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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_ if the _GNU_SOURCE feature test macro	is de-
       fined.)	 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".	  Com-
       mon 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 such as
	      LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC,  and
	      LC_TIME  (see locale(7) for further details of the LC_* environ-
	      ment variables).

       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.

       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 behavior 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,
       and so on influence locale handling; see	 catopen(3),  gettext(3),  and
       locale(7).

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

       LD_LIBRARY_PATH,	 LD_PRELOAD and	other LD_* variables influence the be-
       havior 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
       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 re-
       naming their option to GZIP_OPT.

SEE ALSO
       env(1), bash(1),	csh(1),	login(1),  printenv(1),	 sh(1),	 tcsh(1),  ex-
       ecve(2),	 clearenv(3),  exec(3),	 getenv(3),  putenv(3),	setenv(3), un-
       setenv(3), locale(7), ld.so(8)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			    ENVIRON(7)

NAME | SYNOPSIS | DESCRIPTION | BUGS | SEE ALSO | COLOPHON

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