man7/environ.7

   1 .\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
   2 .\"   Fri Apr  2 11:32:09 MET DST 1993
   3 .\" and Andries Brouwer (aeb@cwi.nl), Fri Feb 14 21:47:50 1997.
   4 .\"
   5 .\" This is free documentation; you can redistribute it and/or
   6 .\" modify it under the terms of the GNU General Public License as
   7 .\" published by the Free Software Foundation; either version 2 of
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  15 .\" This manual is distributed in the hope that it will be useful,
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  20 .\" You should have received a copy of the GNU General Public
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  24 .\"
  25 .\" Modified Sun Jul 25 10:45:30 1993 by Rik Faith (faith@cs.unc.edu)
  26 .\" Modified Sun Jul 21 21:25:26 1996 by Andries Brouwer (aeb@cwi.nl)
  27 .\" Modified Mon Oct 21 17:47:19 1996 by Eric S. Raymond (esr@thyrsus.com)
  28 .\" Modified Wed Aug 27 20:28:58 1997 by Nicolás Lichtmaier (nick@debian.org)
  29 .\" Modified Mon Sep 21 00:00:26 1998 by Andries Brouwer (aeb@cwi.nl)
  30 .\" Modified Wed Jan 24 06:37:24 2001 by Eric S. Raymond (esr@thyrsus.com)
  31 .\" Modified Thu Dec 13 23:53:27 2001 by Martin Schulze <joey@infodrom.org>
  32 .\"
  33 .TH ENVIRON 7 2001-12-14 "Linux" "Linux Programmer's Manual"
  34 .SH NAME
  35 environ \- user environment
  36 .SH SYNOPSIS
  37 .nf
  38 .BI "extern char **" environ ;
  39 .br
  40 .fi
  41 .SH DESCRIPTION
  42 The variable
  43 .I environ
  44 points to an array of strings called the "environment".
  45 (This variable must be declared in the user program,
  46 but is declared in the header file
  47 .I <unistd.h>
  48 in case the header files came from libc4 or libc5, and
  49 in case they came from glibc and
  50 .B _GNU_SOURCE
  51 was defined.)
  52 This array of strings is made available to the process by the
  53 .BR exec (3)
  54 call that started the process.
  55 By convention these strings
  56 have the form "\fIname\fP\fB=\fP\fIvalue\fP".
  57 Common examples are:
  58 .TP
  59 .B USER
  60 The name of the logged-in user (used by some BSD-derived programs).
  61 .TP
  62 .B LOGNAME
  63 The name of the logged-in user (used by some System-V derived programs).
  64 .TP
  65 .B HOME
  66 A user's login directory, set by
  67 .BR login (1)
  68 from the password file
  69 .BR passwd (5).
  70 .TP
  71 .B LANG
  72 The name of a locale to use for locale categories when not overridden
  73 by \fBLC_ALL\fP or more specific environment variables like
  74 \fBLC_COLLATE\fP, \fBLC_CTYPE\fP, \fBLC_MESSAGES\fP, \fBLC_MONETARY\fP,
  75 \fBLC_NUMERIC\fP, \fBLC_TIME\fP, cf.
  76 .BR locale (5).
  77 .TP
  78 .B PATH
  79 The sequence of directory prefixes that
  80 .BR sh (1)
  81 and many other
  82 programs apply in searching for a file known by an incomplete pathname.
  83 The prefixes are separated by '\fB:\fP'.
  84 (Similarly one has \fBCDPATH\fP used by some shells to find the target
  85 of a change directory command, \fBMANPATH\fP used by
  86 .BR man (1)
  87 to
  88 find manual pages, etc.)
  89 .TP
  90 .B PWD
  91 The current working directory.
  92 Set by some shells.
  93 .TP
  94 .B SHELL
  95 The pathname of the user's login shell.
  96 .TP
  97 .B TERM
  98 The terminal type for which output is to be prepared.
  99 .TP
 100 .B PAGER
 101 The user's preferred utility to display text files.
 102 .TP
 103 .BR EDITOR / VISUAL
 104 The user's preferred utility to edit text files.
 105 .\" .TP
 106 .\" .B BROWSER
 107 .\" The user's preferred utility to browse URLs. Sequence of colon-separated
 108 .\" browser commands. See http://www.catb.org/~esr/BROWSER/ .
