{ "mode": "man", "parameter": "sh", "section": "1", "url": "https://www.chedong.com/phpMan.php/man/sh/1/json", "generated": "2026-05-30T04:01:26Z", "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 the\nprocess of being changed to conform with the POSIX 1003.2 and 1003.2a specifications for the\nshell. This version has many features which make it appear similar in some respects to the\nKorn shell, but it is not a Korn shell clone (see ksh(1)). Only features designated by POSIX,\nplus a few Berkeley extensions, are being incorporated into this shell. This man page is not\nintended 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 into\nthe system (although a user can select a different shell with the chsh(1) command). The shell\nimplements a language that has flow control constructs, a macro facility that provides a vari‐\nety of features in addition to data storage, along with built in history and line editing capa‐\nbilities. It incorporates many features to aid interactive use and has the advantage that the\ninterpretative language is common to both interactive and non-interactive use (shell scripts).\nThat is, commands can be typed directly to the running shell or can be put into a file and the\nfile 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 if\nthe -i flag is set), and the -c option is not present, the shell is considered an interactive\nshell. An interactive shell generally prompts before each command and handles programming and\ncommand errors differently (as described below). When first starting, the shell inspects argu‐\nment 0, and if it begins with a dash ‘-’, the shell is also considered a login shell. This is\nnormally done automatically by the system when the user first logs in. A login shell first\nreads commands from the files /etc/profile and .profile if they exist. If the environment\nvariable ENV is set on entry to an interactive shell, or is set in the .profile of a login\nshell, the shell next reads commands from the file named in ENV. Therefore, a user should\nplace commands that are to be executed only at login time in the .profile file, and commands\nthat are executed for every interactive shell inside the ENV file. To set the ENV variable to\nsome file, place the following line in your .profile of your home directory\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 re‐\nmaining arguments are set as the positional parameters of the shell ($1, $2, etc). Otherwise,\nthe 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 description\nbelow. Specifying a dash “-” turns the option on, while using a plus “+” disables the option.\nThe following options can be set from the command line or with the set builtin (described\nlater).\n\n-a allexport Export all variables assigned to.\n\n-c Read commands from the commandstring operand instead of from the stan‐\ndard input. Special parameter 0 will be set from the commandname oper‐\nand and the positional parameters ($1, $2, etc.) set from the remaining\nargument 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. The\nexit status of a command is considered to be explicitly tested if the\ncommand is used to control an if, elif, while, or until; or if the com‐\nmand 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 use‐\nful 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 immediately.\n\n-v verbose The shell writes its input to standard error as it is read. Useful for\ndebugging.\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 shell\nhas 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 has\nbeen 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 is\nnot set by default to help avoid incorrect usage by setuid root programs\nvia 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 called\n“operators”. There are two types of operators: control operators and redirection operators\n(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, such\nas 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 (ex‐\ncept 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 a\nreserved word may occur (see above), and after checking for reserved words, the shell checks\nthe word to see if it matches an alias. If it does, it replaces it in the input stream with\nits value. For example, if there is an alias called “lf” with the value “ls -F”, then the in‐\nput:\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 hav‐\ning to learn how to create functions with arguments. They can also be used to create lexically\nobscure code. This use is discouraged.\n" }, { "name": "Commands", "content": "The shell interprets the words it reads according to a language, the specification of which is\noutside the scope of this man page (refer to the BNF in the POSIX 1003.2 document). Essen‐\ntially though, a line is read and if the first word of the line (or after a control operator)\nis not a reserved word, then the shell has recognized a simple command. Otherwise, a complex\ncommand 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 environ‐\nment of the simple command. Redirection operators and their arguments (as described\nbelow) 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 com‐\nmand name resulted, then the “name=value” variable assignments recognized in item 1\naffect 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 for‐\nmat 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 speci‐\nfied on the initial line is quoted, then the here-doc-text is treated literally, otherwise the\ntext is subjected to parameter expansion, command substitution, and arithmetic expansion (as\ndescribed in the section on “Expansions”). If the operator is “<<-” instead of “<<”, then\nleading 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 – and\nthe command is searched for (by name) in that order. They each are executed in a different\nway.\n\nWhen a shell function is executed, all of the shell positional parameters (except $0, which re‐\nmains unchanged) are set to the arguments of the shell function. The variables which are ex‐\nplicitly placed in the environment of the command (by placing assignments to them before the\nfunction name) are made local to the function and are set to the values given. Then the com‐\nmand given in the function definition is executed. The positional parameters are restored to\ntheir original values when the command completes. This all occurs within the current shell.\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 for\nas a normal program in the file system (as described in the next section). When a normal pro‐\ngram is executed, the shell runs the program, passing the arguments and the environment to the\nprogram. If the program is not a normal executable file (i.e., if it does not begin with the\n\"magic number\" whose ASCII representation is \"#!