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.
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  26 .\" Modified Sun Jul 25 10:45:30 1993 by Rik Faith (faith@cs.unc.edu)
  27 .\" Modified Sun Jul 21 21:25:26 1996 by Andries Brouwer (aeb@cwi.nl)
  28 .\" Modified Mon Oct 21 17:47:19 1996 by Eric S. Raymond (esr@thyrsus.com)
  29 .\" Modified Wed Aug 27 20:28:58 1997 by Nicolás Lichtmaier (nick@debian.org)
  30 .\" Modified Mon Sep 21 00:00:26 1998 by Andries Brouwer (aeb@cwi.nl)
  31 .\" Modified Wed Jan 24 06:37:24 2001 by Eric S. Raymond (esr@thyrsus.com)
  32 .\" Modified Thu Dec 13 23:53:27 2001 by Martin Schulze <joey@infodrom.org>
  33 .\"
  34 .TH ENVIRON 7 2016-10-08 "Linux" "Linux Programmer's Manual"
  35 .SH NAME
  36 environ \- user environment
  37 .SH SYNOPSIS
  38 .nf
  39 .BI "extern char **" environ ;
  40 .br
  41 .fi
  42 .SH DESCRIPTION
  43 The variable
  44 .I environ
  45 points to an array of pointers to strings called the "environment".
  46 The last pointer in this array has the value NULL.
  47 (This variable must be declared in the user program,
  48 but is declared in the header file
  49 .I <unistd.h>
  50 if the
  51 .B _GNU_SOURCE
  52 feature test macro is defined.)
  53 This array of strings is made available to the process by the
  54 .BR exec (3)
  55 call that started the process.
  56 When a child process is created via
  57 .BR fork (2),
  58 it inherits a
  59 .I copy
  60 of its parent's environment.
  61
  62 By convention the strings in
  63 .I environ
  64 have the form "\fIname\fP\fB=\fP\fIvalue\fP".
  65 Common examples are:
  66 .TP
  67 .B USER
  68 The name of the logged-in user (used by some BSD-derived programs).
  69 .TP
  70 .B LOGNAME
  71 The name of the logged-in user (used by some System-V derived programs).
  72 .TP
  73 .B HOME
  74 A user's login directory, set by
  75 .BR login (1)
  76 from the password file
  77 .BR passwd (5).
  78 .TP
  79 .B LANG
  80 The name of a locale to use for locale categories when not overridden
  81 by
  82 .B LC_ALL
  83 or more specific environment variables such as
  84 .BR LC_COLLATE ,
  85 .BR LC_CTYPE ,
  86 .BR LC_MESSAGES ,
  87 .BR LC_MONETARY ,
  88 .BR LC_NUMERIC ,
  89 and
  90 .BR LC_TIME
  91 (see
  92 .BR locale (7)
  93 for further details of the
  94 .BR LC_*
  95 environment variables).
  96 .TP
  97 .B PATH
  98 The sequence of directory prefixes that
  99 .BR sh (1)
 100 and many other
 101 programs apply in searching for a file known by an incomplete pathname.
 102 The prefixes are separated by \(aq\fB:\fP\(aq.
 103 (Similarly one has
 104 .B CDPATH
 105 used by some shells to find the target
 106 of a change directory command,
 107 .B MANPATH
 108 used by
 109 .BR man (1)
 110 to find manual pages, and so on)
 111 .TP
 112 .B PWD
 113 The current working directory.
 114 Set by some shells.
 115 .TP
 116 .B SHELL
 117 The pathname of the user's login shell.
 118 .TP
 119 .B TERM
 120 The terminal type for which output is to be prepared.
 121 .TP
 122 .B PAGER
 123 The user's preferred utility to display text files.
 124 .TP
 125 .BR EDITOR / VISUAL
 126 The user's preferred utility to edit text files.
 127 .\" .TP
 128 .\" .B BROWSER
 129 .\" The user's preferred utility to browse URLs. Sequence of colon-separated
 130 .\" browser commands. See http://www.catb.org/~esr/BROWSER/ .
 131 .PP
 132 Names may be placed in the shell's environment by the
 133 .I export
 134 command in
 135 .BR sh (1),
 136 or by the
 137 .I setenv
 138 command if you use
 139 .BR csh (1).
 140
 141 The initial environment of the shell is populated in various ways,
 142 such as definitions from
 143 .IR /etc/environment
 144 that are processed by
 145 .BR pam_env (8)
 146 for all users at login time (on systems that employ
 147 .BR pam (8)).
