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(2) 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-
The name of the logged-in user (used by some System-V
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
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.
The file name of the user's login shell.
TERM The terminal type for which output is to be prepared.
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 program can
manipulate its environment using the functions getenv(),
putenv(), setenv() and unsetenv().
Note that the behaviour 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.
TMPDIR influences the path prefix of names created by
tmpnam() and other routines, the temporary directory used by
sort(1) and other programs, etc.
LD_LIBRARY_PATH, LD_PRELOAD and other LD_* variables influ-
ence the behaviour of the dynamic loader/linker.
POSIXLY_CORRECT makes certain programs and library routines
follow the prescriptions of POSIX.
The behaviour of malloc() is influenced by MALLOC_* vari-
The variable HOSTALIASES gives the name of a file containing
aliases to be used with gethostbyname().
TZ and TZDIR give time zone information.
TERMCAP gives information on how to address a given terminal
(or gives the name of a file containing such information).
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.
login(1), sh(1), bash(1), csh(1), tcsh(1), exec(2),
getenv(3), putenv(3), setenv(3), unsetenv(3).