof the command search. This is the same as the type builtin.\n\n-v Do not execute the command but search for the command and print the absolute\npathname of utilities, the name for builtins or the expansion of aliases.\n\ncd|chdir -\n\ncd|chdir [-LP] [directory]\nSwitch to the specified directory (default HOME). If an entry for CDPATH appears in\nthe environment of the cd command or the shell variable CDPATH is set and the direc‐\ntory name does not begin with a slash, then the directories listed in CDPATH will be\nsearched for the specified directory. The format of CDPATH is the same as that of\nPATH. If a single dash is specified as the argument, it will be replaced by the value\nof OLDPWD. The cd command will print out the name of the directory that it actually\nswitched to if this is different from the name that the user gave. These may be dif‐\nferent either because the CDPATH mechanism was used or because the argument is a sin‐\ngle dash. The -P option causes the physical directory structure to be used, that is,\nall symbolic links are resolved to their respective values. The -L option turns off\nthe effect of any preceding -P options.\n\necho [-n] args...\nPrint the arguments on the standard output, separated by spaces. Unless the -n option\nis present, a newline is output following the arguments.\n\nIf any of the following sequences of characters is encountered during output, the se‐\nquence is not output. Instead, the specified action is performed:\n\n\\b A backspace character is output.\n\n\\c Subsequent output is suppressed. This is normally used at the end of the last\nargument to suppress the trailing newline that echo would otherwise output.\n\n\\e Outputs an escape character (ESC).\n\n\\f Output a form feed.\n\n\\n Output a newline character.\n\n\\r Output a carriage return.\n\n\\t Output a (horizontal) tab character.\n\n\\v Output a vertical tab.\n\n\\0digits\nOutput the character whose value is given by zero to three octal digits. If\nthere are zero digits, a nul character is output.\n\n\\\\ Output a backslash.\n\nAll other backslash sequences elicit undefined behaviour.\n\neval string ...\nConcatenate all the arguments with spaces. Then re-parse and execute the command.\n\nexec [command arg ...]\nUnless command is omitted, the shell process is replaced with the specified program\n(which must be a real program, not a shell builtin or function). Any redirections on\nthe exec command are marked as permanent, so that they are not undone when the exec\ncommand finishes.\n\nexit [exitstatus]\nTerminate the shell process. If exitstatus is given it is used as the exit status of\nthe shell; otherwise the exit status of the preceding command is used.\n\nexport name ...\n\nexport -p\nThe specified names are exported so that they will appear in the environment of subse‐\nquent commands. The only way to un-export a variable is to unset it. The shell al‐\nlows the value of a variable to be set at the same time it is exported by writing\n\nexport name=value\n\nWith no arguments the export command lists the names of all exported variables. With\nthe -p option specified the output will be formatted suitably for non-interactive use.\n\nfc [-e editor] [first [last]]\n\nfc -l [-nr] [first [last]]\n\nfc -s [old=new] [first]\nThe fc builtin lists, or edits and re-executes, commands previously entered to an in‐\nteractive shell.\n\n-e editor\nUse the editor named by editor to edit the commands. The editor string is a\ncommand name, subject to search via the PATH variable. The value in the FCEDIT\nvariable is used as a default when -e is not specified. If FCEDIT is null or\nunset, the value of the EDITOR variable is used. If EDITOR is null or unset,\ned(1) is used as the editor.\n" }, { "name": "-l (ell)", "content": "List the commands rather than invoking an editor on them. The commands are\nwritten in the sequence indicated by the first and last operands, as affected\nby -r, with each command preceded by the command number.\n\n-n Suppress command numbers when listing with -l.\n\n-r Reverse the order of the commands listed (with -l) or edited (with neither -l\nnor -s).\n\n-s Re-execute the command without invoking an editor.\n\nfirst\n\nlast Select the commands to list or edit. The number of previous commands that can\nbe accessed are determined by the value of the HISTSIZE variable. The value of\nfirst or last or both are one of the following:\n\n[+]number\nA positive number representing a command number; command numbers can be\ndisplayed with the -l option.\n\n-number\nA negative decimal number representing the command that was executed\nnumber of commands previously. For example, -1 is the immediately pre‐\nvious command.\n\nstring\nA string indicating the most recently entered command that begins with that\nstring. If the old=new operand is not also specified with -s, the string form\nof the first operand cannot contain an embedded equal sign.\n\nThe following environment variables affect the execution of fc:\n\nFCEDIT Name of the editor to use.\n\nHISTSIZE The number of previous commands that are accessible.\n\nfg [job]\nMove the specified job or the current job to the foreground.\n\ngetopts optstring var [arg ...]\nThe POSIX getopts command, not to be confused with the Bell Labs-derived getopt(1).\n\nThe first argument should be a series of letters, each of which may be optionally fol‐\nlowed by a colon to indicate that the option requires an argument. The variable spec‐\nified is set to the parsed option.\n\nThe getopts command deprecates the older getopt(1) utility due to its handling of ar‐\nguments containing whitespace.\n\nThe getopts builtin may be used to obtain options and their arguments from a list of\nparameters. When invoked, getopts places the value of the next option from the option\nstring in the list in the shell variable specified by var and its index in the shell\nvariable OPTIND. When the shell is invoked, OPTIND is initialized to 1. For each op‐\ntion that requires an argument, the getopts builtin will place it in the shell vari‐\nable OPTARG. If an option is not allowed for in the optstring, then OPTARG will be\nunset.\n\nBy default, the variables $1, ..., $n are inspected; if args are specified, they'll be\nparsed instead.