 148 In addition, various shell initialization scripts, such as the system-wide
 149 .IR /etc/profile
 150 script and per-user initializations script may include commands
 151 that add variables to the shell's environment;
 152 see the manual page of your preferred shell for details.
 153
 154 Bourne-style shells support the syntax
 155
 156     NAME=value command
 157
 158 to create an environment variable definition only in the scope
 159 of the process that executes
 160 .IR command .
 161 Multiple variable definitions, separated by white space, may precede
 162 .IR command .
 163
 164 Arguments may also be placed in the
 165 environment at the point of an
 166 .BR exec (3).
 167 A C program can manipulate its environment using the functions
 168 .BR getenv (3),
 169 .BR putenv (3),
 170 .BR setenv (3),
 171 and
 172 .BR unsetenv (3).
 173
 174 Note that the behavior of many programs and library routines is
 175 influenced by the presence or value of certain environment variables.
 176 Examples include the following:
 177 .IP * 3
 178 The variables
 179 .BR LANG ", " LANGUAGE ", " NLSPATH ", " LOCPATH ", "
 180 .BR LC_ALL ", " LC_MESSAGES ", "
 181 and so on influence locale handling; see
 182 .BR catopen (3),
 183 .BR gettext (3),
 184 and
 185 .BR locale (7).
 186 .IP *
 187 .B TMPDIR
 188 influences the path prefix of names created by
 189 .BR tmpnam (3)
 190 and other routines, and the temporary directory used by
 191 .BR sort (1)
 192 and other programs.
 193 .IP *
 194 .BR LD_LIBRARY_PATH ", " LD_PRELOAD
 195 and other LD_* variables influence
 196 the behavior of the dynamic loader/linker.
 197 .IP *
 198 .B POSIXLY_CORRECT
 199 makes certain programs and library routines follow
 200 the prescriptions of POSIX.
 201 .IP *
 202 The behavior of
 203 .BR malloc (3)
 204 is influenced by
 205 .B MALLOC_*
 206 variables.
 207 .IP *
 208 The variable
 209 .B HOSTALIASES
 210 gives the name of a file containing aliases
 211 to be used with
 212 .BR gethostbyname (3).
 213 .IP *
 214 .BR TZ " and " TZDIR
 215 give timezone information used by
 216 .BR tzset (3)
 217 and through that by functions like
 218 .BR ctime (3),
 219 .BR localtime (3),
 220 .BR mktime (3),
 221 .BR strftime (3).
 222 See also
 223 .BR tzselect (8).
 224 .IP *
 225 .B TERMCAP
 226 gives information on how to address a given terminal
 227 (or gives the name of a file containing such information).
 228 .IP *
 229 .BR COLUMNS " and " LINES
 230 tell applications about the window size, possibly overriding the actual size.
 231 .IP *
 232 .BR PRINTER " or " LPDEST
 233 may specify the desired printer to use.
 234 See
 235 .BR lpr (1).
 236 .SH BUGS
 237 Clearly there is a security risk here.
 238 Many a system command has been
 239 tricked into mischief by a user who specified unusual values for
 240 .BR IFS " or " LD_LIBRARY_PATH .
 241
 242 There is also the risk of name space pollution.
 243 Programs like
 244 .I make
 245 and
 246 .I autoconf
 247 allow overriding of default utility names from the
 248 environment with similarly named variables in all caps.
 249 Thus one uses
 250 .B CC
 251 to select the desired C compiler (and similarly
 252 .BR MAKE ,
 253 .BR AR ,
 254 .BR AS ,
 255 .BR FC ,
 256 .BR LD ,
 257 .BR LEX ,
 258 .BR RM ,
 259 .BR YACC ,
 260 etc.).
 261 However, in some traditional uses such an environment variable
 262 gives options for the program instead of a pathname.
 263 Thus, one has
 264 .BR MORE ,
 265 .BR LESS ,
 266 and
 267 .BR GZIP .
 268 Such usage is considered mistaken, and to be avoided in new
 269 programs.
 270 The authors of
 271 .I gzip
 272 should consider renaming their option to
 273 .BR GZIP_OPT .
 274 .SH SEE ALSO
 275 .BR bash (1),
 276 .BR csh (1),
 277 .BR env (1),
 278 .BR login (1),
 279 .BR printenv (1),
 280 .BR sh (1),
 281 .BR tcsh (1),
 282 .BR execve (2),
 283 .BR clearenv (3),
 284 .BR exec (3),
 285 .BR getenv (3),
 286 .BR putenv (3),
 287 .BR setenv (3),
 288 .BR unsetenv (3),
 289 .BR locale (7),
 290 .BR ld.so (8),
 291 .BR pam_env (8)