environ(7) — Linux manual page


ENVIRON(7)              Linux Programmer's Manual             ENVIRON(7)

NAME         top

       environ - user environment

SYNOPSIS         top

       extern char **environ;

DESCRIPTION         top

       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 variable must be declared in the user program,
       but is declared in the header file <unistd.h> if the _GNU_SOURCE
       feature test macro is defined.)  This array of strings is made
       available to the process by the exec(3) call that started the
       process.  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".
       Common examples are:

       USER   The name of the logged-in user (used by some BSD-derived

              The name of the logged-in user (used by some System-V
              derived programs).

       HOME   A user's login directory, set by login(1) from the
              password file passwd(5).

       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_MONETARY, LC_NUMERIC, and LC_TIME (see locale(7) for
              further details of the LC_* environment variables).

       PATH   The sequence of directory prefixes that sh(1) and many
              other programs apply in searching for a file known by an
              incomplete pathname.  The prefixes are separated by ':'.
              (Similarly 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    The current working directory.  Set by some shells.

       SHELL  The pathname of the user's login shell.

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

       PAGER  The user's preferred utility to display text files.

              The user's preferred utility to edit text files.

       Names 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 details.

       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 unsetenv(3).

       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 dynamic loader/linker.

       *  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 gethostbyname(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

NOTES         top

       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.

BUGS         top

       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 overriding 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, LESS, and
       GZIP.  Such usage is considered mistaken, and to be avoided in
       new programs.  The authors of gzip should consider renaming their
       option to GZIP_OPT.

SEE ALSO         top

       bash(1), csh(1), env(1), login(1), printenv(1), sh(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)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

Linux                          2020-08-13                     ENVIRON(7)

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