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

       environ - user environment

       extern char **environ;

       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 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).

       What follows is a list of environment variables typically seen on a
       system.  This list is incomplete and includes only common variables seen
       by average users in their day-to-day routine.  Environment variables
       specific to a particular program or library function are documented 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.

              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

       LANG   The name of a locale to use for locale categories when not
              overridden by LC_ALL or more specific environment variables such
              LC_TIME (see locale(7) for further details of the LC_* environment

       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 pathname 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 concatenating 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
              application shall use an explicit pathname (e.g., .)  to specify
              the current working directory.

              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    The current working directory.  Set by some shells.

       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 command-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

              The user's preferred utility to edit text files.  Any string
              acceptable as a command_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:

          and so on influence locale handling; see catopen(3), gettext(3), and

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

       *  LD_LIBRARY_PATH, LD_PRELOAD, and other LD_* variables influence the
          behavior of the dynamic 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 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 lpr(1).

       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.)

       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

       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 and LESS.  Such usage is considered mistaken, and to be avoided
       in new programs.

       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)

       This page is part of release 5.13 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                              2021-03-22                         ENVIRON(7)