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 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>
       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 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 programs).

       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 overridden by LC_ALL or more specific environment vari-
              ables 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  manipu-
       late 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 environ-
       ment 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 programs, etc.

       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).

       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 vari-
       able 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)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-07-25                                                 ENVIRON(7)