Ubuntu Manpage: environ - user environment

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 addition, 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, separated 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 functions 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 variables
specific to a particular program or library function are documented in the ENVIRONMENT section of the
appropriate manual page.

USER The name of the logged-in user (used by some BSD-derived programs). 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 overridden 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_* environment 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 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., .) 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 applications
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 acceptable as a command_string
operand to the sh -c command shall be valid.

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:

• 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), 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).

NOTES

       Historically and by standard, environ must be declared in the user program.  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 LOGNAME 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.

NAME

SYNOPSIS

DESCRIPTION

NOTES

BUGS

SEE ALSO