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).
A users login directory, set by login(1) from the password file passwd(5).
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 users login shell.
The terminal type for which output is to be prepared.
The users preferred utility to display 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 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).