environ

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



NAME
       environ - user environment

SYNOPSIS
       extern char **environ;

DESCRIPTION
       The  variable  environ points to an array of strings called the ‘envi-
       ronment’.  (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	these
       strings have the form ‘name=value’.  Common 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
	      overridden  by  LC_ALL  or  more specific environment variables
	      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
	      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.

       EDITOR/VISUAL
	      The user’s preferred utility to edit text files.

       BROWSER
	      The user’s preferred utility to browse URLs. Sequence of colon-
	      separated		   browser	      commands.		  See
	      http://www.catb.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 func-
       tions getenv(3), putenv(3), setenv(3), and unsetenv(3).

       Note that the behaviour 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 pro-
       grams, 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 POSIX.

       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 gethostbyname(3).

       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 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	envi-
       ronment	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 envi-
       ronment variable gives options for the program instead of a  pathname.
       Thus,  one  has	MORE,  LESS, and GZIP.	Such usage is considered mis-
       taken, and to be avoided in new programs. The authors of	 gzip  should
       consider renaming their option to GZIP_OPT.

SEE ALSO
       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)