ENVIRON(7)                 Linux Programmer's Manual                ENVIRON(7)
NAME
       environ - user environment

SYNOPSIS
       extern char **environ;

DESCRIPTION
       The variable environ points to an array of strings called the 'environ-
       ment'.  (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 pro-
              grams).

       LOGNAME
              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 over-
              ridden  by  LC_ALL  or  more specific environment variables like
              LC_COLLATE,  LC_CTYPE,  LC_MESSAGES,  LC_MONETARY,   LC_NUMERIC,
              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
              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,
              etc.)

       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.

       EDITOR/VISUAL
              The user's preferred utility to edit text files.

       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 func-
       tions getenv(3), putenv(3), setenv(3), and unsetenv(3).

       Note that the behaviour of many programs and library routines is influ-
       enced  by  the  presence  or value of certain environment variables.  A
       random collection:

       The variables LANG, LANGUAGE, NLSPATH,  LOCPATH,  LC_ALL,  LC_MESSAGES,
       etc. 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  pro-
       grams, etc.

       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 tzselect(1).

       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).

       Etc.

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

Linux                             2001-12-14                        ENVIRON(7)