1ENVIRON(7) Linux Programmer's Manual ENVIRON(7)
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6 environ - user environment
7
9 extern char **environ;
10
12 The variable environ points to an array of pointers to strings called
13 the "environment". The last pointer in this array has the value NULL.
14 (This variable must be declared in the user program, but is declared in
15 the header file <unistd.h> if the _GNU_SOURCE feature test macro is
16 defined.) This array of strings is made available to the process by
17 the exec(3) call that started the process. When a child process is
18 created via fork(2), it inherits a copy of its parent's environment.
19 By convention the strings in environ have the form "name=value". Com‐
21 mon examples are:
22 USER The name of the logged-in user (used by some BSD-derived pro‐
24 grams).
25 LOGNAME
27 The name of the logged-in user (used by some System-V derived
28 programs).
29 HOME A user's login directory, set by login(1) from the password file
31 passwd(5).
32 LANG The name of a locale to use for locale categories when not over‐
34 ridden by LC_ALL or more specific environment variables such as
35 LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and
36 LC_TIME (see locale(7) for further details of the LC_* environ‐
37 ment variables).
38 PATH The sequence of directory prefixes that sh(1) and many other
40 programs apply in searching for a file known by an incomplete
41 pathname. The prefixes are separated by ':'. (Similarly one
42 has CDPATH used by some shells to find the target of a change
43 directory command, MANPATH used by man(1) to find manual pages,
44 and so on)
45 PWD The current working directory. Set by some shells.
47 SHELL The pathname of the user's login shell.
49 TERM The terminal type for which output is to be prepared.
51 PAGER The user's preferred utility to display text files.
53 EDITOR/VISUAL
55 The user's preferred utility to edit text files.
56 Names may be placed in the shell's environment by the export command in
58 sh(1), or by the setenv command if you use csh(1).
59 The initial environment of the shell is populated in various ways, such
61 as definitions from /etc/environment that are processed by pam_env(8)
62 for all users at login time (on systems that employ pam(8)). In addi‐
63 tion, various shell initialization scripts, such as the system-wide
64 /etc/profile script and per-user initializations script may include
65 commands that add variables to the shell's environment; see the manual
66 page of your preferred shell for details.
67 Bourne-style shells support the syntax
69 NAME=value command
71 to create an environment variable definition only in the scope of the
73 process that executes command. Multiple variable definitions, sepa‐
74 rated by white space, may precede command.
75 Arguments may also be placed in the environment at the point of an
77 exec(3). A C program can manipulate its environment using the func‐
78 tions getenv(3), putenv(3), setenv(3), and unsetenv(3).
79 Note that the behavior of many programs and library routines is influ‐
81 enced by the presence or value of certain environment variables. Exam‐
82 ples include the following:
83 * The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES,
85 and so on influence locale handling; see catopen(3), gettext(3), and
86 locale(7).
87 * TMPDIR influences the path prefix of names created by tempnam(3) and
89 other routines, and the temporary directory used by sort(1) and
90 other programs.
91 * LD_LIBRARY_PATH, LD_PRELOAD, and other LD_* variables influence the
93 behavior of the dynamic loader/linker.
94 * POSIXLY_CORRECT makes certain programs and library routines follow
96 the prescriptions of POSIX.
97 * The behavior of malloc(3) is influenced by MALLOC_* variables.
99 * The variable HOSTALIASES gives the name of a file containing aliases
101 to be used with gethostbyname(3).
102 * TZ and TZDIR give timezone information used by tzset(3) and through
104 that by functions like ctime(3), localtime(3), mktime(3), strf‐
105 time(3). See also tzselect(8).
106 * TERMCAP gives information on how to address a given terminal (or
108 gives the name of a file containing such information).
109 * COLUMNS and LINES tell applications about the window size, possibly
111 overriding the actual size.
112 * PRINTER or LPDEST may specify the desired printer to use. See
114 lpr(1).
115
117 The prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can
118 be used to control the location of the process's environment.
119
121 Clearly there is a security risk here. Many a system command has been
122 tricked into mischief by a user who specified unusual values for IFS or
123 LD_LIBRARY_PATH.
124 There is also the risk of name space pollution. Programs like make and
126 autoconf allow overriding of default utility names from the environment
127 with similarly named variables in all caps. Thus one uses CC to select
128 the desired C compiler (and similarly MAKE, AR, AS, FC, LD, LEX, RM,
129 YACC, etc.). However, in some traditional uses such an environment
130 variable gives options for the program instead of a pathname. Thus,
131 one has MORE, LESS, and GZIP. Such usage is considered mistaken, and
132 to be avoided in new programs. The authors of gzip should consider
133 renaming their option to GZIP_OPT.
134
136 bash(1), csh(1), env(1), login(1), printenv(1), sh(1), tcsh(1),
137 execve(2), clearenv(3), exec(3), getenv(3), putenv(3), setenv(3),
138 unsetenv(3), locale(7), ld.so(8), pam_env(8)
139
141 This page is part of release 4.15 of the Linux man-pages project. A
142 description of the project, information about reporting bugs, and the
143 latest version of this page, can be found at
144 https://www.kernel.org/doc/man-pages/.
145Linux 2017-09-15 ENVIRON(7)