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+.\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
+.\"   Fri Apr  2 11:32:09 MET DST 1993
+.\" and Andries Brouwer (aeb@cwi.nl), Fri Feb 14 21:47:50 1997.
+.\"
+.\" SPDX-License-Identifier: GPL-2.0-or-later
+.\"
+.\" Modified Sun Jul 25 10:45:30 1993 by Rik Faith (faith@cs.unc.edu)
+.\" Modified Sun Jul 21 21:25:26 1996 by Andries Brouwer (aeb@cwi.nl)
+.\" Modified Mon Oct 21 17:47:19 1996 by Eric S. Raymond (esr@thyrsus.com)
+.\" Modified Wed Aug 27 20:28:58 1997 by Nicolás Lichtmaier (nick@debian.org)
+.\" Modified Mon Sep 21 00:00:26 1998 by Andries Brouwer (aeb@cwi.nl)
+.\" Modified Wed Jan 24 06:37:24 2001 by Eric S. Raymond (esr@thyrsus.com)
+.\" Modified Thu Dec 13 23:53:27 2001 by Martin Schulze <joey@infodrom.org>
+.\"
+.TH environ 7 2024-05-02 "Linux man-pages (unreleased)"
+.SH NAME
+environ \- user environment
+.SH SYNOPSIS
+.nf
+.BI "extern char **" environ ;
+.fi
+.SH DESCRIPTION
+The variable
+.I 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
+.BR execve (2)
+call when a new program is started.
+When a child process is created via
+.BR fork (2),
+it inherits a
+.I copy
+of its parent's environment.
+.P
+By convention, the strings in
+.I environ
+have the form "\fIname\fP\fB=\fP\fIvalue\fP".
+The name is case-sensitive and may not contain
+the character "\fB=\fP".
+The value can be anything that can be represented as a string.
+The name and the value may not contain an embedded null byte (\[aq]\e0\[aq]),
+since this is assumed to terminate the string.
+.P
+Environment variables may be placed in the shell's environment by the
+.I export
+command in
+.BR sh (1),
+or by the
+.I setenv
+command if you use
+.BR csh (1).
+.P
+The initial environment of the shell is populated in various ways,
+such as definitions from
+.I /etc/environment
+that are processed by
+.BR pam_env (8)
+for all users at login time (on systems that employ
+.BR pam (8)).
+In addition, various shell initialization scripts, such as the system-wide
+.I /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.
+.P
+Bourne-style shells support the syntax
+.P
+.in +4n
+.EX
+NAME=value command
+.EE
+.in
+.P
+to create an environment variable definition only in the scope
+of the process that executes
+.IR command .
+Multiple variable definitions, separated by white space, may precede
+.IR command .
+.P
+Arguments may also be placed in the
+environment at the point of an
+.BR exec (3).
+A C program can manipulate its environment using the functions
+.BR getenv (3),
+.BR putenv (3),
+.BR setenv (3),
+and
+.BR unsetenv (3).
+.P
+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 variables specific to a particular program or library function
+are documented in the ENVIRONMENT section of the appropriate manual page.
+.TP
+.B USER
+The name of the logged-in user (used by some BSD-derived programs).
+Set at login time, see section NOTES below.
+.TP
+.B LOGNAME
+The name of the logged-in user (used by some System-V derived programs).
+Set at login time, see section NOTES below.
+.TP
+.B HOME
+A user's login directory.
+Set at login time, see section NOTES below.
+.TP
+.B LANG
+The name of a locale to use for locale categories when not overridden
+by
+.B LC_ALL
+or more specific environment variables such as
+.BR LC_COLLATE ,
+.BR LC_CTYPE ,
+.BR LC_MESSAGES ,
+.BR LC_MONETARY ,
+.BR LC_NUMERIC ,
+and
+.B LC_TIME
+(see
+.BR locale (7)
+for further details of the
+.B LC_*
+environment variables).
+.TP
+.B PATH
+The sequence of directory prefixes that
+.BR 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 (\fB:\fP).
+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.
+.IP
+As a legacy feature, a zero-length prefix
+(specified as two adjacent colons, or an initial or terminating colon)
+is interpreted 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.,
+.IR . )
+to specify the current working directory.
+.IP
+Analogously to
+.BR PATH ,
+one has
+.B CDPATH
+used by some shells to find the target
+of a change directory command,
+.B MANPATH
+used by
+.BR man (1)
+to find manual pages, and so on.
+.TP
+.B PWD
+Absolute path to the current working directory;
+required to be partially canonical (no
+.I .\&
+or
+.I ..\&
+components).
+.TP
+.B SHELL
+The absolute pathname of the user's login shell.
+Set at login time, see section NOTES below.
+.TP
+.B TERM
+The terminal type for which output is to be prepared.
+.TP
+.B PAGER
+The user's preferred utility to display text files.
