If you haven’t tried it yet, you might be surprised by the many features of shopt. While it works like a Linux command, it’s actually a bash shell builtin that allows you to change many things about that shell’s behavior.
One option, for example, allows the shell to fix minor typos when you type directory names. To demonstrate, in the first cd command shown below, the directory name, bin, is typed with an extra letter and the shell complains and gives up:
$ cd binn -bash: cd: binn: No such file or directory
This next command enables the cdspell option that gets bash to attempt to correct minor typos in directory names.
$ shopt -s cdspell
Once cdspell is enabled, if some directory name is misspelled, bash will display the correct spelling and then run the corrected cd command. As you can see below, pwd confirms that the cd command was indeed run.
$ cd binn bin $ pwd /home/shs/bin
If you’re wondering about the name, try suppressing any impulse to call it “shop tea”. It will be easier to remember if you, instead, think “shell option” and pronounce it the same as “shopped”. That is undoubtedly what the authors had in mind.
The basic syntax for shopt is:
shopt [-pqsu] [-o] [optname …]
This syntax summary shows that all arguments are optional and that more than one “optname” (the setting being displayed, enabled or disabled) can be manipulated with one command. Keep in mind when reading about shopt that both the “optname” list and the -pqsu arguments are often both referred to as “options” — command options and shell options.
Another very useful bash setting is histappend which allows more than one shell to write to the same history file. Just remember that bash adds commands from a session to its history file only when the session is ended.
Once you test a setting and decide you like it, add the required command to your .bashrc file so that it will be enabled whenever you log in. You may only have to remove a comment marker for some:
# append to the history file, don't overwrite it #shopt -s histappend <==
To see a complete list of the settings available for shopt and whether they are currently enabled, simply type “shopt”. In the command below, the output is being passed to the column command simply to condense the output for this post.
$ shopt | column autocd off globstar off assoc_expand_once off gnu_errfmt off cdable_vars off histappend off cdspell on histreedit off checkhash off histverify off checkjobs off hostcomplete off checkwinsize on huponexit off cmdhist on inherit_errexit off compat31 off interactive_comments on compat32 off lastpipe off compat40 off lithist off compat41 off localvar_inherit off compat42 off localvar_unset off compat43 off login_shell on compat44 off mailwarn off complete_fullquote on no_empty_cmd_completion off direxpand off nocaseglob off dirspell off nocasematch off dotglob off nullglob off execfail off progcomp on expand_aliases on progcomp_alias off extdebug off promptvars on extglob on restricted_shell off extquote on shift_verbose off failglob off sourcepath on force_fignore on xpg_echo off globasciiranges on
The list above contains 53 settings. If you want to display only those settings that are enabled, use the shopt -s command:
$ shopt -s | column cdspell on force_fignore on checkwinsize on globasciiranges on cmdhist on interactive_comments on complete_fullquote on login_shell on expand_aliases on progcomp on extglob on promptvars on extquote on sourcepath on
To view the status of an individual setting, you can type “shopt” following by the setting name. For example:
$ shopt cdspell cdspell on
The options for the shopt built-in include:
- -s enable (set) the setting
- -u disable (unset) the setting
- -q suppress the normal output
- -o restricts values of optname to those defined for the -o option with set
The -q option might be a little difficult to understand until you try it out. In the second example below, we use this option when trying to determine if the cdspell setting is enabled, but don’t see any confirmation because the -q suppresses the output. We can, however, still determine whether the setting is enabled by displaying the return code. If the setting had been disabled, the echo $? command would have returned a 1.
$ shopt cdspell cdspell on $ shopt -q cdspell $ echo $? 0
To view a list of the shopt settings (a.k.a. options) with explanations about how each of them affects how bash behaves, look at the bash man page and scroll down to the section that begins like this:
The list of shopt options is:
assoc_expand_once
If set, the shell suppresses multiple evaluation of as‐
sociative array subscripts during arithmetic expression
evaluation, while executing builtins that can perform
variable assignments, and while executing builtins that
perform array dereferencing.
autocd If set, a command name that is the name of a directory
is executed as if it were the argument to the cd com‐
mand. This option is only used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command that is
not a directory is assumed to be the name of a variable
whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory com‐
ponent in a cd command will be corrected. The errors
checked for are transposed characters, a missing charac‐
ter, and one character too many. If a correction is
found, the corrected filename is printed, and the com‐
mand proceeds. This option is only used by interactive
shells.
Since this post mentions the histappend setting, here is the man page’s explanation for that one:
histappend
If set, the history list is appended to the file named
by the value of the HISTFILE variable when the shell ex‐
its, rather than overwriting the file.
The shopt builtin may not become one of your favorite Linux tricks, but some of the ways that it allows you to tweak the way bash behaves might please you.
