man > environ(7)

environ(7)                        Miscellaneous Information Manual                        environ(7)

NAME
       environ - user environment

SYNOPSIS
       extern char **environ;

DESCRIPTION
       The variable environ points to an array of pointers to strings called the "environment".  The
       last  pointer  in  this array has the value NULL.  This array of strings is made available to
       the process by the execve(2) call when a new program is started.  When  a  child  process  is
       created via fork(2), it inherits a copy of its parent's environment.

       By convention, the strings in environ have the form "name=value".  The name is case-sensitive
       and  may not contain the character "=".  The value can be anything that can be represented as
       a string.  The name and the value may not contain an embedded null byte ('\0'), since this is
       assumed to terminate the string.

       Environment variables may be placed in the shell's  environment  by  the  export  command  in
       sh(1), or by the setenv command if you use csh(1).

       The  initial  environment of the shell is populated in various ways, such as definitions from
       /etc/environment that are processed by pam_env(8) for all users at  login  time  (on  systems
       that  employ pam(8)).  In addition, various shell initialization scripts, such as the system-
       wide /etc/profile script and per-user initializations script may include  commands  that  add
       variables  to  the  shell's  environment; see the manual page of your preferred shell for de‐
       tails.

       Bourne-style shells support the syntax

           NAME=value command

       to create an environment variable definition only in the scope of the process  that  executes
       command.  Multiple variable definitions, separated by white space, may precede command.

       Arguments  may also be placed in the environment at the point of an exec(3).  A C program can
       manipulate its environment using the  functions  getenv(3),  putenv(3),  setenv(3),  and  un‐
       setenv(3).

       What follows is a list of environment variables typically seen on a system.  This list is in‐
       complete  and  includes  only common variables seen by average users in their day-to-day rou‐
       tine.  Environment variables specific to a particular program or library function  are  docu‐
       mented in the ENVIRONMENT section of the appropriate manual page.

       USER   The  name  of  the  logged-in  user (used by some BSD-derived programs).  Set at login
              time, see section NOTES below.

       LOGNAME
              The name of the logged-in user (used by some System-V derived programs).  Set at login
              time, see section NOTES below.

       HOME   A user's login directory.  Set at login time, see section NOTES below.

       LANG   The name of a locale to use for locale categories when not  overridden  by  LC_ALL  or
              more specific environment variables such as LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MON‐
              ETARY, LC_NUMERIC, and LC_TIME (see locale(7) for further details of the LC_* environ‐
              ment variables).

       PATH   The  sequence  of  directory  prefixes  that sh(1) and many other programs employ when
              searching for an executable file that is specified as a simple filename (i.a., a path‐
              name that contains no slashes).  The prefixes are separated by colons (:).   The  list
              of prefixes is searched from beginning to end, by checking the pathname formed by con‐
              catenating  a  prefix, a slash, and the filename, until a file with execute permission
              is found.

              As a legacy feature, a zero-length prefix (specified as two  adjacent  colons,  or  an
              initial  or  terminating  colon) is interpreted to mean the current working directory.
              However, use of this feature is deprecated, and POSIX notes that a conforming applica‐
              tion shall use an explicit pathname (e.g., .)  to specify the current  working  direc‐
              tory.

              Analogously to PATH, one has CDPATH used by some shells to find the target of a change
              directory command, MANPATH used by man(1) to find manual pages, and so on.

       PWD    Absolute path to the current working directory; required to be partially canonical (no
              . or .. components).

       SHELL  The absolute pathname of the user's login shell.  Set at login time, see section NOTES
              below.

       TERM   The terminal type for which output is to be prepared.

       PAGER  The  user's  preferred utility to display text files.  Any string acceptable as a com‐
              mand-string operand to the sh -c command shall be valid.  If PAGER is null or  is  not
              set,  then  applications that launch a pager will default to a program such as less(1)
              or more(1).

       EDITOR/VISUAL
              The user's preferred utility to edit text files.  Any  string  acceptable  as  a  com‐
              mand_string operand to the sh -c command shall be valid.

       Note that the behavior of many programs and library routines is influenced by the presence or
       value of certain environment variables.  Examples include the following:

       •  The  variables  LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES, and so on influence
          locale handling; see catopen(3), gettext(3), and locale(7).

       •  TMPDIR influences the path prefix of names created by tempnam(3) and other  routines,  and
          the temporary directory used by sort(1) and other programs.

       •  LD_LIBRARY_PATH,  LD_PRELOAD,  and  other LD_* variables influence the behavior of the dy‐
          namic loader/linker.  See also ld.so(8).

       •  POSIXLY_CORRECT makes certain programs and library routines follow  the  prescriptions  of
          POSIX.

       •  The behavior of malloc(3) is influenced by MALLOC_* variables.

       •  The variable HOSTALIASES gives the name of a file containing aliases to be used with geth‐
          ostbyname(3).

       •  TZ and TZDIR give timezone information used by tzset(3) and through that by functions like
          ctime(3), localtime(3), mktime(3), strftime(3).  See also tzselect(8).

       •  TERMCAP  gives information on how to address a given terminal (or gives the name of a file
          containing such information).

       •  COLUMNS and LINES tell applications about the window size, possibly overriding the  actual
          size.

       •  PRINTER or LPDEST may specify the desired printer to use.  See lpr(1).

NOTES
       Historically  and  by  standard, environ must be declared in the user program.  However, as a
       (nonstandard) programmer convenience, environ is declared in the header  file  <unistd.h>  if
       the _GNU_SOURCE feature test macro is defined (see feature_test_macros(7)).

       The  prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can be used to control the
       location of the process's environment.

       The HOME, LOGNAME, SHELL, and USER variables are set when the user is changed via  a  session
       management  interface,  typically by a program such as login(1) from a user database (such as
       passwd(5)).  (Switching to the root user using su(1) may result in a mixed environment  where
       LOGNAME and USER are retained from old user; see the su(1) manual page.)

BUGS
       Clearly  there is a security risk here.  Many a system command has been tricked into mischief
       by a user who specified unusual values for IFS or LD_LIBRARY_PATH.

       There is also the risk of name space pollution.  Programs like make and autoconf allow  over‐
       riding  of  default  utility names from the environment with similarly named variables in all
       caps.  Thus one uses CC to select the desired C compiler (and similarly MAKE, AR, AS, FC, LD,
       LEX, RM, YACC, etc.).  However, in some traditional uses such an environment  variable  gives
       options  for  the program instead of a pathname.  Thus, one has MORE and LESS.  Such usage is
       considered mistaken, and to be avoided in new programs.

SEE ALSO
       bash(1),  csh(1),  env(1),  login(1),  printenv(1),   sh(1),   su(1),   tcsh(1),   execve(2),
       clearenv(3),  exec(3),  getenv(3),  putenv(3),  setenv(3),  unsetenv(3), locale(7), ld.so(8),
       pam_env(8)

Linux man-pages 6.7                          2023-10-31                                   environ(7)