.\" Copyright (c) 1993 Michael Haardt (michael@moria.de),

.\"   Fri Apr  2 11:32:09 MET DST 1993

.\" and Andries Brouwer (aeb@cwi.nl), Fri Feb 14 21:47:50 1997.

.\"

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.\" Modified Sun Jul 25 10:45:30 1993 by Rik Faith (faith@cs.unc.edu)

.\" Modified Sun Jul 21 21:25:26 1996 by Andries Brouwer (aeb@cwi.nl)

.\" Modified Mon Oct 21 17:47:19 1996 by Eric S. Raymond (esr@thyrsus.com)

.\" Modified Wed Aug 27 20:28:58 1997 by Nicolás Lichtmaier (nick@debian.org)

.\" Modified Mon Sep 21 00:00:26 1998 by Andries Brouwer (aeb@cwi.nl)

.\" Modified Wed Jan 24 06:37:24 2001 by Eric S. Raymond (esr@thyrsus.com)

.\" Modified Thu Dec 13 23:53:27 2001 by Martin Schulze <joey@infodrom.org>

.\"

.TH ENVIRON 7 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

environ \- user environment

.SH SYNOPSIS

.nf

.BI "extern char **" environ ;

.fi

.SH DESCRIPTION

The variable

.I 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

.I <unistd.h>

if the

.B _GNU_SOURCE

feature test macro is defined.)

This array of strings is made available to the process by the

.BR exec (3)

call that started the process.

When a child process is created via

.BR fork (2),

it inherits a

.I copy

of its parent's environment.

.PP

By convention the strings in

.I environ

have the form "\fIname\fP\fB=\fP\fIvalue\fP".

Common examples are:

.TP

.B USER

The name of the logged-in user (used by some BSD-derived programs).

.TP

.B LOGNAME

The name of the logged-in user (used by some System-V derived programs).

.TP

.B HOME

A user's login directory, set by

.BR login (1)

from the password file

.BR passwd (5).

.TP

.B LANG

The name of a locale to use for locale categories when not overridden

by

.B LC_ALL

or more specific environment variables such as

.BR LC_COLLATE ,

.BR LC_CTYPE ,

.BR LC_MESSAGES ,

.BR LC_MONETARY ,

.BR LC_NUMERIC ,

and

.BR LC_TIME

(see

.BR locale (7)

for further details of the

.BR LC_*

environment variables).

.TP

.B PATH

The sequence of directory prefixes that

.BR sh (1)

and many other

programs apply in searching for a file known by an incomplete pathname.

The prefixes are separated by \(aq\fB:\fP\(aq.

(Similarly one has

.B CDPATH

used by some shells to find the target

of a change directory command,

.B MANPATH

used by

.BR man (1)

to find manual pages, and so on)

.TP

.B PWD

The current working directory.

Set by some shells.

.TP

.B SHELL

The pathname of the user's login shell.

.TP

.B TERM

The terminal type for which output is to be prepared.

.TP

.B PAGER

The user's preferred utility to display text files.

.TP

.BR EDITOR / VISUAL

The user's preferred utility to edit text files.

.\" .TP

.\" .B BROWSER

.\" The user's preferred utility to browse URLs. Sequence of colon-separated

.\" browser commands. See http://www.catb.org/~esr/BROWSER/ .

.PP

Names may be placed in the shell's environment by the

.I export

command in

.BR sh (1),

or by the

.I setenv

command if you use

.BR csh (1).

.PP

The initial environment of the shell is populated in various ways,

such as definitions from

.IR /etc/environment

that are processed by

.BR pam_env (8)

for all users at login time (on systems that employ

.BR pam (8)).

In addition, various shell initialization scripts, such as the system-wide

.IR /etc/profile

script and per-user initializations script may include commands

that add variables to the shell's environment;

see the manual page of your preferred shell for details.

.PP

Bourne-style shells support the syntax

.PP

    NAME=value command

.PP

to create an environment variable definition only in the scope

of the process that executes

.IR command .

Multiple variable definitions, separated by white space, may precede

.IR command .

.PP

Arguments may also be placed in the

environment at the point of an

.BR exec (3).

A C program can manipulate its environment using the functions

.BR getenv (3),

.BR putenv (3),

.BR setenv (3),

and

.BR unsetenv (3).

.PP

Note that the behavior of many programs and library routines is

influenced by the presence or value of certain environment variables.

Examples include the following:

.IP * 3

The variables

.BR LANG ", " LANGUAGE ", " NLSPATH ", " LOCPATH ", "

.BR LC_ALL ", " LC_MESSAGES ", "

and so on influence locale handling; see

.BR catopen (3),

.BR gettext (3),

and

.BR locale (7).

.IP *

.B TMPDIR

influences the path prefix of names created by

.BR tempnam (3)

and other routines, and the temporary directory used by

.BR sort (1)

and other programs.

.IP *

.BR LD_LIBRARY_PATH ", " LD_PRELOAD ,

and other

.BR LD_*

variables influence the behavior of the dynamic loader/linker.

.IP *

.B POSIXLY_CORRECT

makes certain programs and library routines follow

the prescriptions of POSIX.

.IP *

The behavior of

.BR malloc (3)

is influenced by

.B MALLOC_*

variables.

.IP *

The variable

.B HOSTALIASES

gives the name of a file containing aliases

to be used with

.BR gethostbyname (3).

.IP *

.BR TZ " and " TZDIR

give timezone information used by

.BR tzset (3)

and through that by functions like

.BR ctime (3),

.BR localtime (3),

.BR mktime (3),

.BR strftime (3).

See also

.BR tzselect (8).

.IP *

.B TERMCAP

gives information on how to address a given terminal

(or gives the name of a file containing such information).

.IP *

.BR COLUMNS " and " LINES

tell applications about the window size, possibly overriding the actual size.

.IP *

.BR PRINTER " or " LPDEST

may specify the desired printer to use.

See

.BR lpr (1).

.SH NOTES

The

.BR prctl (2)

.B PR_SET_MM_ENV_START

and

.B PR_SET_MM_ENV_END

operations can be used to control the location of the process's environment.

.SH 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

.BR IFS " or " LD_LIBRARY_PATH .

.PP

There is also the risk of name space pollution.

Programs like

.I make

and

.I autoconf

allow overriding of default utility names from the

environment with similarly named variables in all caps.

Thus one uses

.B CC

to select the desired C compiler (and similarly

.BR MAKE ,

.BR AR ,

.BR AS ,

.BR FC ,

.BR LD ,

.BR LEX ,

.BR RM ,

.BR YACC ,

etc.).

However, in some traditional uses such an environment variable

gives options for the program instead of a pathname.

Thus, one has

.BR MORE ,

.BR LESS ,

and

.BR GZIP .

Such usage is considered mistaken, and to be avoided in new

programs.

The authors of

.I gzip

should consider renaming their option to

.BR GZIP_OPT .

.SH SEE ALSO

.BR bash (1),

.BR csh (1),

.BR env (1),

.BR login (1),

.BR printenv (1),

.BR sh (1),

.BR tcsh (1),

.BR execve (2),

.BR clearenv (3),

.BR exec (3),

.BR getenv (3),

.BR putenv (3),

.BR setenv (3),

.BR unsetenv (3),

.BR locale (7),

.BR ld.so (8),

.BR pam_env (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.