extern char **environ;
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 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).
TMPDIR influences the path prefix of names created by tmpnam(3) and other routines, the temporary directory used by sort(1) and other programs, etc.
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 used by tzset(3) and through that by functions like ctime(), localtime(), mktime(), strftime(). See also tzselect(1).
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).
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.