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
macro is defined.) This array of strings is made available to the process by
(3) call that started the process. When a child process is
created via fork
(2), it inherits a copy
of its parent's
By convention the strings in environ
have the form "
". Common examples are:
- The name of the logged-in user (used by some BSD-derived programs).
- The name of the logged-in user (used by some System-V derived
- A user's login directory, set by login(1) from the password file
- 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 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
- The current working directory. Set by some shells.
- The pathname of the user's login shell.
- The terminal type for which output is to be prepared.
- The user's preferred utility to display text files.
- The user's preferred utility to edit text files.
Names may be placed in the shell's environment by the export
(1), or by the setenv
command if you use csh
The initial environment of the shell is populated in various ways, such as
definitions from /etc/environment
that are processed by
(8) for all users at login time (on systems that employ
(8)). In addition, various shell initialization scripts, such as the
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
(3). A C program can manipulate its environment using the functions
(3), and unsetenv
Note that the behavior of many programs and library routines is influenced by
the presence or value of certain environment variables. Examples include the
- 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.
- 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.
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
There is also the risk of name space pollution. Programs like make
allow overriding of default utility names from the environment
with similarly named variables in all caps. Thus one uses CC
the desired C compiler (and similarly MAKE
, etc.). However, in
some traditional uses such an environment variable gives options for the
program instead of a pathname. Thus, one has MORE
. Such usage is considered mistaken, and to be avoided in new
programs. The authors of gzip
should consider renaming their option to