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 Nicol<6F>s 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 2001-12-14 "Linux" "Linux Programmer's Manual"
.SH NAME
environ \- user environment
.SH SYNOPSIS
.nf
.BI "extern char **" environ ;
.br
.fi
.SH DESCRIPTION
The variable
.I environ
points to an array of strings called the "environment".
(This variable must be declared in the user program,
but is declared in the header file
.I <unistd.h>
in case the header files came from libc4 or libc5, and
in case they came from glibc and
.B _GNU_SOURCE
was defined.)
This array of strings is made available to the process by the
.BR exec (3)
call that started the process.
By convention these strings
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 \fBLC_ALL\fP or more specific environment variables like
\fBLC_COLLATE\fP, \fBLC_CTYPE\fP, \fBLC_MESSAGES\fP, \fBLC_MONETARY\fP,
\fBLC_NUMERIC\fP, \fBLC_TIME\fP, cf.
.BR locale (5).
.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 \fBCDPATH\fP used by some shells to find the target
of a change directory command, \fBMANPATH\fP used by
.BR man (1)
to
find manual pages, etc.)
.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
Further names may be placed in the environment by the \fIexport\fP
command and "name=value" in
.BR sh (1),
or by the \fIsetenv\fP command if you use
.BR csh (1).
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).

Note that the behavior of many programs and library routines is
influenced by the presence or value of certain environment variables.
A random collection:
.LP
The variables
.BR LANG ", " LANGUAGE ", " NLSPATH ", " LOCPATH ", "
.BR LC_ALL ", " LC_MESSAGES ", "
etc. influence locale handling, cf.
.BR locale (5).
.LP
.B TMPDIR
influences the path prefix of names created by
.BR tmpnam (3)
and other routines, the temporary directory used by
.BR sort (1)
and other programs, etc.
.LP
.BR LD_LIBRARY_PATH ", " LD_PRELOAD
and other LD_* variables influence
the behavior of the dynamic loader/linker.
.LP
.B POSIXLY_CORRECT
makes certain programs and library routines follow
the prescriptions of POSIX.
.LP
The behavior of
.BR malloc (3)
is influenced by
.B MALLOC_*
variables.
.LP
The variable
.B HOSTALIASES
gives the name of a file containing aliases
to be used with
.BR gethostbyname (3).
.LP
.BR TZ " and " TZDIR
give time zone 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).
.LP
.B TERMCAP
gives information on how to address a given terminal
(or gives the name of a file containing such information).
.LP
.BR COLUMNS " and " LINES
tell applications about the window size, possibly overriding the actual size.
.LP
.BR PRINTER " or " LPDEST
may specify the desired printer to use.
See
.BR lpr (1).
.LP
Etc.
.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 .

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 login (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 (5)