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 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 prepared.
PAGER
The user's preferred utility to display text files.
EDITOR / VISUAL
The user's preferred utility to edit text files.
BROWSER
The user's preferred utility to browse URLs. Sequence of colon-separated
browser commands. See http://www.catb.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(3) . A C program 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 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.
BUGS
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 .
