From Linux Man Pages
environ - user environment
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 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 environ-
ment 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 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 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 behaviour of the dynamic loader/linker.
POSIXLY_CORRECT makes certain programs and library routines follow the prescriptions of POSIX.
The behaviour 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(), localtime(),
mktime(), strftime(). See also tzselect(1).
TERMCAP gives information on how to address a given terminal (or gives the name of a file containing such infor-
mation).
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.
BUGS
Clearly there is a security risk here. Many a system command has been tricked into mischief by a user who speci-
fied 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.
RELATED
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)
CATEGORY