environ - user environment
extern char **environ;
The variable environ
points to an array of pointers to strings called the
"environment". The last pointer in this array has the value NULL.
(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
defined.) This array of strings is made available to the process by the
call that started the process.
By convention the strings in environ
have the form "
". Common examples are:
- The name of the logged-in user (used by some BSD-derived
- The name of the logged-in user (used by some System-V
- A user's login directory, set by login(1) from the
password file passwd(5).
- 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.
- 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, and so
- The current working directory. Set by some shells.
- The pathname of the user's login shell.
- The terminal type for which output is to be prepared.
- The user's preferred utility to display text files.
- The user's preferred utility to edit text files.
Further names may be placed in the environment by the export
"name=value" in sh(1)
, or by the setenv
command if you
(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)
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
, etc. influence locale handling, cf.
influences the path prefix of names created by tmpnam(3)
and other routines, the temporary directory used by sort(1)
and other LD_* variables influence the
behavior of the dynamic loader/linker.
makes certain programs and library routines follow the
prescriptions of POSIX.
The behavior of malloc(3)
is influenced by MALLOC_*
The variable HOSTALIASES
gives the name of a file containing aliases to
be used with gethostbyname(3)
give timezone information used by tzset(3)
through that by functions like ctime(3)
. See also tzselect(8)
gives information on how to address a given terminal (or gives
the name of a file containing such information).
tell applications about the window size,
possibly overriding the actual size.
may specify the desired printer to use. See
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
There is also the risk of name space pollution. Programs like make
allow overriding of default utility names from the environment
with similarly named variables in all caps. Thus one uses CC
the desired C compiler (and similarly MAKE
, etc.). However, in
some traditional uses such an environment variable gives options for the
program instead of a pathname. Thus, one has MORE
. Such usage is considered mistaken, and to be avoided in new
programs. The authors of gzip
should consider renaming their option to
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