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NAME | SYNOPSIS | DESCRIPTION | NOTES | BUGS | SEE ALSO | COLOPHON |
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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 5.10 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 2020-08-13 ENVIRON(7)
Pages that refer to this page: login(1), pcp2json(1), pcp2template(1), pcp2xlsx(1), pcp2xml(1), pcp-atopsar(1), pcp-free(1), pcpintro(1), pcp-mpstat(1), pcp-pidstat(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), pmseries(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)
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