\", so execve(2) returns ENOEXEC then) the shell\nwill interpret the program in a subshell. The child shell will reinitialize itself in this\ncase, so that the effect will be as if a new shell had been invoked to handle the ad-hoc shell\nscript, except that the location of hashed commands located in the parent shell will be remem‐\nbered 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 name.\nThen it looks for a builtin command by that name. If a builtin command is not found, one of\ntwo 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 di‐\nrectory 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. The\nparadigm is that a command exits with zero for normal or success, and non-zero for failure, er‐\nror, or a false indication. The man page for each command should indicate the various exit\ncodes and what they mean. Additionally, the builtin commands return exit codes, as does an ex‐\necuted 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 words,\ntogether creating a larger complex command. More generally, a command is one of the following:\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 com‐\nmand. 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 to\ncomplete.\n\nIf the reserved word ! does not precede the pipeline, the exit status is the exit status of the\nlast command specified in the pipeline. Otherwise, the exit status is the logical NOT of the\nexit status of the last command. That is, if the last command returns zero, the exit status is\n1; 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 re‐\ndirection, 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\nBackground Commands –– &\nIf 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 execut‐\ning 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\nLists –– Generally Speaking\nA list is a sequence of zero or more commands separated by newlines, semicolons, or ampersands,\nand optionally terminated by one of these three characters. The commands in a list are exe‐\ncuted in the order they are written. If command is followed by an ampersand, the shell starts\nthe command and immediately proceeds onto the next command; otherwise it waits for the command\nto 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 simi‐\nlar, 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\nFlow-Control Constructs –– if, while, for, case\nThe 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 un‐\ntil command is similar, but has the word until in place of while, which causes it to repeat un‐\ntil 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 variable\nset 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 itera‐\ntion 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), sepa‐\nrated 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 (list)\nwill not affect the current shell. The second form does not fork another shell so is slightly\nmore efficient. Grouping commands together this way allows you to redirect their output as\nthough 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 reserved\nword 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 ap‐\npear 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, the\nvariable is initially unset. The shell uses dynamic scoping, so that if you make the variable\nx local to function f, which then calls function g, references to the variable x made inside g\nwill refer to the variable x declared inside f, not to the global variable named 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 val‐\nues 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 un‐\nderscores - the first of which must not be numeric. A parameter can also be denoted by a num‐\nber 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 of\neach 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 example,\nis if $1 is “abc” and $2 is “def ghi”, then \"$@\" expands to the two arguments:\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 implic‐\nitly by the shell.\n\n$ Expands to the process ID of the invoked shell. A subshell retains the same value\nof $ as its parent.\n\n! Expands to the process ID of the most recent background command executed from the\ncurrent shell. For a pipeline, the process ID is that of the last command in the\npipeline.\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 quote\nremovals that occur within a single word expand to a single field. It is only field splitting\nor pathname expansion that can create multiple fields from a single word. The single exception\nto this rule is the expansion of the special parameter @ within double-quotes, as was described\nabove.\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 the\ncharacters up to a slash (/) or the end of the word are treated as a username and are replaced\nwith the user's home directory. If the username is missing (as in ~/foobar), the tilde is re‐\nplaced 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${expression}\n\nwhere expression consists of all characters until the matching “}”. Any “}” escaped by a back‐\nslash or within a quoted string, and characters in embedded arithmetic expansions, command sub‐\nstitutions, and variable expansions, are not examined in determining the matching “}”.\n\nThe simplest form for parameter expansion is:\n\n${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\nIn addition, a parameter expansion can be modified by using one of the following formats.\n\n${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word\nis substituted; otherwise, the value of parameter is substituted.\n\n${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 parame‐\nter is substituted. Only variables, not positional parameters or special\nparameters, can be assigned in this way.\n\n${parameter:?[word]} Indicate Error if Null or Unset. If parameter is unset or null, the ex‐\npansion of word (or a message indicating it is unset if word is omitted)\nis written to standard error and the shell exits with a nonzero exit sta‐\ntus. Otherwise, the value of parameter is substituted. An interactive\nshell need not exit.\n\n${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 test\nfor a parameter that is unset or null; omission of the colon results in a test for a parameter\nthat is only unset.\n\n${#parameter} String Length. The length in characters of the value of parameter.