Section: Linux Programmer's Manual (7)
Return to Main Contents
environ - user environment
extern char **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
feature test macro is defined.)
This array of strings is made available to the process by the
call that started the process.
When a child process is created via
it inherits a
of its parent's environment.
By convention the strings in
have the form "name=value".
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 programs).
A user's login directory, set by
from the password file
The name of a locale to use for locale categories when not overridden
or more specific environment variables such as
for further details of the
The sequence of directory prefixes that
and many other
programs apply in searching for a file known by an incomplete pathname.
The prefixes are separated by ':'.
(Similarly one has
used by some shells to find the target
of a change directory command,
to find manual pages, and so on)
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
or by the
command if you use
The initial environment of the shell is populated in various ways,
such as definitions from
that are processed by
for all users at login time (on systems that employ
In addition, various shell initialization scripts, such as the system-wide
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
Multiple variable definitions, separated by white space, may precede
Arguments may also be placed in the
environment at the point of an
A C program can manipulate its environment using the functions
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:
LANG, LANGUAGE, NLSPATH, LOCPATH,
and so on influence locale handling; see
influences the path prefix of names created by
and other routines, and the temporary directory used by
and other programs.
variables influence the behavior of the dynamic loader/linker.
makes certain programs and library routines follow
the prescriptions of POSIX.
The behavior of
is influenced by
gives the name of a file containing aliases
to be used with
TZ and TZDIR
give timezone information used by
and through that by functions like
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 or LD_LIBRARY_PATH.
There is also the risk of name space pollution.
allow overriding of default utility names from the
environment with similarly named variables in all caps.
Thus one uses
to select the desired C compiler (and similarly
However, in some traditional uses such an environment variable
gives options for the program instead of a pathname.
Thus, one has
Such usage is considered mistaken, and to be avoided in new
The authors of
should consider renaming their option to
This page is part of release 4.15 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- SEE ALSO
This document was created by
using the manual pages.
Time: 19:30:00 GMT, September 19, 2018