ENVIRON(5)                  Linux Programmer's Manual                 ENVIRON(5)

       environ - user environment

       extern char **environ;

       The variable environ points to an array of strings called the
       `environment'.  (This variable must be declared in the user program, but
       is declared in the header file unistd.h in case the header files came
       from libc4 or libc5, and in case they came from glibc and _GNU_SOURCE was
       defined.)  This array of strings is made available to the process by the
       exec(3) call that started the process.  By convention these strings 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 like
              LC_TIME, cf.  locale(5).

       PATH   The sequence of directory prefixes that sh(1) and many other
              programs apply in searching for a file known by an incomplete path
              name.  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,

       PWD    The current working directory. Set by some shells.

       SHELL  The file name 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.

              The user's preferred utility to browse URLs. Sequence of colon-
              separated browser commands. See http://www.catb.org/~esr/BROWSER/

       Further names may be placed in the environment by the export command and
       `name=value' in sh(1), or by the setenv command if you use csh(1).
       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 behaviour of many programs and library routines is
       influenced by the presence or value of certain environment variables.  A
       random collection:

       influence locale handling, cf.  locale(5).

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

       LD_LIBRARY_PATH, LD_PRELOAD and other LD_* variables influence the
       behaviour of the dynamic loader/linker.

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

       The behaviour 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 time zone information used by tzset(3) and through that
       by functions like ctime(), localtime(), mktime(), strftime().  See also

       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).


       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.

       login(1), sh(1), bash(1), csh(1), tcsh(1), execve(2), exec(3), getenv(3),
       putenv(3), setenv(3), clearenv(3), unsetenv(3), locale(5)

Linux                              2001-12-14                         ENVIRON(5)