environ — user environment
extern char **environ;
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:
USERThe name of the logged-in user (used by some BSD-derived programs).
LOGNAMEThe name of the logged-in user (used by some System-V derived programs).
HOMEA user's login directory, set by login(1) from the password file passwd(5).
LANGThe 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).
PATHThe 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.)
PWDThe current working directory. Set by some shells.
SHELLThe pathname of the user's login shell.
TERMThe terminal type for which output is to be prepared.
PAGERThe user's preferred utility to display text files.
EDITOR/VISUALThe 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 time zone 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)