+Any string acceptable as a command-string operand to the
+.I sh\ \-c
+command shall be valid.
+If
+.B PAGER
+is null or is not set,
+then applications that launch a pager will default to a program such as
+.BR less (1)
+or
+.BR more (1).
+.TP
+.BR EDITOR / VISUAL
+The user's preferred utility to edit text files.
+Any string acceptable as a command_string operand to the
+.I sh\ \-c
+command shall be valid.
+.\" .TP
+.\" .B BROWSER
+.\" The user's preferred utility to browse URLs. Sequence of colon-separated
+.\" browser commands. See http://www.catb.org/\[ti]esr/BROWSER/ .
+.P
+Note that the behavior of many programs and library routines is
+influenced by the presence or value of certain environment variables.
+Examples include the following:
+.IP \[bu] 3
+The variables
+.BR LANG ", " LANGUAGE ", " NLSPATH ", " LOCPATH ,
+.BR LC_ALL ", " LC_MESSAGES ,
+and so on influence locale handling; see
+.BR catopen (3),
+.BR gettext (3),
+and
+.BR locale (7).
+.IP \[bu]
+.B TMPDIR
+influences the path prefix of names created by
+.BR tempnam (3)
+and other routines, and the temporary directory used by
+.BR sort (1)
+and other programs.
+.IP \[bu]
+.BR LD_LIBRARY_PATH ", " LD_PRELOAD ,
+and other
+.B LD_*
+variables influence the behavior of the dynamic loader/linker.
+See also
+.BR ld.so (8).
+.IP \[bu]
+.B POSIXLY_CORRECT
+makes certain programs and library routines follow
+the prescriptions of POSIX.
+.IP \[bu]
+The behavior of
+.BR malloc (3)
+is influenced by
+.B MALLOC_*
+variables.
+.IP \[bu]
+The variable
+.B HOSTALIASES
+gives the name of a file containing aliases
+to be used with
+.BR gethostbyname (3).
+.IP \[bu]
+.BR TZ " and " TZDIR
+give timezone information used by
+.BR tzset (3)
+and through that by functions like
+.BR ctime (3),
+.BR localtime (3),
+.BR mktime (3),
+.BR strftime (3).
+See also
+.BR tzselect (8).
+.IP \[bu]
+.B TERMCAP
+gives information on how to address a given terminal
+(or gives the name of a file containing such information).
+.IP \[bu]
+.BR COLUMNS " and " LINES
+tell applications about the window size, possibly overriding the actual size.
+.IP \[bu]
+.BR PRINTER " or " LPDEST
+may specify the desired printer to use.
+See
+.BR lpr (1).
+.SH NOTES
+Historically and by standard,
+.I environ
+must be declared in the user program.
+However, as a (nonstandard) programmer convenience,
+.I environ
+is declared in the header file
+.I <unistd.h>
+if the
+.B _GNU_SOURCE
+feature test macro is defined (see
+.BR feature_test_macros (7)).
+.P
+The
+.BR prctl (2)
+.B PR_SET_MM_ENV_START
+and
+.B PR_SET_MM_ENV_END
+operations can be used to control the location of the process's environment.
+.P
+The
+.BR HOME ,
+.BR LOGNAME ,
+.BR SHELL ,
+and
+.B USER
+variables are set when the user is changed via a
+session management interface, typically by a program such as
+.BR login (1)
+from a user database (such as
+.BR passwd (5)).
+(Switching to the root user using
+.BR su (1)
+may result in a mixed environment where
+.B LOGNAME
+and
+.B USER
+are retained from old user; see the
+.BR su (1)
+manual page.)
+.SH 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
+.BR IFS " or " LD_LIBRARY_PATH .
+.P
+There is also the risk of name space pollution.
+Programs like
+.I make
+and
+.I autoconf
+allow overriding of default utility names from the
+environment with similarly named variables in all caps.
+Thus one uses
+.B CC
+to select the desired C compiler (and similarly
+.BR MAKE ,
+.BR AR ,
+.BR AS ,
+.BR FC ,
+.BR LD ,
+.BR LEX ,
+.BR RM ,
+.BR YACC ,
+etc.).
+However, in some traditional uses such an environment variable
+gives options for the program instead of a pathname.
+Thus, one has
+.B MORE
+and
+.BR LESS .
+Such usage is considered mistaken, and to be avoided in new
+programs.
+.SH SEE ALSO
+.BR bash (1),
+.BR csh (1),
+.BR env (1),
+.BR login (1),
+.BR printenv (1),
+.BR sh (1),
+.BR su (1),
+.BR tcsh (1),
+.BR execve (2),
+.BR clearenv (3),
+.BR exec (3),
+.BR getenv (3),
+.BR putenv (3),
+.BR setenv (3),
+.BR unsetenv (3),
+.BR locale (7),
+.BR ld.so (8),
+.BR pam_env (8)