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NAME | SYNOPSIS | DESCRIPTION | NOTES | BUGS | SEE ALSO | COLOPHON |
ENVIRON(7) Linux Programmer's Manual ENVIRON(7)
environ - user environment
extern char **environ;
The variable 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 <unistd.h> if the _GNU_SOURCE feature
test macro is defined.) This array of strings is made available to
the process by the exec(3) call that started the process. When a
child process is created via fork(2), it inherits a copy of its
parent's environment.
By convention the strings in environ have the form "name=value".
Common examples are:
USER The name of the logged-in user (used by some BSD-derived
programs).
LOGNAME
The name of the logged-in user (used by some System-V derived
programs).
HOME A user's login directory, set by login(1) from the password
file passwd(5).
LANG The name of a locale to use for locale categories when not
overridden by LC_ALL or more specific environment variables
such as LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY,
LC_NUMERIC, and LC_TIME (see locale(7) for further details of
the LC_* environment variables).
PATH The sequence of directory prefixes that sh(1) and many other
programs apply in searching for a file known by an incomplete
pathname. The prefixes are separated by ':'. (Similarly one
has CDPATH used by some shells to find the target of a change
directory command, MANPATH used by man(1) to find manual
pages, and so on)
PWD The current working directory. Set by some shells.
SHELL The pathname of the user's login shell.
TERM The terminal type for which output is to be prepared.
PAGER The user's preferred utility to display text files.
EDITOR/VISUAL
The user's preferred utility to edit text files.
Names may be placed in the shell's environment by the export command
in sh(1), or by the setenv command if you use csh(1).
The initial environment of the shell is populated in various ways,
such as definitions from /etc/environment that are processed by
pam_env(8) for all users at login time (on systems that employ
pam(8)). In addition, various shell initialization scripts, such as
the system-wide /etc/profile script and per-user initializations
script may include commands that add variables to the shell's
environment; see the manual page of your preferred shell for details.
Bourne-style shells support the syntax
NAME=value command
to create an environment variable definition only in the scope of the
process that executes command. Multiple variable definitions,
separated by white space, may precede command.
Arguments may also be placed in the environment at the point of an
exec(3). A C program can manipulate its environment using the
functions getenv(3), putenv(3), setenv(3), and unsetenv(3).
Note that the behavior of many programs and library routines is
influenced by the presence or value of certain environment variables.
Examples include the following:
* The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL,
LC_MESSAGES, and so on influence locale handling; see catopen(3),
gettext(3), and locale(7).
* TMPDIR influences the path prefix of names created by tempnam(3)
and other routines, and the temporary directory used by sort(1)
and other programs.
* LD_LIBRARY_PATH, LD_PRELOAD, and other LD_* variables influence
the behavior of the dynamic loader/linker.
* POSIXLY_CORRECT makes certain programs and library routines follow
the prescriptions of POSIX.
* The behavior of malloc(3) is influenced by MALLOC_* variables.
* The variable HOSTALIASES gives the name of a file containing
aliases to be used with gethostbyname(3).
* TZ and TZDIR give timezone information used by tzset(3) and
through that by functions like ctime(3), localtime(3), mktime(3),
strftime(3). See also tzselect(8).
* TERMCAP gives information on how to address a given terminal (or
gives the name of a file containing such information).
* COLUMNS and LINES tell applications about the window size,
possibly overriding the actual size.
* PRINTER or LPDEST may specify the desired printer to use. See
lpr(1).
The prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can
be used to control the location of the process's environment.
Clearly there is a security risk here. Many a system command has
been tricked into mischief by a user who specified unusual values for
IFS or LD_LIBRARY_PATH.
There is also the risk of name space pollution. Programs like make
and autoconf allow overriding of default utility names from the
environment with similarly named variables in all caps. Thus one
uses CC to select the desired C compiler (and similarly MAKE, AR, AS,
FC, LD, LEX, RM, YACC, etc.). However, in some traditional uses such
an environment variable gives options for the program instead of a
pathname. Thus, one has MORE, LESS, and GZIP. Such usage is
considered mistaken, and to be avoided in new programs. The authors
of gzip should consider renaming their option to GZIP_OPT.
bash(1), csh(1), env(1), login(1), printenv(1), sh(1), tcsh(1),
execve(2), clearenv(3), exec(3), getenv(3), putenv(3), setenv(3),
unsetenv(3), locale(7), ld.so(8), pam_env(8)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 ENVIRON(7)
Pages that refer to this page: login(1), pcp-free(1), pcpintro(1), pcp-iostat(1), pcp-mpstat(1), pcp-pidstat(1), pcp-tapestat(1), pcp-uptime(1), pmchart(1), pmclient(1), pmdiff(1), pmdumplog(1), pmdumptext(1), pmie(1), pmlc(1), pmlogcheck(1), pmlogextract(1), pmloglabel(1), pmlogreduce(1), pmlogsummary(1), pmrep(1), pmstat(1), pmval(1), pmview(1), tset(1), execve(2), clearenv(3), exec(3), getenv(3), intro(3), pmgetconfig(3), pmnewcontextzone(3), pmnewzone(3), putenv(3), sd_bus_default(3), setenv(3), udev_device_new_from_syspath(3), localtime(5), nfs.conf(5), pam_env.conf(5), proc(5), systemd.exec(5), systemd-system.conf(5), file-hierarchy(7), pam_env(8)