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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> 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 variables like LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MON-
	      ETARY, 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, 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 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 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 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 geth-
       ostbyname(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 mis-
       chief 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 3.55 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)
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