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
       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

       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 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:

          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.

       *  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).

       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.

       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, 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.

       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)

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       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)