The variable environ points to an array of strings called the `environment'. (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 these strings have the form `name=value'. Common examples are:
The name of the logged-in user (used by some BSD-derived programs).
The name of the logged-in user (used by some System-V derived programs).
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).
The sequence of directory prefixes that sh(1) and many other programs apply in searching for a file known by an incomplete path name. 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, etc.)
The current working directory. Set by some shells.
The file name of the user's login shell.
The terminal type for which output is to be prepared.
The user's preferred utility to display text files.
The user's preferred utility to edit text files.
The user's preferred utility to browse URLs. Sequence of colon-separated browser commands. See http://www.tuxedo.org/esr/BROWSER/ .
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(2)?. A C program can manipulate its environment using the functions getenv(3), putenv(3), setenv(3) and unsetenv(3).
Note that the behaviour 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).
LD_LIBRARY_PATH, LD_PRELOAD and other LD_* variables influence the behaviour of the dynamic loader/linker.
POSIXLY_CORRECT makes certain programs and library routines follow the prescriptions of POSIX.
The behaviour 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 time zone information.
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.
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.