Controlling bash Features and Options

This section explains how to controlbash features and options using command line options and the set andshopt builtins.

Command Line Options

Two kinds of command line options are available: short and long. Short options consist of a hyphen followed by a letter; long options have two hyphens followed by multiple characters. Long options must appear before short options on a command line that calls bash. Table 8-12 lists some commonly used command line options.

Table 8-12. Command line options

Option	Explanation	Syntax
Help	Displays a usage message.	--help
No edit	Prevents users from using the Readline Library (page 304) to edit command lines in an interactive shell.	--noediting
No profile	Prevents reading these startup files (page 259): /etc/profile, ~/.bash_profile, ~/.bash_login, and ~/.profile.	--noprofile
No rc	Prevents reading the ~/.bashrc startup file (page 260). This option is on by default if the shell is called as sh.	--norc
POSIX	Runs bash in POSIX mode.	--posix
Version	Displays bash version information and exits.	--version
Login	Causes bash to run as though it were a login shell.	--I (lowercase "I")
shopt	Runs a shell with the opt shopt option (page 318). A -0 (uppercase "0") sets the option; +0 unsets it.	[±]0 [ opt ]
End of options	On the command line, signals the end of options. Subsequent tokens are treated as arguments even if they begin with a hyphen (-).

Shell Features

You can control the behavior of the Bourne Again Shell by turning features on and off. Different features use different methods to turn features on and off. The set builtin controls one group of features, while the shopt builtin controls another group. You can also control many features from the command line you use to call bash.

Tip: Features, options, variables?

To avoid confusing terminology, this book refers to the various shell behaviors that you can control as features. The bash info page refers to them as "options" and "values of variables controlling optional shell behavior."

set ±o: Turns Shell Features On and Off

The set builtin (there is a set builtin in tcsh, but it works differently), when used with the -o or +ooption, enables, disables, and lists certain bash features. For example, the following command turns on the noclobber feature (page 125):

   $ set -o noclobber

You can turn this feature off (the default) by giving the command

   $ set +o noclobber

The command set -o without an option lists each of the features controlled by set followed by its state (on or off). The command set +o without an option lists the same features in a form that you can use as input to the shell. Table 8-13 lists bash features.

Table 8-13. bash features

Feature	Description	Syntax	Alternate syntax
allexport	Automatically exports all variables and functions that you create or modify after giving this command.	set –o allexport	set –a
braceexpand	Causes bash to perform brace expansion (the default; page 323).	set –o braceexpand	set –B
cdspell	Corrects minor spelling errors in directory names used as arguments to cd.	shopt –s cdspell
cmdhist	Saves all lines of a multiline command in the same history entry, adding semicolons as needed.	shopt –s cmdhist
dotglob	Causes shell special characters (wildcards; page 133) in an ambiguous file reference to match a leading period in a filename. By default special characters do not to match a leading period. You must always specify the filenames . and ..explicitly because no pattern ever matches them.	shopt –s dotglob
emacs	Specifies emacs editing mode for command line editing (the default; page 305).	set –o emacs
errexit	Causes bash to exit when a simple command (not a control structure) fails.	set –o errexit	set –e
execfail	Causes a shell script to continue running when it cannot find the file that is given as an argument to exec. By default a script terminates whenexec cannot find the file that is given as its argument.	shopt –s execfail
expand_aliases	Causes aliases (page 311) to be expanded (by default it is on for interactive shells and off for noninteractive shells).	shopt –s expand_alias
hashall	Causes bash to remember where commands it has found using PATH (page 285) are located (default).	set –o hashall	set –h
histappend	Causes bash to append the history list to the file named by HISTFILE (page 295) when the shell exits. By default bash overwrites this file.	shopt –s histappend
histexpand	Causes the history mechanism (which uses exclamation points; page 299) to work (default). Turn this feature off to turn off history expansion.	set –o histexpand	set –H
history	Enable command history (on by default; page 295).	set –o history
ignoreeof	Specifies that bash must receive ten EOF characters before it exits. Useful on noisy dial-up lines.	set –o ignoreeof
monitor	Enables job control (on by default, page 272).	set –o monitor	set –m
nocaseglob	Causes ambiguous file references (page 133) to match filenames without regard to case (off by default).	shopt –s nocaseglob
noclobber	Helps prevent overwriting files (off by default; page 125).	set –o noclobber	set –C
noglob	Disables pathname expansion (off by default; page 133).	set –o noglob	set –f
notify	With job control (page 272) enabled, reports the termination status of background jobs immediately. The default behavior is to display the status just before the next prompt.	set –o notify	set –b
nounset	Displays an error and exits from a shell script when you use an unset variable in an interactive shell. The default is to display a null value for an unset variable.	set –o nounset	set –u
nullglob	Causes bash to expand ambiguous file references (page 133) that do not match a filename to a null string. By default bash passes these file references without expanding them.	shopt –s nullglob
posix	Runs bash in POSIX mode.	set –o posix
verbose	Displays command lines as bash reads them.	set –o verbose	set –v
vi	Specifies vi editing mode for command line editing (page 304).	set –o vi
xpg_echo	Causes the echo builtin to expand back-slash escape sequences without the need for the –eoption (page 548).	shopt –s xpg_echo
xtrace	Turns on shell debugging (page 536).	set –o xtrace	set –x

shopt: Turns Shell Features On and Off

The shopt (shell option) builtin (not available in tcsh) enables, disables, and lists certain bashfeatures that control the behavior of the shell. For example, the following command causes bashto include filenames that begin with a period (.) when it expands ambiguous file references (the -sstands for set):

   $ shopt -s dotglob

You can turn this feature off (the default) by giving the command (the –u stands for unset)

$ shopt -u dotglob

The shell displays how a feature is set if you give the name of the feature as the only argument toshopt:

$ shopt dotglob
dotglob           off

The command shopt without any options or arguments lists the features controlled by shopt and their state. The command shopt -s without an argument lists the features controlled by shopt that are set or on. The command shopt –u lists the features that are unset or off. Table 8-13 lists bashfeatures.

Tip: Setting set ±0 features using shopt

You can use shopt to set/unset features that are otherwise controlled by set ±o. Use the regular shopt syntax with –s or –u and include the –o option. For example, the following command turns on the noclobber feature:

$ shopt -o -s noclobber

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