\n\noptstring is a string of recognized option letters (see getopt(3)). If a letter is\nfollowed by a colon, the option is expected to have an argument which may or may not\nbe separated from it by white space. If an option character is not found where ex‐\npected, getopts will set the variable var to a “?”; getopts will then unset OPTARG and\nwrite output to standard error. By specifying a colon as the first character of\noptstring all errors will be ignored.\n\nAfter the last option getopts will return a non-zero value and set var to “?”.\n\nThe following code fragment shows how one might process the arguments for a command\nthat can take the options [a] and [b], and the option [c], which requires an argument.\n\nwhile getopts abc: f\ndo\ncase $f in\na | b) flag=$f;;\nc) carg=$OPTARG;;\n\\?) echo $USAGE; exit 1;;\nesac\ndone\nshift $((OPTIND - 1))\n\nThis code will accept any of the following as equivalent:\n\ncmd -acarg file file\ncmd -a -c arg file file\ncmd -carg -a file file\ncmd -a -carg -- file file\n\nhash [command ...]\n\nhash -r\nThe shell maintains a hash table which remembers the locations of commands. With no\narguments whatsoever, the hash command prints out the contents of this table. Entries\nwhich have not been looked at since the last cd command are marked with an asterisk;\nit is possible for these entries to be invalid.\n\nWith arguments, the hash command removes the specified commands from the hash table\n(unless they are functions) and then locates them. The -r option causes the hash com‐\nmand to delete all the entries in the hash table except for functions.\n\njobs [-lp] [job ...]\nDisplay the status of all, or just the specified, jobs:\nBy default display the job number, currency (+-) status, if any, the job state,\nand its shell command.\n-l also output the PID of the group leader, and just the PID and shell\ncommands of other members of the job.\n-p Display only leader PIDs, one per line.\n\nkill [-s sigspec | -signum | -sigspec] [pid | job ...]\nEquivalent to kill(1), but a job spec may also be specified. Signals can be either\ncase-insensitive names without SIG prefixes or decimal numbers; the default is TERM.\n\nkill -l [signum | exitstatus]\nList available signal names without the SIG prefix (sigspecs). If signum specified,\ndisplay just the sigspec for that signal. If exitstatus specified (> 128), display\njust the sigspec that caused it.\n\npwd [-LP]\nbuiltin command remembers what the current directory is rather than recomputing it\neach time. This makes it faster. However, if the current directory is renamed, the\nbuiltin version of pwd will continue to print the old name for the directory. The -P\noption causes the physical value of the current working directory to be shown, that\nis, all symbolic links are resolved to their respective values. The -L option turns\noff the effect of any preceding -P options.\n\nread [-p prompt] [-r] variable [...]\nThe prompt is printed if the -p option is specified and the standard input is a termi‐\nnal. Then a line is read from the standard input. The trailing newline is deleted\nfrom the line and the line is split as described in the section on word splitting\nabove, and the pieces are assigned to the variables in order. At least one variable\nmust be specified. If there are more pieces than variables, the remaining pieces\n(along with the characters in IFS that separated them) are assigned to the last vari‐\nable. If there are more variables than pieces, the remaining variables are assigned\nthe null string. The read builtin will indicate success unless EOF is encountered on\ninput, in which case failure is returned.\n\nBy default, unless the -r option is specified, the backslash “\\” acts as an escape\ncharacter, causing the following character to be treated literally. If a backslash is\nfollowed by a newline, the backslash and the newline will be deleted.\n\nreadonly name ...\n\nreadonly -p\nThe specified names are marked as read only, so that they cannot be subsequently modi‐\nfied or unset. The shell allows the value of a variable to be set at the same time it\nis marked read only by writing\n\nreadonly name=value\n\nWith no arguments the readonly command lists the names of all read only variables.\nWith the -p option specified the output will be formatted suitably for non-interactive\nuse.\n\nprintf format [value]...\nprintf formats and prints its arguments according to format, a character string which\ncontains three types of objects: plain characters, which are simply copied to standard\noutput, character escape sequences which are converted and copied to the standard out‐\nput, and format specifications, each of which causes printing of the next successive\nvalue.\n\nEach value is treated as a string if the corresponding format specification is either\nb, c, or s; otherwise it is evaluated as a C constant, with the following additions:\n• A leading plus or minus sign is allowed.\n• If the leading character is a single or double quote, the value of the next\nbyte.\n\nThe format string is reused as often as necessary until all values are consumed. Any\nextra format specifications are evaluated with zero or the null string.\n\nCharacter escape sequences are in backslash notation as defined in ANSI X3.159-1989\n(“ANSI C89”). The characters and their meanings are as follows:\n\n\\a Write a character.\n\n\\b Write a character.\n\n\\e Write an (ESC) character.\n\n\\f Write a character.\n\n\\n Write a character.\n\n\\r Write a character.\n\n\\t Write a character.\n\n\\v Write a character.\n\n\\\\ Write a backslash character.\n\n\\num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit oc‐\ntal number num.\n\nEach format specification is introduced by the percent character (``%''). The remain‐\nder of the format specification includes, in the following order:\n\nZero or more of the following flags:\n\n# A `#' character specifying that the value should be printed in an\n``alternative form''. For b, c, d, and s formats, this option has no\neffect. For the o format the precision of the number is increased to\nforce the first character of the output string to a zero. For the x\n(X) format, a non-zero result has the string 0x (0X) prepended to it.\nFor e, E, f, g, and G formats, the result will always contain a deci‐\nmal point, even if no digits follow the point (normally, a decimal\npoint only appears in the results of those formats if a digit follows\nthe decimal point). For g and G formats, trailing zeros are not re‐\nmoved from the result as they would otherwise be.