Section: Linux Programmer's Manual (7)Updated: 2009-07-25Local indexUp
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
in case the header files came from libc4 or libc5, and
in case they came from glibc and
This array of strings is made available to the process by the
call that started the process.
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
by LC_ALL or more specific environment variables like
LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY,
LC_NUMERIC, LC_TIME, cf.
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 CDPATH used by some shells to find the target
of a change directory command, MANPATH used by
find manual pages, etc.)
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.
Further names may be placed in the environment by the export
command and "name=value" in
or by the setenv command if you use
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.
A random collection:
LANG, LANGUAGE, NLSPATH, LOCPATH,
etc. influence locale handling, cf.
influences the path prefix of names created by
and other routines, the temporary directory used by
and other programs, etc.
and other LD_* 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
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.
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