environ(7) Miscellaneous Information Manual environ(7)
NAME
environ - user environment
SYNOPSIS
extern char **environ;
DESCRIPTION
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 array of strings is made available to the process by the execve(2)
call when a new program is started. When a child process is created
via fork(2), it inherits a copy of its parent's environment.
By convention, the strings in environ have the form "name=value". The
name is case-sensitive and may not contain the character "=". The
value can be anything that can be represented as a string. The name
and the value may not contain an embedded null byte ('\0'), since this
is assumed to terminate the string.
Environment variables may be placed in the shell's environment by the
export command in sh(1), or by the setenv command if you use csh(1).
The initial environment of the shell is populated in various ways, such
as definitions from /etc/environment that are processed by pam_env(8)
for all users at login time (on systems that employ pam(8)). In addi-
tion, various shell initialization scripts, such as the system-wide
/etc/profile script and per-user initializations script may include
commands that add variables to the shell's environment; see the manual
page of your preferred shell for details.
Bourne-style shells support the syntax
NAME=value command
to create an environment variable definition only in the scope of the
process that executes command. Multiple variable definitions, sepa-
rated by white space, may precede command.
Arguments may also be placed in the environment at the point of an
exec(3). A C program can manipulate its environment using the func-
tions getenv(3), putenv(3), setenv(3), and unsetenv(3).
What follows is a list of environment variables typically seen on a
system. This list is incomplete and includes only common variables
seen by average users in their day-to-day routine. Environment vari-
ables specific to a particular program or library function are docu-
mented in the ENVIRONMENT section of the appropriate manual page.
USER The name of the logged-in user (used by some BSD-derived pro-
grams). Set at login time, see section NOTES below.
LOGNAME
The name of the logged-in user (used by some System-V derived
programs). Set at login time, see section NOTES below.
HOME A user's login directory. Set at login time, see section NOTES
below.
LANG The name of a locale to use for locale categories when not over-
ridden by LC_ALL or more specific environment variables such as
LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and
LC_TIME (see locale(7) for further details of the LC_* environ-
ment variables).
PATH The sequence of directory prefixes that sh(1) and many other
programs employ when searching for an executable file that is
specified as a simple filename (i.a., a pathname that contains
no slashes). The prefixes are separated by colons (:). The
list of prefixes is searched from beginning to end, by checking
the pathname formed by concatenating a prefix, a slash, and the
filename, until a file with execute permission is found.
As a legacy feature, a zero-length prefix (specified as two ad-
jacent colons, or an initial or terminating colon) is inter-
preted to mean the current working directory. However, use of
this feature is deprecated, and POSIX notes that a conforming
application shall use an explicit pathname (e.g., .) to specify
the current working directory.
Analogously to PATH, 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 on.
PWD Absolute path to the current working directory; required to be
partially canonical (no . or .. components).
SHELL The absolute pathname of the user's login shell. Set at login
time, see section NOTES below.
TERM The terminal type for which output is to be prepared.
PAGER The user's preferred utility to display text files. Any string
acceptable as a command-string operand to the sh -c command
shall be valid. If PAGER is null or is not set, then applica-
tions that launch a pager will default to a program such as
less(1) or more(1).
EDITOR/VISUAL
The user's preferred utility to edit text files. Any string ac-
ceptable as a command_string operand to the sh -c command shall
be valid.
Note that the behavior of many programs and library routines is influ-
enced by the presence or value of certain environment variables. Exam-
ples include the following:
• The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES,
and so on influence locale handling; see catopen(3), gettext(3), and
locale(7).
• TMPDIR influences the path prefix of names created by tempnam(3) and
other routines, and the temporary directory used by sort(1) and
other programs.
• LD_LIBRARY_PATH, LD_PRELOAD, and other LD_* variables influence the
behavior of the dynamic loader/linker. See also ld.so(8).
• 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), strf-
time(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).
NOTES
Historically and by standard, environ must be declared in the user pro-
gram. However, as a (nonstandard) programmer convenience, environ is
declared in the header file <unistd.h> if the _GNU_SOURCE feature test
macro is defined (see feature_test_macros(7)).
The prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can
be used to control the location of the process's environment.
The HOME, LOGNAME, SHELL, and USER variables are set when the user is
changed via a session management interface, typically by a program such
as login(1) from a user database (such as passwd(5)). (Switching to
the root user using su(1) may result in a mixed environment where LOG-
NAME and USER are retained from old user; see the su(1) manual page.)
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 and LESS. Such usage is considered mistaken, and to be
avoided in new programs.
SEE ALSO
bash(1), csh(1), env(1), login(1), printenv(1), sh(1), su(1), tcsh(1),
execve(2), clearenv(3), exec(3), getenv(3), putenv(3), setenv(3), un-
setenv(3), locale(7), ld.so(8), pam_env(8)
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