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 variable must be declared in the user program, but is declared in the header file <unistd.h> if the _GNU_SOURCE feature test macro is defined.) This array of strings is made available to the process by the exec(3) call that started the process.
By convention the strings in environ 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 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 apply in searching for a file known by an incomplete pathname. 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, and so on)
The current working directory. Set by some shells.
The pathname 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.
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(3). A C program can manipulate its environment using the functions getenv(3), putenv(3), setenv(3), and unsetenv(3).
Note that the behavior of many programs and library routines is influenced by the presence or value of certain environment variables. A random collection:
LD_LIBRARY_PATH, LD_PRELOAD and other LD_* variables influence the behavior of the dynamic loader/linker.
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).
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).
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, LESS, and GZIP. Such usage is considered mistaken, and to be avoided in new programs. The authors of gzip should consider renaming their option to GZIP_OPT.
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