\n\nThe following four varieties of parameter expansion provide for substring processing. In each\ncase, pattern matching notation (see Shell Patterns), rather than regular expression notation,\nis used to evaluate the patterns. If parameter is * or @, the result of the expansion is un‐\nspecified. Enclosing the full parameter expansion string in double-quotes does not cause the\nfollowing four varieties of pattern characters to be quoted, whereas quoting characters within\nthe braces has this effect.\n\n${parameter%word} Remove Smallest Suffix Pattern. The word is expanded to produce a pat‐\ntern. The parameter expansion then results in parameter, with the small‐\nest 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 small‐\nest 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 sequences\nof one or more ⟨newline⟩s at the end of the substitution. (Embedded ⟨newline⟩s before the end\nof the output are not removed; however, during field splitting, they may be translated into\n⟨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 the\nexpression is not treated specially. The shell expands all tokens in the expression for param‐\neter 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 splitting\nand multiple fields can result.\n\nThe shell treats each character of the IFS as a delimiter and uses the delimiters to split the\nresults 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 complete.\nEach word is viewed as a series of patterns, separated by slashes. The process of expansion\nreplaces the word with the names of all existing files whose names can be formed by replacing\neach pattern with a string that matches the specified pattern. There are two restrictions on\nthis: first, a pattern cannot match a string containing a slash, and second, a pattern cannot\nmatch a string starting with a period unless the first character of the pattern is a period.\nThe next section describes the patterns used for both Pathname Expansion and 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 charac‐\nter. A left bracket (“[”) introduces a character class. The end of the character class is in‐\ndicated by a (“]”); if the “]” is missing then the “[” matches a “[” rather than introducing a\ncharacter class. A character class matches any of the characters between the square brackets.\nA range of characters may be specified using a minus sign. The character class may be comple‐\nmented by making an exclamation point the first character of the character class.\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 op‐\neration that can't be performed by a separate process. In addition to these, there are several\nother commands that may be builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).\n\n:\n\ntrue A null command that returns a 0 (true) 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, the\nalias 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 background.\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 of\nthe 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 -\n\ncd [-LP] [directory]\nSwitch to the specified directory (default HOME). If an entry for CDPATH appears in the\nenvironment of the cd command or the shell variable CDPATH is set and the directory name\ndoes not begin with a slash, then the directories listed in CDPATH will be searched for\nthe specified directory. The format of CDPATH is the same as that of PATH. If a single\ndash is specified as the argument, it will be replaced by the value of OLDPWD. The cd\ncommand will print out the name of the directory that it actually switched to if this is\ndifferent from the name that the user gave. These may be different either because the\nCDPATH mechanism was used or because the argument is a single dash. The -P option\ncauses the physical directory structure to be used, that is, all symbolic links are re‐\nsolved to their respective values. The -L option turns off the effect of any preceding\n-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 com‐\nmand 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 allows\nthe 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 inter‐\nactive shell.\n\n-e editor\nUse the editor named by editor to edit the commands. The editor string is a com‐\nmand 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 un‐\nset, the value of the EDITOR variable is used. If EDITOR is null or unset, ed(1)\nis used as the editor.\n" }, { "name": "-l (ell)", "content": "List the commands rather than invoking an editor on them. The commands are writ‐\nten in the sequence indicated by the first and last operands, as affected by -r,\nwith 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 nor\n-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 be\naccessed 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 num‐\nber of commands previously. For example, -1 is the immediately previous\ncommand.\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 of\nthe 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\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 speci‐\nfied is set to the parsed option.\n\nThe getopts command deprecates the older getopt(1) utility due to its handling of argu‐\nments containing whitespace.\n\nThe getopts builtin may be used to obtain options and their arguments from a list of pa‐\nrameters. 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 variable\nOPTARG. If an option is not allowed for in the optstring, then OPTARG will be unset.\n\noptstring is a string of recognized option letters (see getopt(3)). If a letter is fol‐\nlowed by a colon, the option is expected to have an argument which may or may not be\nseparated from it by white space. If an option character is not found where expected,\ngetopts will set the variable var to a “?”; getopts will then unset OPTARG and write\noutput to standard error. By specifying a colon as the first character of optstring all\nerrors 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 that\ncan 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 `expr $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 -rv command ...\nThe shell maintains a hash table which remembers the locations of commands. With no ar‐\nguments 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; it\nis possible for these entries to be invalid.\n\nWith arguments, the hash command removes the specified commands from the hash table (un‐\nless they are functions) and then locates them. With the -v option, hash prints the lo‐\ncations of the commands as it finds them. The -r option causes the hash command to\ndelete all the entries in the hash table except for functions.