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

       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
              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

       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 5.08 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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