environ - user environment
SYNOPSIS
extern char **environ;
DESCRIPTION
The variable environ points to an array of strings called
the `environment'. (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
these strings 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 Sys
tem-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 path name. 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 file name of the user's login shell.
TERM The terminal type for which output is to be pre
pared.
PAGER The user's preferred utility to display text files.
EDITOR/VISUAL
The user's preferred utility to edit text files.
The user's preferred utility to browse URLs.
Sequence of colon-separated browser commands. See
http://www.tuxedo.org/~esr/BROWSER/ .
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(2). A C pro
gram can manipulate its environment using the functions
getenv(3), putenv(3), setenv(3) and unsetenv(3).
Note that the behaviour of many programs and library rou
tines 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 tmp
nam(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 behaviour of the dynamic loader/linker.
POSIXLY_CORRECT makes certain programs and library rou
tines follow the prescriptions of POSIX.
The behaviour of malloc(3) is influenced by MALLOC_* vari
ables.
The variable HOSTALIASES gives the name of a file contain
ing aliases to be used with gethostbyname(3).
TZ and TZDIR give time zone information.
TERMCAP gives information on how to address a given termi
nal (or gives the name of a file containing such informa
tion).
COLUMNS and LINES tell applications about the window size,
possibly overriding the actual size.
PRINTER or LPDEST may specify the desired printer to use.
Etc. etc.
BUGS
Clearly there is a security risk here. Many a system com
mand has been tricked into mischief by a user who speci
fied unusual values for IFS or LD_LIBRARY_PATH.
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.
SEE ALSO
login(1), sh(1), bash(1), csh(1), tcsh(1), execve(2),
exec(3), getenv(3), putenv(3), setenv(3), unsetenv(3),
locale(5)
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