Harley Hahn's Guide to Unix and Linux

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Appendix G...

Shell Options & Shell Variables

Every shell supports a variety of ways for you to control its behavior. The Bourne Shell family (Bash, Korn Shell) uses shell options; the C-Shell family (Tcsh, C-Shell) uses both shell options and shell variables. This appendix summarizes these options and variables.

For a detailed discussion of both shell options and shell variables, see Chapter 12.

Bourne Shell Family
C-Shell Family

Bourne Shell Family

The Bourne Shell family uses a large number of options to control the behavior of the shell. In particular, interactive shells require different options than non-interactive shells. (In fact, this is what makes them interactive.)

There are two ways to specify such options. First, they may be invoked in the standard manner by specifying options on the command line when the shell is started. This is the case for non-interactive shells that are run automatically, and for interactive shells that you yourself start by entering a command. For example:

bash -vx

(By the way, these two options are particularly useful when you are testing or debugging a shell script.)

Within an interactive shell — that is, at the shell prompt — you can turn options on or off as you wish. To set (turn on) an option, use the set ‑o command. To unset (turn off) an option, use the set +o command. When you use these commands, you specify the long name for the option, not its abbreviation. For example:

set -o verbose
set +o xtrace

To display the current state of all the options, use either command by itself:

set -o
set +o

The -o variation prints human-readable output; +o prints output suitable for a script. (Try them.)

In all, the Bourne Shell family uses a great many shell options. In most cases, you will not need to change them, because the defaults will work just fine. For reference, however, the table in Figure G-1 summarizes the shell options for Bash and the Korn Shell. Not all options are available with all shells, so be sure to look at the leftmost column, which shows you which shells support each option.

— hint —

With Bash, there are extra options available to control a variety of features. To set and unset these options, you use the shopt ("shell options") command. The shopt options are so esoteric that you are unlikely to need them. If you have a moment, though, and you'd like to take a look at them, see the shopt man page.

If you have a moment, though, and you'd like to take a look at them, the best place to read about the shopt options is the Bash man page. To find the relevant discussion, search for "shopt".

Figure G-1: Bourne Shell family: Shell options

This table summarizes the important shell options available with Bourne family shells. The leftmost column shows which shells support each option: B = Bash; K = Korn Shell. A dot indicates that a shell does not support that option. Note that some options, such as bgnice, have a long name but not a short option name. For more information, see the man page for your particular shell.

Notes: (1) Although Bash supports the emacs and vi options, it does not use -E and -V. (2) The Korn shell uses -h and -t, but does not support the long names hashall and onecmd.

Shells	Option	Long Name	Meaning
B K	-a	allexport	export all subsequently defined variables and functions
• K		bgnice	run background jobs at lower priority
B •	-B	braceexpand	brace expansion: enable
B K	-c	—	read commands from string argument
B K	-E	emacs	command line editor: Emacs mode; turns off vi mode
B K	-e	errexit	if a command fails, abort and exit script
B K	-h	hashall	hash (remember) locations of commands as they are found
B •	-H	histexpand	command history: enable ! style history substitution
B •		history	command history: enable
B K	-I	ignoreeof	ignore eof signal ^D; use exit to quit shell (see Chapter 7)
B K	-k	keyword	place all keyword arguments in the environment
• K		markdirs	when globbing, append / to directory names
B K	-m	monitor	job control: enable
B K	-C	noclobber	do not allow redirected output to replace a file
B K	-n	noexec	debug: read commands, check syntax, but do not execute
B K	-f	noglob	globbing (filename expansion): disable
• K		nolog	do not save function definitions in history file
B K	-b	notify	job control: notify immediately when background jobs finish
B K	-u	nounset	treat using an unset variable as an error
B K	-t	onecmd	read and execute one command, then exit
B •		posix	conform to POSIX standard
B •	-p	privileged	use privileged mode to run a script or start a shell
B •	-r	—	start shell in restricted mode
• K	-s	—	sort positional parameters
• K		trackall	aliases: substitute full pathnames for commands
B K	-v	verbose	debug: echo each command to stderr (standard error)
B K	-V	vi	command line editor: vi mode; turns off Emacs mode
• K		viraw	in vi mode: process each character as it is typed
B K	-x	xtrace	debug: as command executes, echo to stderr (that is, trace)
Shells	Option	Long Name	Meaning

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C-Shell Family

The C-Shell family also uses options to control the behavior of the shell. These options are summarized in Figure G-2. If you compare the options in Figure G-2 with those in Figure G-1, you will notice that most of the C-Shell options are also used by the Bourne Shell family. However, there is an important difference.

In the Bourne Shell family the behavior of the shell is controlled entirely by options, of which there are three types: regular command-line options (such as -v), "long name" options used with the set +o command, and the large number of extra options used only with the Bash shopt command.

