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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 was defined.)
This array of strings is made available to the process by the exec(3)
call that started the process.
By convention the strings in environ have the form "name=value".
Common examples are:
USER The name of the logged-in user (used by some BSD-derived
The name of the logged-in user (used by some System-V derived
HOME A user’s login directory, set by login(1) from the password file
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
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,
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.
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 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
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 timezone 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).
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
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)
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