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ENVIRON(7)		   Linux Programmers Manual		   ENVIRON(7)

       environ - user environment

       extern char **environ;

       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  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

	      The name of the logged-in user (used by  some  System-V  derived

       HOME   A users login directory, set by login(1) from the password file

       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_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,

       PWD    The current working directory.  Set by some shells.

       SHELL  The pathname of the users login shell.

       TERM   The terminal type for which output is to be prepared.

       PAGER  The users preferred utility to display text files.

	      The users 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 behavior of many programs and library routines is	influ
       enced  by  the  presence  or value of certain environment variables.  A
       random collection:

       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
       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 time zone 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).


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

       This page is part of release 3.05 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				  2001-12-14			    ENVIRON(7)

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