man-pages/man7/environ.7

.\" 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 2020-08-13 "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/\(tiesr/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)