\n\n- A minus sign `-' which specifies left adjustment of the output in the\nindicated field;\n\n+ A `+' character specifying that there should always be a sign placed\nbefore the number when using signed formats.\n\n‘ ’ A space specifying that a blank should be left before a positive num‐\nber for a signed format. A `+' overrides a space if both are used;\n\n0 A zero `0' character indicating that zero-padding should be used\nrather than blank-padding. A `-' overrides a `0' if both are used;\n\nField Width:\nAn optional digit string specifying a field width; if the output string has\nfewer characters than the field width it will be blank-padded on the left (or\nright, if the left-adjustment indicator has been given) to make up the field\nwidth (note that a leading zero is a flag, but an embedded zero is part of a\nfield width);\n\nPrecision:\nAn optional period, ‘.’, followed by an optional digit string giving a\nprecision which specifies the number of digits to appear after the decimal\npoint, for e and f formats, or the maximum number of bytes to be printed from\na string (b and s formats); if the digit string is missing, the precision is\ntreated as zero;\n\nFormat:\nA character which indicates the type of format to use (one of diouxXfwEgGbcs).\n\nA field width or precision may be ‘*’ instead of a digit string. In this case an\nargument supplies the field width or precision.\n\nThe format characters and their meanings are:\n\ndiouXx The argument is printed as a signed decimal (d or i), unsigned octal, un‐\nsigned decimal, or unsigned hexadecimal (X or x), respectively.\n\nf The argument is printed in the style [-]ddd.ddd where the number of d's\nafter the decimal point is equal to the precision specification for the\nargument. If the precision is missing, 6 digits are given; if the preci‐\nsion is explicitly 0, no digits and no decimal point are printed.\n\neE The argument is printed in the style [-]d.ddde±dd where there is one digit\nbefore the decimal point and the number after is equal to the precision\nspecification for the argument; when the precision is missing, 6 digits\nare produced. An upper-case E is used for an `E' format.\n\ngG The argument is printed in style f or in style e (E) whichever gives full\nprecision in minimum space.\n\nb Characters from the string argument are printed with backslash-escape se‐\nquences expanded.\nThe following additional backslash-escape sequences are supported:\n\n\\c Causes dash to ignore any remaining characters in the string\noperand containing it, any remaining string operands, and any ad‐\nditional characters in the format operand.\n\n\\0num Write an 8-bit character whose ASCII value is the 1-, 2-, or\n3-digit octal number num.\n\nc The first character of argument is printed.\n\ns Characters from the string argument are printed until the end is reached\nor until the number of bytes indicated by the precision specification is\nreached; if the precision is omitted, all characters in the string are\nprinted.\n\n% Print a `%'; no argument is used.\n\nIn no case does a non-existent or small field width cause truncation of a field;\npadding takes place only if the specified field width exceeds the actual width.\n\nset [{ -options | +options | -- }] arg ...\nThe set command performs three different functions.\n\nWith no arguments, it lists the values of all shell variables.\n\nIf options are given, it sets the specified option flags, or clears them as described\nin the section called “Argument List Processing”. As a special case, if the option is\n-o or +o and no argument is supplied, the shell prints the settings of all its op‐\ntions. If the option is -o, the settings are printed in a human-readable format; if\nthe option is +o, the settings are printed in a format suitable for reinput to the\nshell to affect the same option settings.\n\nThe third use of the set command is to set the values of the shell's positional para‐\nmeters to the specified args. To change the positional parameters without changing\nany options, use “--” as the first argument to set. If no args are present, the set\ncommand will clear all the positional parameters (equivalent to executing “shift $#”.)\n\nshift [n]\nShift the positional parameters n times. A shift sets the value of $1 to the value of\n$2, the value of $2 to the value of $3, and so on, decreasing the value of $# by one.\nIf n is greater than the number of positional parameters, shift will issue an error\nmessage, and exit with return status 2.\n\ntest expression\n\n[ expression ]\nThe test utility evaluates the expression and, if it evaluates to true, returns a zero\n(true) exit status; otherwise it returns 1 (false). If there is no expression, test\nalso returns 1 (false).\n\nAll operators and flags are separate arguments to the test utility.\n\nThe following primaries are used to construct expression:\n\n-b file True if file exists and is a block special file.\n\n-c file True if file exists and is a character special file.\n\n-d file True if file exists and is a directory.\n\n-e file True if file exists (regardless of type).\n\n-f file True if file exists and is a regular file.\n\n-g file True if file exists and its set group ID flag is set.\n\n-h file True if file exists and is a symbolic link.\n\n-k file True if file exists and its sticky bit is set.\n\n-n string True if the length of string is nonzero.\n\n-p file True if file is a named pipe (FIFO).\n\n-r file True if file exists and is readable.\n\n-s file True if file exists and has a size greater than zero.\n\n-t filedescriptor\nTrue if the file whose file descriptor number is filedescriptor is open\nand is associated with a terminal.\n\n-u file True if file exists and its set user ID flag is set.\n\n-w file True if file exists and is writable. True indicates only that the write\nflag is on. The file is not writable on a read-only file system even if\nthis test indicates true.\n\n-x file True if file exists and is executable. True indicates only that the ex‐\necute flag is on. If file is a directory, true indicates that file can\nbe searched.\n\n-z string True if the length of string is zero.\n\n-L file True if file exists and is a symbolic link. This operator is retained\nfor compatibility with previous versions of this program. Do not rely\non its existence; use -h instead.\n\n-O file True if file exists and its owner matches the effective user id of this\nprocess.