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ENVIRON(7)		   Linux Programmer's Manual		    ENVIRON(7)

       environ - user environment

       extern char **environ;

       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".  Com‐
       mon 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 user's 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 such  as
	      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.

	      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:

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

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

       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  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).


       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(7)

       This page is part of release 3.65 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				  2014-01-18			    ENVIRON(7)

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