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NAME | SYNOPSIS | DESCRIPTION | NOTES | BUGS | SEE ALSO | COLOPHON |
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environ(7) Miscellaneous Information 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 array of strings is made available to the
process by the execve(2) call when a new program is started.
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".
The name is case-sensitive and may not contain the character "=".
The value can be anything that can be represented as a string.
The name and the value may not contain an embedded null byte
('\0'), since this is assumed to terminate the string.
Environment variables 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).
What follows is a list of environment variables typically seen on
a system. This list is incomplete and includes only common
variables seen by average users in their day-to-day routine.
Environment variables specific to a particular program or library
function are documented in the ENVIRONMENT section of the
appropriate manual page.
USER The name of the logged-in user (used by some BSD-derived
programs). Set at login time, see section NOTES below.
LOGNAME
The name of the logged-in user (used by some System-V
derived programs). Set at login time, see section NOTES
below.
HOME A user's login directory. Set at login time, see section
NOTES below.
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 employ when searching for an executable
file that is specified as a simple filename (i.a., a
pathname that contains no slashes). The prefixes are
separated by colons (:). The list of prefixes is searched
from beginning to end, by checking the pathname formed by
concatenating a prefix, a slash, and the filename, until a
file with execute permission is found.
As a legacy feature, a zero-length prefix (specified as
two adjacent colons, or an initial or terminating colon)
is interpreted to mean the current working directory.
However, use of this feature is deprecated, and POSIX
notes that a conforming application shall use an explicit
pathname (e.g., .) to specify the current working
directory.
Analogously to PATH, 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 Absolute path to the current working directory; required
to be partially canonical (no . or .. components).
SHELL The absolute pathname of the user's login shell. Set at
login time, see section NOTES below.
TERM The terminal type for which output is to be prepared.
PAGER The user's preferred utility to display text files. Any
string acceptable as a command-string operand to the sh -c
command shall be valid. If PAGER is null or is not set,
then applications that launch a pager will default to a
program such as less(1) or more(1).
EDITOR/VISUAL
The user's preferred utility to edit text files. Any
string acceptable as a command_string operand to the sh -c
command shall be valid.
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. See also
ld.so(8).
• 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).
Historically and by standard, environ must be declared in the
user program. However, as a (nonstandard) programmer
convenience, environ is declared in the header file <unistd.h> if
the _GNU_SOURCE feature test macro is defined (see
feature_test_macros(7)).
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.
The HOME, LOGNAME, SHELL, and USER variables are set when the
user is changed via a session management interface, typically by
a program such as login(1) from a user database (such as
passwd(5)). (Switching to the root user using su(1) may result
in a mixed environment where LOGNAME and USER are retained from
old user; see the su(1) manual page.)
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 and LESS.
Such usage is considered mistaken, and to be avoided in new
programs.
bash(1), csh(1), env(1), login(1), printenv(1), sh(1), su(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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
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man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 environ(7)
Pages that refer to this page: login(1), pcp2arrow(1), pcp2json(1), pcp2openmetrics(1), pcp2template(1), pcp2xlsx(1), pcp2xml(1), pcp-atopsar(1), pcp-buddyinfo(1), pcp-free(1), pcpintro(1), pcp-meminfo(1), pcp-mpstat(1), pcp-netstat(1), pcp-pidstat(1), pcp-ps(1), pcp-slabinfo(1), pcp-uptime(1), pcp-zoneinfo(1), pmchart(1), pmclient(1), pmdiff(1), pmdumptext(1), pmie(1), pminfo(1), pmlc(1), pmlogcheck(1), pmlogdump(1), pmlogextract(1), pmloglabel(1), pmlogreduce(1), pmlogsummary(1), pmprobe(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_pid_environ(5), systemd.exec(5), systemd-system.conf(5), file-hierarchy(7), pam_env(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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