\n\n-G file True if file exists and its group matches the effective group id of this\nprocess.\n\n-S file True if file exists and is a socket.\n\nfile1 -nt file2\nTrue if file1 and file2 exist and file1 is newer than file2.\n\nfile1 -ot file2\nTrue if file1 and file2 exist and file1 is older than file2.\n\nfile1 -ef file2\nTrue if file1 and file2 exist and refer to the same file.\n\nstring True if string is not the null string.\n\ns1 = s2 True if the strings s1 and s2 are identical.\n\ns1 != s2 True if the strings s1 and s2 are not identical.\n\ns1 < s2 True if string s1 comes before s2 based on the ASCII value of their\ncharacters.\n\ns1 > s2 True if string s1 comes after s2 based on the ASCII value of their char‐\nacters.\n\nn1 -eq n2 True if the integers n1 and n2 are algebraically equal.\n\nn1 -ne n2 True if the integers n1 and n2 are not algebraically equal.\n\nn1 -gt n2 True if the integer n1 is algebraically greater than the integer n2.\n\nn1 -ge n2 True if the integer n1 is algebraically greater than or equal to the in‐\nteger n2.\n\nn1 -lt n2 True if the integer n1 is algebraically less than the integer n2.\n\nn1 -le n2 True if the integer n1 is algebraically less than or equal to the inte‐\nger n2.\n\nThese primaries can be combined with the following operators:\n\n! expression True if expression is false.\n\nexpression1 -a expression2\nTrue if both expression1 and expression2 are true.\n\nexpression1 -o expression2\nTrue if either expression1 or expression2 are true.\n\n(expression) True if expression is true.\n\nThe -a operator has higher precedence than the -o operator.\n\ntimes Print the accumulated user and system times for the shell and for processes run from\nthe shell. The return status is 0.\n\ntrap [action signal ...]\nCause the shell to parse and execute action when any of the specified signals are re‐\nceived. The signals are specified by signal number or as the name of the signal. If\nsignal is 0 or EXIT, the action is executed when the shell exits. action may be empty\n(''), which causes the specified signals to be ignored. With action omitted or set to\n`-' the specified signals are set to their default action. When the shell forks off a\nsubshell, it resets trapped (but not ignored) signals to the default action. The trap\ncommand has no effect on signals that were ignored on entry to the shell. trap with‐\nout any arguments cause it to write a list of signals and their associated action to\nthe standard output in a format that is suitable as an input to the shell that\nachieves the same trapping results.\n\nExamples:\n\ntrap\n\nList trapped signals and their corresponding action\n\ntrap '' INT QUIT tstp 30\n\nIgnore signals INT QUIT TSTP USR1\n\ntrap date INT\n\nPrint date upon receiving signal INT\n\ntype [name ...]\nInterpret each name as a command and print the resolution of the command search. Pos‐\nsible resolutions are: shell keyword, alias, shell builtin, command, tracked alias and\nnot found. For aliases the alias expansion is printed; for commands and tracked\naliases the complete pathname of the command is printed.\n\nulimit [-H | -S] [-a | -tfdscmlpnvwr [value]]\nInquire about or set the hard or soft limits on processes or set new limits. The\nchoice between hard limit (which no process is allowed to violate, and which may not\nbe raised once it has been lowered) and soft limit (which causes processes to be sig‐\nnaled but not necessarily killed, and which may be raised) is made with these flags:\n\n-H set or inquire about hard limits\n\n-S set or inquire about soft limits. If neither -H nor -S is specified, the\nsoft limit is displayed or both limits are set. If both are specified,\nthe last one wins.\n\nThe limit to be interrogated or set, then, is chosen by specifying any one of these\nflags:\n\n-a show all the current limits\n\n-t show or set the limit on CPU time (in seconds)\n\n-f show or set the limit on the largest file that can be created (in 512-byte\nblocks)\n\n-d show or set the limit on the data segment size of a process (in kilobytes)\n\n-s show or set the limit on the stack size of a process (in kilobytes)\n\n-c show or set the limit on the largest core dump size that can be produced\n(in 512-byte blocks)\n\n-m show or set the limit on the total physical memory that can be in use by a\nprocess (in kilobytes)\n\n-l show or set the limit on how much memory a process can lock with mlock(2)\n(in kilobytes)\n\n-p show or set the limit on the number of processes this user can have at one\ntime\n\n-n show or set the limit on the number files a process can have open at once\n\n-v show or set the limit on the total virtual memory that can be in use by a\nprocess (in kilobytes)\n\n-w show or set the limit on the total number of locks held by a process\n\n-r show or set the limit on the real-time scheduling priority of a process\n\nIf none of these is specified, it is the limit on file size that is shown or set. If\nvalue is specified, the limit is set to that number; otherwise the current limit is\ndisplayed.\n\nLimits of an arbitrary process can be displayed or set using the sysctl(8) utility.\n\numask [mask]\nSet the value of umask (see umask(2)) to the specified octal value. If the argument\nis omitted, the umask value is printed.\n\nunalias [-a] [name]\nIf name is specified, the shell removes that alias. If -a is specified, all aliases\nare removed.\n\nunset [-fv] name ...\nThe specified variables and functions are unset and unexported. If -f or -v is speci‐\nfied, the corresponding function or variable is unset, respectively. If a given name\ncorresponds to both a variable and a function, and no options are given, only the\nvariable is unset.\n\nwait [job]\nWait for the specified job to complete and return the exit status of the last process\nin the job. If the argument is omitted, wait for all jobs to complete and return an\nexit status of zero.\n", "flag": "-l" }, { "name": "Command Line Editing", "content": "When dash is being used interactively from a terminal, the current command and the command\nhistory (see fc in “Builtins”) can be edited using vi-mode command-line editing. This mode\nuses commands, described below, similar to a subset of those described in the vi man page.\nThe command ‘set -o vi’ enables vi-mode editing and places sh into vi insert mode. With vi-\nmode enabled, sh can be switched between insert mode and command mode. It is similar to vi:\ntyping ⟨ESC⟩ enters vi command mode. Hitting ⟨return⟩ while in command mode will pass the\nline to the shell.