environ - user environment
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
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
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
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.
- The name of the logged-in user (used by some BSD-derived programs). Set at
login time, see section NOTES below.
- The name of the logged-in user (used by some System-V derived programs).
Set at login time, see section NOTES below.
- A user's login directory. Set at login time, see section NOTES below.
- 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
- 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.
- The current working directory. Set by some shells.
- The absolute pathname of the user's login shell. Set at login time, see
section NOTES below.
- The terminal type for which output is to be prepared.
- 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
- The user's preferred utility to edit text files. Any string acceptable as
a command_string operand to the sh -c command shall be
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:
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 variables LANG, LANGUAGE, NLSPATH,
LOCPATH, LC_ALL, LC_MESSAGES, and so on influence
locale handling; see catopen(3), gettext(3), and
- 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
- POSIXLY_CORRECT makes certain programs and library routines follow
the prescriptions of POSIX.
- The behavior of malloc(3) is influenced by MALLOC_*
- 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
- 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.
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
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
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),
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