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ENVIRON(5)			    Linux Programmer's Manual			       ENVIRON(5)

       environ - user environment

       extern char **environ;

       The  variable environ points to an array of strings called the `environment'.  (This vari-
       able 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 programs).

	      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 path name.  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 file name 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.

	      The user's preferred utility to edit text files.

	      The  user's  preferred  utility to browse URLs. Sequence of colon-separated browser
	      commands. See http://www.tuxedo.org/~esr/BROWSER/ .

       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 behaviour of many programs and library routines is influenced by the pres-
       ence 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  behaviour  of  the
       dynamic loader/linker.

       POSIXLY_CORRECT	makes  certain	programs and library routines follow the prescriptions of

       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 geth-

       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

       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  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.

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

Linux					    2001-12-14				       ENVIRON(5)
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