\n" } ] }, "EXIT STATUS": { "content": "Errors that are detected by the shell, such as a syntax error, will cause the shell to exit\nwith a non-zero exit status. If the shell is not an interactive shell, the execution of the\nshell file will be aborted. Otherwise the shell will return the exit status of the last com‐\nmand executed, or if the exit builtin is used with a numeric argument, it will return the ar‐\ngument.\n", "subsections": [] }, "ENVIRONMENT": { "content": "HOME Set automatically by login(1) from the user's login directory in the password file\n(passwd(4)). This environment variable also functions as the default argument for\nthe cd builtin.\n\nPATH The default search path for executables. See the above section “Path Search”.\n\nCDPATH The search path used with the cd builtin.\n\nMAIL The name of a mail file, that will be checked for the arrival of new mail. Over‐\nridden by MAILPATH.\n\nMAILCHECK The frequency in seconds that the shell checks for the arrival of mail in the\nfiles specified by the MAILPATH or the MAIL file. If set to 0, the check will oc‐\ncur at each prompt.\n\nMAILPATH A colon “:” separated list of file names, for the shell to check for incoming\nmail. This environment setting overrides the MAIL setting. There is a maximum of\n10 mailboxes that can be monitored at once.\n\nPS1 The primary prompt string, which defaults to “$ ”, unless you are the superuser,\nin which case it defaults to “# ”.\n\nPS2 The secondary prompt string, which defaults to “> ”.\n\nPS4 Output before each line when execution trace (set -x) is enabled, defaults to\n“+ ”.\n\nIFS Input Field Separators. This is normally set to ⟨space⟩, ⟨tab⟩, and ⟨newline⟩.\nSee the “White Space Splitting” section for more details.\n\nTERM The default terminal setting for the shell. This is inherited by children of the\nshell, and is used in the history editing modes.\n\nHISTSIZE The number of lines in the history buffer for the shell.\n\nPWD The logical value of the current working directory. This is set by the cd com‐\nmand.\n\nOLDPWD The previous logical value of the current working directory. This is set by the\ncd command.\n\nPPID The process ID of the parent process of the shell.\n", "subsections": [] }, "FILES": { "content": "$HOME/.profile\n\n/etc/profile\n", "subsections": [] }, "SEE ALSO": { "content": "csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1), getopt(3), passwd(5),\nenviron(7), sysctl(8)\n", "subsections": [] }, "HISTORY": { "content": "dash is a POSIX-compliant implementation of /bin/sh that aims to be as small as possible.\ndash is a direct descendant of the NetBSD version of ash (the Almquist SHell), ported to\nLinux in early 1997. It was renamed to dash in 2002.\n", "subsections": [] }, "BUGS": { "content": "Setuid shell scripts should be avoided at all costs, as they are a significant security risk.\n\nPS1, PS2, and PS4 should be subject to parameter expansion before being displayed.\n\nDebian January 19, 2003 DASH(1)", "subsections": [] } }, "summary": "dash — command interpreter (shell)", "flags": [ { "flag": "-l", "long": null, "arg": null, "description": "List the commands rather than invoking an editor on them. The commands are written in the sequence indicated by the first and last operands, as affected by -r, with each command preceded by the command number. -n Suppress command numbers when listing with -l. -r Reverse the order of the commands listed (with -l) or edited (with neither -l nor -s). -s Re-execute the command without invoking an editor. first last Select the commands to list or edit. The number of previous commands that can be accessed are determined by the value of the HISTSIZE variable. The value of first or last or both are one of the following: [+]number A positive number representing a command number; command numbers can be displayed with the -l option. -number A negative decimal number representing the command that was executed number of commands previously. For example, -1 is the immediately pre‐ vious command. string A string indicating the most recently entered command that begins with that string. If the old=new operand is not also specified with -s, the string form of the first operand cannot contain an embedded equal sign. The following environment variables affect the execution of fc: FCEDIT Name of the editor to use. HISTSIZE The number of previous commands that are accessible. fg [job] Move the specified job or the current job to the foreground. getopts optstring var [arg ...] The POSIX getopts command, not to be confused with the Bell Labs-derived getopt(1). The first argument should be a series of letters, each of which may be optionally fol‐ lowed by a colon to indicate that the option requires an argument. The variable spec‐ ified is set to the parsed option. The getopts command deprecates the older getopt(1) utility due to its handling of ar‐ guments containing whitespace. The getopts builtin may be used to obtain options and their arguments from a list of parameters. When invoked, getopts places the value of the next option from the option string in the list in the shell variable specified by var and its index in the shell variable OPTIND. When the shell is invoked, OPTIND is initialized to 1. For each op‐ tion that requires an argument, the getopts builtin will place it in the shell vari‐ able OPTARG. If an option is not allowed for in the optstring, then OPTARG will be unset. By default, the variables $1, ..., $n are inspected; if args are specified, they'll be parsed instead. optstring is a string of recognized option letters (see getopt(3)). If a letter is followed by a colon, the option is expected to have an argument which may or may not be separated from it by white space. If an option character is not found where ex‐ pected, getopts will set the variable var to a “?”; getopts will then unset OPTARG and write output to standard error. By specifying a colon as the first character of optstring all errors will be ignored. After the last option getopts will return a non-zero value and set var to “?”