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 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
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.
The name of the logged-in user (used by some BSD-derived programs). Set at login time, see section Notes below.
The name of the logged-in user (used by some System-V derived programs). Set at login time, see section Notes below.
A user's login directory. Set at login time, see section Notes below.
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).
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.
The current working directory. Set by some shells.
The absolute pathname of the user's login shell. Set at login time, see section Notes below.
The terminal type for which output is to be prepared.
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).
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).
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.)
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.
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), unsetenv(3), locale(7), ld.so(8), pam_env(8)
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