\n\npwd [-LP]\nbuiltin command remembers what the current directory is rather than recomputing it each\ntime. This makes it faster. However, if the current directory is renamed, the builtin\nversion of pwd will continue to print the old name for the directory. The -P option\ncauses the physical value of the current working directory to be shown, that is, all\nsymbolic links are resolved to their respective values. The -L option turns off the ef‐\nfect 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 from\nthe line and the line is split as described in the section on word splitting above, and\nthe pieces are assigned to the variables in order. At least one variable must be speci‐\nfied. If there are more pieces than variables, the remaining pieces (along with the\ncharacters in IFS that separated them) are assigned to the last variable. If there are\nmore variables than pieces, the remaining variables are assigned the null string. The\nread builtin will indicate success unless EOF is encountered on input, in which case\nfailure is returned.\n\nBy default, unless the -r option is specified, the backslash “\\” acts as an escape char‐\nacter, causing the following character to be treated literally. If a backslash is fol‐\nlowed 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. With\nthe -p option specified the output will be formatted suitably for non-interactive use.\n\nprintf format [arguments ...]\nprintf formats and prints its arguments, after the first, under control of the format.\nThe format is a character string which contains three types of objects: plain charac‐\nters, which are simply copied to standard output, character escape sequences which are\nconverted and copied to the standard output, and format specifications, each of which\ncauses printing of the next successive argument.\n\nThe arguments after the first are treated as strings if the corresponding format is ei‐\nther b, c or s; otherwise it is evaluated as a C constant, with the following exten‐\nsions:\n\n•• A leading plus or minus sign is allowed.\n•• If the leading character is a single or double quote, the value is the ASCII\ncode of the next character.\n\nThe format string is reused as often as necessary to satisfy the arguments. Any extra\nformat 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 octal\nnumber num.\n\nEach format specification is introduced by the percent character (``%''). The remainder\nof 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 ``al‐\nternative form''. For b, c, d, and s formats, this option has no ef‐\nfect. 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 (X)\nformat, a non-zero result has the string 0x (0X) prepended to it. For\ne, E, f, g, and G formats, the result will always contain a decimal\npoint, even if no digits follow the point (normally, a decimal point\nonly appears in the results of those formats if a digit follows the dec‐\nimal point). For g and G formats, trailing zeros are not removed from\nthe 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 be‐\nfore the number when using signed formats.\n\n‘ ’ A space specifying that a blank should be left before a positive number\nfor a signed format. A `+' overrides a space if both are used;\n\n0 A zero `0' character indicating that zero-padding should be used rather\nthan 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 precision\nwhich specifies the number of digits to appear after the decimal point, for e\nand f formats, or the maximum number of bytes to be printed from a string (b and\ns formats); if the digit string is missing, the precision is treated 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 af‐\nter the decimal point is equal to the precision specification for the argu‐\nment. If the precision is missing, 6 digits are given; if the precision is\nexplicitly 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 are\nproduced. 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 operand\ncontaining it, any remaining string operands, and any additional\ncharacters in the format operand.\n\n\\0num Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit\noctal 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 or\nuntil 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; padding\ntakes 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 in\nthe section called Argument List Processing. As a special case, if the option is -o or\n+o and no argument is supplied, the shell prints the settings of all its options. If\nthe option is -o, the settings are printed in a human-readable format; if the option is\n+o, the settings are printed in a format suitable for reinput to the shell to affect the\nsame option settings.\n\nThe third use of the set command is to set the values of the shell's positional parame‐\nters to the specified args. To change the positional parameters without changing any\noptions, use “--” as the first argument to set. If no args are present, the set command\nwill 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 mes‐\nsage, 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 exe‐\ncute flag is on. If file is a directory, true indicates that file can be\nsearched.\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 for\ncompatibility with previous versions of this program. Do not rely on its\nexistence; 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 char‐\nacters.\n\ns1 > s2 True if string s1 comes after s2 based on the ASCII value of their charac‐\nters.\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 inte‐\nger 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 integer\nn2.\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 the\nshell. 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 without\nany arguments cause it to write a list of signals and their associated action to the\nstandard output in a format that is suitable as an input to the shell that achieves the\nsame 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. Possi‐\nble resolutions are: shell keyword, alias, shell builtin, command, tracked alias and not\nfound. For aliases the alias expansion is printed; for commands and tracked aliases the\ncomplete pathname of the command is printed.\n\nulimit [-H | -S] [-a | -tfdscmlpnv [value]]\nInquire about or set the hard or soft limits on processes or set new limits. The choice\nbetween hard limit (which no process is allowed to violate, and which may not be raised\nonce it has been lowered) and soft limit (which causes processes to be signaled but not\nnecessarily 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, the\nlast 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 (in\n512-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-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 dis‐\nplayed.\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 is\nomitted, the umask value is printed.