. The following code fragment shows how one might process the arguments for a command that can take the options [a] and [b], and the option [c], which requires an argument. while getopts abc: f do case $f in a | b) flag=$f;; c) carg=$OPTARG;; \\?) echo $USAGE; exit 1;; esac done shift $((OPTIND - 1)) This code will accept any of the following as equivalent: cmd -acarg file file cmd -a -c arg file file cmd -carg -a file file cmd -a -carg -- file file hash [command ...] hash -r The shell maintains a hash table which remembers the locations of commands. With no arguments whatsoever, the hash command prints out the contents of this table. Entries which have not been looked at since the last cd command are marked with an asterisk; it is possible for these entries to be invalid. With arguments, the hash command removes the specified commands from the hash table (unless they are functions) and then locates them. The -r option causes the hash com‐ mand to delete all the entries in the hash table except for functions. jobs [-lp] [job ...] Display the status of all, or just the specified, jobs: By default display the job number, currency (+-) status, if any, the job state, and its shell command. -l also output the PID of the group leader, and just the PID and shell commands of other members of the job. -p Display only leader PIDs, one per line. kill [-s sigspec | -signum | -sigspec] [pid | job ...] Equivalent to kill(1), but a job spec may also be specified. Signals can be either case-insensitive names without SIG prefixes or decimal numbers; the default is TERM. kill -l [signum | exitstatus] List available signal names without the SIG prefix (sigspecs). If signum specified, display just the sigspec for that signal. If exitstatus specified (> 128), display just the sigspec that caused it. pwd [-LP] builtin command remembers what the current directory is rather than recomputing it each time. This makes it faster. However, if the current directory is renamed, the builtin version of pwd will continue to print the old name for the directory. The -P option causes the physical value of the current working directory to be shown, that is, all symbolic links are resolved to their respective values. The -L option turns off the effect of any preceding -P options. read [-p prompt] [-r] variable [...] The prompt is printed if the -p option is specified and the standard input is a termi‐ nal. Then a line is read from the standard input. The trailing newline is deleted from the line and the line is split as described in the section on word splitting above, and the pieces are assigned to the variables in order. At least one variable must be specified. If there are more pieces than variables, the remaining pieces (along with the characters in IFS that separated them) are assigned to the last vari‐ able. If there are more variables than pieces, the remaining variables are assigned the null string. The read builtin will indicate success unless EOF is encountered on input, in which case failure is returned. By default, unless the -r option is specified, the backslash “\\” acts as an escape character, causing the following character to be treated literally. If a backslash is followed by a newline, the backslash and the newline will be deleted. readonly name ... readonly -p The specified names are marked as read only, so that they cannot be subsequently modi‐ fied or unset. The shell allows the value of a variable to be set at the same time it is marked read only by writing readonly name=value With no arguments the readonly command lists the names of all read only variables. With the -p option specified the output will be formatted suitably for non-interactive use. printf format [value]... printf formats and prints its arguments according to format, a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences which are converted and copied to the standard out‐ put, and format specifications, each of which causes printing of the next successive value. Each value is treated as a string if the corresponding format specification is either b, c, or s; otherwise it is evaluated as a C constant, with the following additions: • A leading plus or minus sign is allowed. • If the leading character is a single or double quote, the value of the next byte. The format string is reused as often as necessary until all values are consumed. Any extra format specifications are evaluated with zero or the null string. Character escape sequences are in backslash notation as defined in ANSI X3.159-1989 (“ANSI C89”). The characters and their meanings are as follows: \\a Write a character. \\b Write a character. \\e Write an (ESC) character. \\f Write a character. \\n Write a character. \\r Write a character. \\t Write a character. \\v Write a character. \\\\ Write a backslash character. \\num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit oc‐ tal number num. Each format specification is introduced by the percent character (``%''). The remain‐ der of the format specification includes, in the following order: Zero or more of the following flags: # A `#' character specifying that the value should be printed in an ``alternative form''. For b, c, d, and s formats, this option has no effect. For the o format the precision of the number is increased to force the first character of the output string to a zero. For the x (X) format, a non-zero result has the string 0x (0X) prepended to it. For e, E, f, g, and G formats, the result will always contain a deci‐ mal point, even if no digits follow the point (normally, a decimal point only appears in the results of those formats if a digit follows the decimal point). For g and G formats, trailing zeros are not re‐ moved from the result as they would otherwise be. - A minus sign `-' which specifies left adjustment of the output in the indicated field; + A `+' character specifying that there should always be a sign placed before the number when using signed formats. ‘ ’ A space specifying that a blank should be left before a positive num‐ ber for a signed format. A `+' overrides a space if both are used; 0 A zero `0' character indicating that zero-padding should be used rather than blank-padding. A `-' overrides a `0' if both are used; Field Width: An optional digit string specifying a field width; if the output string has fewer characters than the field width it will be blank-padded on the left (or right, if the left-adjustment indicator has been given) to make up the field width (note that a leading zero is a flag, but an embedded zero is part of a field width); Precision: An optional period, ‘.’, followed by an optional digit string giving a precision which