 109 .PP
 110 Further names may be placed in the environment by the \fIexport\fP
 111 command and "name=value" in
 112 .BR sh (1),
 113 or by the \fIsetenv\fP command if you use
 114 .BR csh (1).
 115 Arguments may also be placed in the
 116 environment at the point of an
 117 .BR exec (3).
 118 A C program can manipulate its environment using the functions
 119 .BR getenv (3),
 120 .BR putenv (3),
 121 .BR setenv (3),
 122 and
 123 .BR unsetenv (3).
 124
 125 Note that the behavior of many programs and library routines is
 126 influenced by the presence or value of certain environment variables.
 127 A random collection:
 128 .LP
 129 The variables
 130 .BR LANG ", " LANGUAGE ", " NLSPATH ", " LOCPATH ", "
 131 .BR LC_ALL ", " LC_MESSAGES ", "
 132 etc. influence locale handling, cf.
 133 .BR locale (5).
 134 .LP
 135 .B TMPDIR
 136 influences the path prefix of names created by
 137 .BR tmpnam (3)
 138 and other routines, the temporary directory used by
 139 .BR sort (1)
 140 and other programs, etc.
 141 .LP
 142 .BR LD_LIBRARY_PATH ", " LD_PRELOAD
 143 and other LD_* variables influence
 144 the behavior of the dynamic loader/linker.
 145 .LP
 146 .B POSIXLY_CORRECT
 147 makes certain programs and library routines follow
 148 the prescriptions of POSIX.
 149 .LP
 150 The behavior of
 151 .BR malloc (3)
 152 is influenced by
 153 .B MALLOC_*
 154 variables.
 155 .LP
 156 The variable
 157 .B HOSTALIASES
 158 gives the name of a file containing aliases
 159 to be used with
 160 .BR gethostbyname (3).
 161 .LP
 162 .BR TZ " and " TZDIR
 163 give time zone information used by
 164 .BR tzset (3)
 165 and through that by functions like
 166 .BR ctime (3),
 167 .BR localtime (3),
 168 .BR mktime (3),
 169 .BR strftime (3).
 170 See also
 171 .BR tzselect (8).
 172 .LP
 173 .B TERMCAP
 174 gives information on how to address a given terminal
 175 (or gives the name of a file containing such information).
 176 .LP
 177 .BR COLUMNS " and " LINES
 178 tell applications about the window size, possibly overriding the actual size.
 179 .LP
 180 .BR PRINTER " or " LPDEST
 181 may specify the desired printer to use.
 182 See
 183 .BR lpr (1).
 184 .LP
 185 Etc.
 186 .SH BUGS
 187 Clearly there is a security risk here.
 188 Many a system command has been
 189 tricked into mischief by a user who specified unusual values for
 190 .BR IFS " or " LD_LIBRARY_PATH .
 191
 192 There is also the risk of name space pollution.
 193 Programs like
 194 .I make
 195 and
 196 .I autoconf
 197 allow overriding of default utility names from the
 198 environment with similarly named variables in all caps.
 199 Thus one uses
 200 .B CC
 201 to select the desired C compiler (and similarly
 202 .BR MAKE ,
 203 .BR AR ,
 204 .BR AS ,
 205 .BR FC ,
 206 .BR LD ,
 207 .BR LEX ,
 208 .BR RM ,
 209 .BR YACC ,
 210 etc.).
 211 However, in some traditional uses such an environment variable
 212 gives options for the program instead of a pathname.
 213 Thus, one has
 214 .BR MORE ,
 215 .BR LESS ,
 216 and
 217 .BR GZIP .
 218 Such usage is considered mistaken, and to be avoided in new
 219 programs.
 220 The authors of
 221 .I gzip
 222 should consider renaming their option to
 223 .BR GZIP_OPT .
 224 .SH "SEE ALSO"
 225 .BR bash (1),
 226 .BR csh (1),
 227 .BR login (1),
 228 .BR sh (1),
 229 .BR tcsh (1),
 230 .BR execve (2),
 231 .BR clearenv (3),
 232 .BR exec (3),
 233 .BR getenv (3),
 234 .BR putenv (3),
 235 .BR setenv (3),
 236 .BR unsetenv (3),
 237 .BR locale (5)