| NAME | SYNOPSIS | DESCRIPTION | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
ENVIRON(7) Linux Programmer's 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
variable must be declared in the user program, but is declared in the header
file <unistd.h> in case the header files came from libc4 or libc5, and in case
they came from glibc and _GNU_SOURCE was defined.) This array of strings is
made available to the process by the exec(3) call that started the process.
By convention the strings in environ have the form "name=value". Common
examples are:
USER The name of the logged-in user (used by some BSD-derived programs).
LOGNAME
The name of the logged-in user (used by some System-V derived
programs).
HOME A user's login directory, set by login(1) from the password file
passwd(5).
LANG 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.
locale(5).
PATH 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, etc.)
PWD The current working directory. Set by some shells.
SHELL The pathname of the user's login shell.
TERM The terminal type for which output is to be prepared.
PAGER The user's preferred utility to display text files.
EDITOR/VISUAL
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 sh(1), or by the setenv command if you use csh(1). 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).
Note that the behavior of many programs and library routines is influenced by
the presence or value of certain environment variables. A random collection:
The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES, etc.
influence locale handling, cf. locale(5).
TMPDIR influences the path prefix of names created by tmpnam(3) and other
routines, the temporary directory used by sort(1) and other programs, etc.
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_* 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).
Etc.
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, LESS, and GZIP.
Such usage is considered mistaken, and to be avoided in new programs. The
authors of gzip should consider renaming their option to GZIP_OPT.
bash(1), csh(1), login(1), sh(1), tcsh(1), execve(2), clearenv(3), exec(3),
getenv(3), putenv(3), setenv(3), unsetenv(3), locale(5)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-07-25 ENVIRON(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface