B.3 Built-in Shell Variables

Variable	Chapter	Meaning
#	4	Number of arguments given to current process.
-		Options given to shell on invocation.
?	5	Exit status of previous command.
$	8	Process ID of shell process.
_		Last argument to previous command.
!	8	Process ID of last background command.
CDPATH	3	List of directories for cd command to search.
COLUMNS	3	Width of display in columns (for editing modes and select).
EDITOR	2	Used to set editing mode; also used by mail and other programs.
ERRNO	A	Error number of last system call that failed.
ENV	3	Name of file to run as environment file when shell is invoked.
FCEDIT	2	Default editor for fc command.
FPATH	4	Search path for autoloaded functions.
IFS	7	Internal field separator: list of characters that act as word separators. Normally set to SPACE, TAB, and NEWLINE.
HISTFILE	2	Name of command history file.
HISTSIZE	2	Number of lines kept in history file.
HOME	3	Home (login) directory.
LINENO	9	Number of line in script or function that just ran.
LINES	3	Height of display in lines (for select command).
MAIL	3	Name of file to check for new mail.
MAILCHECK	3	How often (in seconds) to check for new mail.
MAILPATH	3	List of file names to check for new mail, if MAIL is not set.
OLDPWD	3	Previous working directory.
OPTARG	6	Argument to option being processed by getopts.
OPTIND	6	Number of first argument after options.
PATH	3	Search path for commands.
PS1	3	Primary command prompt string.
PS2	3	Prompt string for line continuations.
PS3	5	Prompt string for select command.
PS4	9	Prompt string for xtrace option.
PPID	8	Process ID of parent process.
PWD	3	Current working directory.
RANDOM	9	Random number between 0 and 32767 (2215-1).
REPLY	5,7	User's response to select command; result of read command if no variable names given.
SECONDS	3	Number of seconds since shell was invoked.
SHELL	3	Full pathname of shell.
TMOUT	10	If set to a positive integer, number of seconds between commands after which shell automatically terminates.
VISUAL	2	Used to set editing mode.