\n\nunalias [-a] [name]\nIf name is specified, the shell removes that alias. If -a is specified, all aliases are\nremoved.\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 vari‐\nable is unset.\n\nwait [job]\nWait for the specified job to complete and return the exit status of the last process in\nthe job. If the argument is omitted, wait for all jobs to complete and return an exit\nstatus 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 his‐\ntory (see fc in Builtins) can be edited using vi-mode command-line editing. This mode uses\ncommands, described below, similar to a subset of those described in the vi man page. The com‐\nmand ‘set -o vi’ enables vi-mode editing and places sh into vi insert mode. With vi-mode en‐\nabled, sh can be switched between insert mode and command mode. It is similar to vi: typing\n⟨ESC⟩ enters vi command mode. Hitting ⟨return⟩ while in command mode will pass the line to the\nshell.\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 argu‐\nment.\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. Overrid‐\nden by MAILPATH.\n\nMAILCHECK The frequency in seconds that the shell checks for the arrival of mail in the files\nspecified by the MAILPATH or the MAIL file. If set to 0, the check will occur at\neach prompt.\n\nMAILPATH A colon “:” separated list of file names, for the shell to check for incoming mail.\nThis environment setting overrides the MAIL setting. There is a maximum of 10 mail‐\nboxes that can be monitored at once.\n\nPS1 The primary prompt string, which defaults to “$ ”, unless you are the superuser, in\nwhich 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\nIFS Input Field Separators. This is normally set to ⟨space⟩, ⟨tab⟩, and ⟨newline⟩. See\nthe 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 command.\n\nOLDPWD The previous logical value of the current working directory. This is set by the cd\ncommand.\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. dash\nis a direct descendant of the NetBSD version of ash (the Almquist SHell), ported to Linux in\nearly 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\nBSD January 19, 2003 BSD", "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 writ‐ ten 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 num‐ ber of commands previously. For example, -1 is the immediately previous 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 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 speci‐ fied is set to the parsed option. The getopts command deprecates the older getopt(1) utility due to its handling of argu‐ ments containing whitespace. The getopts builtin may be used to obtain options and their arguments from a list of pa‐ rameters. 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 variable OPTARG. If an option is not allowed for in the optstring, then OPTARG will be unset. optstring is a string of recognized option letters (see getopt(3)). If a letter is fol‐ lowed 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 expected, 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 `expr $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 -rv command ... The shell maintains a hash table which remembers the locations of commands. With no ar‐ guments 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 (un‐ less they are functions) and then locates them. With the -v option, hash prints the lo‐ cations of the commands as it finds them. The -r option causes the hash command to delete all the entries in the hash table except for functions. 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 ef‐ fect 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 speci‐ fied. If there are more pieces than variables, the remaining pieces (along with the characters in IFS that separated them) are assigned to the last variable. 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 char‐ acter, causing the following character to be treated literally. If a backslash is fol‐ lowed 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 [arguments ...] printf formats and prints its arguments, after the first, under control of the format. The format is a character string which contains three types of objects: plain charac‐ ters, which are simply copied to standard output, character escape sequences which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. The arguments after the first are treated as strings if the corresponding format is ei‐ ther b, c or s; otherwise it is evaluated as a C constant, with the following exten‐ sions: •• A leading plus or minus sign is allowed. •• If the leading character is a single or double quote, the value is the ASCII code of the next character. The format string is reused as often as necessary to satisfy the arguments. 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 octal number num. Each format specification is introduced by the percent character (``%''). The remainder 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 ``al‐ ternative form''. For b, c, d, and s formats, this option has no ef‐ fect. 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 decimal 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 dec‐ imal point). For g and G formats, trailing zeros are not removed 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 be‐ fore the number when using signed formats. ‘ ’ A space specifying that a blank should be left before a positive number 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 af‐ ter the decimal point is equal to the precision specification for the argu‐ ment. If the precision is missing, 6 digits are given; if the precision 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 additional 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 options. 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 parame‐ ters 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 mes‐ sage, 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 exe‐ cute 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 char‐ acters. s1 > s2 True if string s1 comes after s2 based on the ASCII value of their charac‐ ters. 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 inte‐ ger 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 integer 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 without 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. Possi‐ ble 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 | -tfdscmlpnv [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 signaled 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) -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 dis‐ played. 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 vari‐ able 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" } ] }