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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  like
	      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,

       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:

       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
       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(5)

       This page is part of release 3.22 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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