specifies the number of digits to appear after the decimal point, for e and f formats, or the maximum number of bytes to be printed from a string (b and s formats); if the digit string is missing, the precision is treated as zero; Format: A character which indicates the type of format to use (one of diouxXfwEgGbcs). A field width or precision may be ‘*’ instead of a digit string. In this case an argument supplies the field width or precision. The format characters and their meanings are: diouXx The argument is printed as a signed decimal (d or i), unsigned octal, un‐ signed decimal, or unsigned hexadecimal (X or x), respectively. f The argument is printed in the style [-]ddd.ddd where the number of d's after the decimal point is equal to the precision specification for the argument. If the precision is missing, 6 digits are given; if the preci‐ sion is explicitly 0, no digits and no decimal point are printed. eE The argument is printed in the style [-]d.ddde±dd where there is one digit before the decimal point and the number after is equal to the precision specification for the argument; when the precision is missing, 6 digits are produced. An upper-case E is used for an `E' format. gG The argument is printed in style f or in style e (E) whichever gives full precision in minimum space. b Characters from the string argument are printed with backslash-escape se‐ quences expanded. The following additional backslash-escape sequences are supported: \\c Causes dash to ignore any remaining characters in the string operand containing it, any remaining string operands, and any ad‐ ditional characters in the format operand. \\0num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number num. c The first character of argument is printed. s Characters from the string argument are printed until the end is reached or until the number of bytes indicated by the precision specification is reached; if the precision is omitted, all characters in the string are printed. % Print a `%'; no argument is used. In no case does a non-existent or small field width cause truncation of a field; padding takes place only if the specified field width exceeds the actual width. set [{ -options | +options | -- }] arg ... The set command performs three different functions. With no arguments, it lists the values of all shell variables. If options are given, it sets the specified option flags, or clears them as described in the section called “Argument List Processing”. As a special case, if the option is -o or +o and no argument is supplied, the shell prints the settings of all its op‐ tions. If the option is -o, the settings are printed in a human-readable format; if the option is +o, the settings are printed in a format suitable for reinput to the shell to affect the same option settings. The third use of the set command is to set the values of the shell's positional para‐ meters to the specified args. To change the positional parameters without changing any options, use “--” as the first argument to set. If no args are present, the set command will clear all the positional parameters (equivalent to executing “shift $#”.) shift [n] Shift the positional parameters n times. A shift sets the value of $1 to the value of $2, the value of $2 to the value of $3, and so on, decreasing the value of $# by one. If n is greater than the number of positional parameters, shift will issue an error message, and exit with return status 2. test expression [ expression ] The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false). If there is no expression, test also returns 1 (false). All operators and flags are separate arguments to the test utility. The following primaries are used to construct expression: -b file True if file exists and is a block special file. -c file True if file exists and is a character special file. -d file True if file exists and is a directory. -e file True if file exists (regardless of type). -f file True if file exists and is a regular file. -g file True if file exists and its set group ID flag is set. -h file True if file exists and is a symbolic link. -k file True if file exists and its sticky bit is set. -n string True if the length of string is nonzero. -p file True if file is a named pipe (FIFO). -r file True if file exists and is readable. -s file True if file exists and has a size greater than zero. -t filedescriptor True if the file whose file descriptor number is filedescriptor is open and is associated with a terminal. -u file True if file exists and its set user ID flag is set. -w file True if file exists and is writable. True indicates only that the write flag is on. The file is not writable on a read-only file system even if this test indicates true. -x file True if file exists and is executable. True indicates only that the ex‐ ecute flag is on. If file is a directory, true indicates that file can be searched. -z string True if the length of string is zero. -L file True if file exists and is a symbolic link. This operator is retained for compatibility with previous versions of this program. Do not rely on its existence; use -h instead. -O file True if file exists and its owner matches the effective user id of this process. -G file True if file exists and its group matches the effective group id of this process. -S file True if file exists and is a socket. file1 -nt file2 True if file1 and file2 exist and file1 is newer than file2. file1 -ot file2 True if file1 and file2 exist and file1 is older than file2. file1 -ef file2 True if file1 and file2 exist and refer to the same file. string True if string is not the null string. s1 = s2 True if the strings s1 and s2 are identical. s1 != s2 True if the strings s1 and s2 are not identical. s1 < s2 True if string s1 comes before s2 based on the ASCII value of their characters. s1 > s2 True if string s1 comes after s2 based on the ASCII value of their char‐ acters. n1 -eq n2 True if the integers n1 and n2 are algebraically equal. n1 -ne n2 True if the integers n1 and n2 are not algebraically equal. n1 -gt n2 True if the integer n1 is algebraically greater than the integer n2. n1 -ge n2 True if the integer n1 is algebraically greater than or equal to the in‐ teger n2. n1 -lt n2 True if the integer n1 is algebraically less than the integer n2. n1 -le n2 True if the integer n1 is algebraically less than or equal to the inte‐ ger n2. These primaries can be combined with the