.\" 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. .\" .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, write to the Free .\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, .\" USA. .\" .\" 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 .\" .TH ENVIRON 7 2009-07-25 "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 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 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 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 \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, 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 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 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). .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) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.