Although the C-Shell family does use a small number of command-line options (Figure G-2), for the most part, the behavior of a shell is controlled by shell variables. This is one of the important differences between C-Shell family and the Bourne Shell family, and is explained in detail in Chapter 12. For reference, Figure G-3 contains a summary of the shell variables used by the C-Shell and Tcsh.

Figure G-2: C-Shell family: Shell options

This table summarizes the important shell options available with C-Shell family. The leftmost column shows which shells support each option: C = C-Shell; T = Tcsh. A dot indicates that a shell does not support that option. For more information, see the man page for your particular shell.

Note: The -v and -V options work by setting the verbose shell variable. The -x and -X options work by setting the echo shell variable.

Shells	Option	Shell Variable	Meaning
C T	-c	—	read commands from string argument; compare to -s
• T	-d	—	load directory stack from .cshdirs file
C T	-e	—	if a command fails, abort and exit script
C T	-i	—	interactive mode (affects prompts, error handling, etc.)
C T	-n	—	debug: read commands, check syntax, but do not execute
C T	-s	—	read commands from stdin (standard input); compare to -c
C T	-t	—	read and execute one command, then exit
C T	-v	verbose	debug: echo each command, after history substitution only
C T	-V	verbose	debug: set verbose before reading .tcshc/.cshrc file
C T	-x	echo	debug: echo each command, after all substitutions
C T	-X	echo	debug: set echo before reading .tcshc/.cshrc file

Figure G-3: C-Shell family: Shell variables

This table summarizes the important shell variables used with the C-Shell family to control the behavior of the shell. The leftmost column shows which shells support each option: C = C-Shell; T = Tcsh. A dot indicates that a shell does not support that option.

For more information, see the man page for your shell. For information about autocompletion, spell-correction, or command line editing, see the Tcsh man page. For a detailed discussion of shell variables, see Chapter 12.

Shells	Shell Variable	Meaning
• T	addsuffix	autocomplete: append / (slash) after directory names
• T	ampm	show times in 12-hour AM/PM format
• T	autocorrect	spell-correct: invoke spell-word editor before completion attempt
• T	autoexpand	autocomplete: invoke expand-history editor before completion attempt
• T	autolist	autocomplete: when completion fails, list remaining choices
• T	autologout	time (in minutes) until auto-logout, if you don't type a command
C T	cdpath	directories to be searched by cd, chdir, popd
• T	color	cause ls-F command to use color
• T	complete	autocomplete: igncase=ignore case; enhance=also . - _ are separators
• T	correct	spell-correct: cmd=command; all=entire line; complete=complete instead
C T	cwd	your current working directory (compare to owd)
C T	echo	debug: echo each command, after all substitutions
• T	echo_style	echo command: bsd (support -n); sysv (support \ escape); both; none
• T	edit	command line editor: enable
C T	fignore	autocomplete: suffixes to ignore
C •	filec	autocomplete: enable (always on with Tcsh)
• T	group	current groupid
C T	hardpaths	directory stack: resolve pathnames to contain no symbolic links
C T	history	command history: number of lines in history list
C T	home	your home directory
C T	ignoreeof	do not quit shell upon eof signal (^D)
• T	implicitcd	typing directory name by itself means change to that directory
• T	inputmode	command line editor: set initial mode at start of line, insert or overwrite
• T	listjobs	job control: list all jobs whenever a job is suspended; long = long format
• T	loginsh	set to indicate a login shell
C T	mail	list of files to check for new email
• T	matchbeep	autocomplete: make a sound; ambiguous, notunique, never, nomatch
C T	nobeep	autocomplete: never make a sound
C T	noclobber	do not allow redirected output to replace a file
C T	noglob	globbing (filename expansion): disable
C T	notify	job control: notify immediately when background jobs finished
• T	owd	your most recent [old] working directory (compare to cwd)
C T	path	directories to search for programs
C T	prompt	your shell prompt (customize by changing this variable)
• T	pushdsilent	directory stack: pushd and popd do not list directory stack
• T	pushdtohome	directory stack: pushd without arguments assumes home directory (like cd)
• T	recexact	autocomplete: match exact match even if longer match is possible
• T	rmstar	force user to confirm before executing rm * (remove all files)
• T	rprompt	when prompting, string to print on right side of screen (hint: set to %/)
• T	savedirs	directory stack: before logout, save directory stack
C T	savehist	command history: before logout, save this number of lines in history list
C T	shell	pathname of your login shell
C T	term	type of terminal you are using
C T	user	current userid
C T	verbose	debug: echo each command, after history substitution only
• T	visiblebell	use a screen flash instead of an audible sound
Shells	Shell Variable	Meaning

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The Unix/Linux Timeline for Students

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