1environ(7) Miscellaneous Information Manual environ(7)
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6 environ - user environment
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9 extern char **environ;
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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 array of strings is made available to the process by the execve(2)
15 call when a new program is started. When a child process is created
16 via fork(2), it inherits a copy of its parent's environment.
17 By convention, the strings in environ have the form "name=value". The
19 name is case-sensitive and may not contain the character "=". The
20 value can be anything that can be represented as a string. The name
21 and the value may not contain an embedded null byte ('\0'), since this
22 is assumed to terminate the string.
23 Environment variables may be placed in the shell's environment by the
25 export command in sh(1), or by the setenv command if you use csh(1).
26 The initial environment of the shell is populated in various ways, such
28 as definitions from /etc/environment that are processed by pam_env(8)
29 for all users at login time (on systems that employ pam(8)). In addi‐
30 tion, various shell initialization scripts, such as the system-wide
31 /etc/profile script and per-user initializations script may include
32 commands that add variables to the shell's environment; see the manual
33 page of your preferred shell for details.
34 Bourne-style shells support the syntax
36 NAME=value command
38 to create an environment variable definition only in the scope of the
40 process that executes command. Multiple variable definitions, sepa‐
41 rated by white space, may precede command.
42 Arguments may also be placed in the environment at the point of an
44 exec(3). A C program can manipulate its environment using the func‐
45 tions getenv(3), putenv(3), setenv(3), and unsetenv(3).
46 What follows is a list of environment variables typically seen on a
48 system. This list is incomplete and includes only common variables
49 seen by average users in their day-to-day routine. Environment vari‐
50 ables specific to a particular program or library function are docu‐
51 mented in the ENVIRONMENT section of the appropriate manual page.
52 USER The name of the logged-in user (used by some BSD-derived pro‐
54 grams). Set at login time, see section NOTES below.
55 LOGNAME
57 The name of the logged-in user (used by some System-V derived
58 programs). Set at login time, see section NOTES below.
59 HOME A user's login directory. Set at login time, see section NOTES
61 below.
62 LANG The name of a locale to use for locale categories when not over‐
64 ridden by LC_ALL or more specific environment variables such as
65 LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and
66 LC_TIME (see locale(7) for further details of the LC_* environ‐
67 ment variables).
68 PATH The sequence of directory prefixes that sh(1) and many other
70 programs employ when searching for an executable file that is
71 specified as a simple filename (i.a., a pathname that contains
72 no slashes). The prefixes are separated by colons (:). The
73 list of prefixes is searched from beginning to end, by checking
74 the pathname formed by concatenating a prefix, a slash, and the
75 filename, until a file with execute permission is found.
76 As a legacy feature, a zero-length prefix (specified as two ad‐
78 jacent colons, or an initial or terminating colon) is inter‐
79 preted to mean the current working directory. However, use of
80 this feature is deprecated, and POSIX notes that a conforming
81 application shall use an explicit pathname (e.g., .) to specify
82 the current working directory.
83 Analogously to PATH, one has CDPATH used by some shells to find
85 the target of a change directory command, MANPATH used by man(1)
86 to find manual pages, and so on.
87 PWD Absolute path to the current working directory; required to be
89 partially canonical (no . or .. components).
90 SHELL The absolute pathname of the user's login shell. Set at login
92 time, see section NOTES below.
93 TERM The terminal type for which output is to be prepared.
95 PAGER The user's preferred utility to display text files. Any string
97 acceptable as a command-string operand to the sh -c command
98 shall be valid. If PAGER is null or is not set, then applica‐
99 tions that launch a pager will default to a program such as
100 less(1) or more(1).
101 EDITOR/VISUAL
103 The user's preferred utility to edit text files. Any string ac‐
104 ceptable as a command_string operand to the sh -c command shall
105 be valid.
106 Note that the behavior of many programs and library routines is influ‐
108 enced by the presence or value of certain environment variables. Exam‐
109 ples include the following:
110 • The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES,
112 and so on influence locale handling; see catopen(3), gettext(3), and
113 locale(7).
114 • TMPDIR influences the path prefix of names created by tempnam(3) and
116 other routines, and the temporary directory used by sort(1) and
117 other programs.
118 • LD_LIBRARY_PATH, LD_PRELOAD, and other LD_* variables influence the
120 behavior of the dynamic loader/linker. See also ld.so(8).
121 • POSIXLY_CORRECT makes certain programs and library routines follow
123 the prescriptions of POSIX.
124 • The behavior of malloc(3) is influenced by MALLOC_* variables.
126 • The variable HOSTALIASES gives the name of a file containing aliases
128 to be used with gethostbyname(3).
129 • TZ and TZDIR give timezone information used by tzset(3) and through
131 that by functions like ctime(3), localtime(3), mktime(3), strf‐
132 time(3). See also tzselect(8).
133 • TERMCAP gives information on how to address a given terminal (or
135 gives the name of a file containing such information).
136 • COLUMNS and LINES tell applications about the window size, possibly
138 overriding the actual size.
139 • PRINTER or LPDEST may specify the desired printer to use. See
141 lpr(1).
142
144 Historically and by standard, environ must be declared in the user pro‐
145 gram. However, as a (nonstandard) programmer convenience, environ is
146 declared in the header file <unistd.h> if the _GNU_SOURCE feature test
147 macro is defined (see feature_test_macros(7)).
148 The prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can
150 be used to control the location of the process's environment.
151 The HOME, LOGNAME, SHELL, and USER variables are set when the user is
153 changed via a session management interface, typically by a program such
154 as login(1) from a user database (such as passwd(5)). (Switching to
155 the root user using su(1) may result in a mixed environment where LOG‐
156 NAME and USER are retained from old user; see the su(1) manual page.)
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159 Clearly there is a security risk here. Many a system command has been
160 tricked into mischief by a user who specified unusual values for IFS or
161 LD_LIBRARY_PATH.
162 There is also the risk of name space pollution. Programs like make and
164 autoconf allow overriding of default utility names from the environment
165 with similarly named variables in all caps. Thus one uses CC to select
166 the desired C compiler (and similarly MAKE, AR, AS, FC, LD, LEX, RM,
167 YACC, etc.). However, in some traditional uses such an environment
168 variable gives options for the program instead of a pathname. Thus,
169 one has MORE and LESS. Such usage is considered mistaken, and to be
170 avoided in new programs.
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173 bash(1), csh(1), env(1), login(1), printenv(1), sh(1), su(1), tcsh(1),
174 execve(2), clearenv(3), exec(3), getenv(3), putenv(3), setenv(3), un‐
175 setenv(3), locale(7), ld.so(8), pam_env(8)
176Linux man-pages 6.04 2023-02-05 environ(7)