following operators: ! expression True if expression is false. expression1 -a expression2 True if both expression1 and expression2 are true. expression1 -o expression2 True if either expression1 or expression2 are true. (expression) True if expression is true. The -a operator has higher precedence than the -o operator. times Print the accumulated user and system times for the shell and for processes run from the shell. The return status is 0. trap [action signal ...] Cause the shell to parse and execute action when any of the specified signals are re‐ ceived. The signals are specified by signal number or as the name of the signal. If signal is 0 or EXIT, the action is executed when the shell exits. action may be empty (''), which causes the specified signals to be ignored. With action omitted or set to `-' the specified signals are set to their default action. When the shell forks off a subshell, it resets trapped (but not ignored) signals to the default action. The trap command has no effect on signals that were ignored on entry to the shell. trap with‐ out any arguments cause it to write a list of signals and their associated action to the standard output in a format that is suitable as an input to the shell that achieves the same trapping results. Examples: trap List trapped signals and their corresponding action trap '' INT QUIT tstp 30 Ignore signals INT QUIT TSTP USR1 trap date INT Print date upon receiving signal INT type [name ...] Interpret each name as a command and print the resolution of the command search. Pos‐ sible resolutions are: shell keyword, alias, shell builtin, command, tracked alias and not found. For aliases the alias expansion is printed; for commands and tracked aliases the complete pathname of the command is printed. ulimit [-H | -S] [-a | -tfdscmlpnvwr [value]] Inquire about or set the hard or soft limits on processes or set new limits. The choice between hard limit (which no process is allowed to violate, and which may not be raised once it has been lowered) and soft limit (which causes processes to be sig‐ naled but not necessarily killed, and which may be raised) is made with these flags: -H set or inquire about hard limits -S set or inquire about soft limits. If neither -H nor -S is specified, the soft limit is displayed or both limits are set. If both are specified, the last one wins. The limit to be interrogated or set, then, is chosen by specifying any one of these flags: -a show all the current limits -t show or set the limit on CPU time (in seconds) -f show or set the limit on the largest file that can be created (in 512-byte blocks) -d show or set the limit on the data segment size of a process (in kilobytes) -s show or set the limit on the stack size of a process (in kilobytes) -c show or set the limit on the largest core dump size that can be produced (in 512-byte blocks) -m show or set the limit on the total physical memory that can be in use by a process (in kilobytes) -l show or set the limit on how much memory a process can lock with mlock(2) (in kilobytes) -p show or set the limit on the number of processes this user can have at one time -n show or set the limit on the number files a process can have open at once -v show or set the limit on the total virtual memory that can be in use by a process (in kilobytes) -w show or set the limit on the total number of locks held by a process -r show or set the limit on the real-time scheduling priority of a process If none of these is specified, it is the limit on file size that is shown or set. If value is specified, the limit is set to that number; otherwise the current limit is displayed. Limits of an arbitrary process can be displayed or set using the sysctl(8) utility. umask [mask] Set the value of umask (see umask(2)) to the specified octal value. If the argument is omitted, the umask value is printed. unalias [-a] [name] If name is specified, the shell removes that alias. If -a is specified, all aliases are removed. unset [-fv] name ... The specified variables and functions are unset and unexported. If -f or -v is speci‐ fied, the corresponding function or variable is unset, respectively. If a given name corresponds to both a variable and a function, and no options are given, only the variable is unset. wait [job] Wait for the specified job to complete and return the exit status of the last process in the job. If the argument is omitted, wait for all jobs to complete and return an exit status of zero." } ], "examples": [], "see_also": [ { "name": "csh", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/csh/1/json" }, { "name": "echo", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/echo/1/json" }, { "name": "getopt", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/getopt/1/json" }, { "name": "ksh", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/ksh/1/json" }, { "name": "login", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/login/1/json" }, { "name": "printf", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/printf/1/json" }, { "name": "test", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/test/1/json" }, { "name": "getopt", "section": "3", "url": "https://www.chedong.com/phpMan.php/man/getopt/3/json" }, { "name": "passwd", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/passwd/5/json" }, { "name": "environ", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/environ/7/json" }, { "name": "sysctl", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/sysctl/8/json" } ], "tldr": { "source": "official", "description": "Debian Almquist Shell, a modern, POSIX-compliant implementation of `sh` (not Bash-compatible).", "examples": [ { "description": "Start an interactive shell session", "command": "dash" }, { "description": "Execute specific [c]ommands", "command": "dash -c \"{{echo 'dash is executed'}}\"" }, { "description": "Execute a specific script", "command": "dash {{path/to/script.sh}}" }, { "description": "Check a specific script for syntax errors", "command": "dash -n {{path/to/script.sh}}" }, { "description": "Execute a specific script while printing each command before executing it", "command": "dash -x {{path/to/script.sh}}" }, { "description": "Execute a specific script and stop at the first [e]rror", "command": "dash -e {{path/to/script.sh}}" }, { "description": "Execute specific commands from `stdin`", "command": "{{echo \"echo 'dash is executed'\"}} | dash" } ] } }