The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete the command sequence. If multiple completions are possible, then [Tab] lists them all. Let's see how it works.
bash$ xtra[Tab] xtraceroute xtrapin xtrapproto xtraceroute.real xtrapinfo xtrapreset xtrapchar xtrapout xtrapstats
bash$ xtrac[Tab] xtraceroute xtraceroute.real
bash$ xtraceroute.r[Tab] xtraceroute.real
Tab completion also works for variables and path names.
bash$ echo $BASH[Tab] $BASH $BASH_COMPLETION $BASH_SUBSHELL $BASH_ARGC $BASH_COMPLETION_DIR $BASH_VERSINFO $BASH_ARGV $BASH_LINENO $BASH_VERSION $BASH_COMMAND $BASH_SOURCE
bash$ echo /usr/local/[Tab] bin/ etc/ include/ libexec/ sbin/ src/ doc/ games/ lib/ man/ share/
The Bash complete and compgen builtins make it
possible for tab completion to recognize partial parameters and
options to commands. In a very simple case, we can use
complete from the command-line to specify a short list of
acceptable parameters.
bash$ touch sample_command bash$ touch file1.txt file2.txt file2.doc file30.txt file4.zzz bash$ chmod +x sample_command bash$ complete -f -X '!*.txt' sample_command
bash$ ./sample[Tab][Tab] sample_command file1.txt file2.txt file30.txt
The -f option to complete specifies filenames, and -X
the filter pattern.
For anything more complex, we could write a script that specifies a list of acceptable command-line parameters. The compgen builtin expands a list of arguments to generate completion matches.
Let us take a modified version of the UseGetOpt.sh script as
an example command. This script accepts a number of command-line
parameters, preceded by either a single or double dash. And here is
the corresponding completion script, by convention given a filename
corresponding to its associated command.
# file: UseGetOpt-2 # UseGetOpt-2.sh parameter-completion
_UseGetOpt-2 () # By convention, the function name { #+ starts with an underscore. local cur # Pointer to current completion word. # By convention, it's named "cur" but this isn't strictly necessary.
COMPREPLY=() # Array variable storing the possible completions. cur=${COMP_WORDS[COMP_CWORD]}
case "$cur" in -*) COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \ --file --log --test --help --' -- $cur ) );; # Generate the completion matches and load them into $COMPREPLY array. # xx) May add more cases here. # yy) # zz) esac
return 0 }
complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh # ^^ ^^^^^^^^^^^^ Invokes the function _UseGetOpt-2.
Now, let's try it.
bash$ source UseGetOpt-2
bash$ ./UseGetOpt-2.sh -[Tab] -- --aoption --debug --file --help --log --test -a -d -f -h -l -t
bash$ ./UseGetOpt-2.sh --[Tab] -- --aoption --debug --file --help --log --test
We begin by sourcing the "completion script." This sets the command-line parameters. [1]
In the first instance, hitting [Tab] after a single dash, the output is all the possible parameters preceded by one or more dashes. Hitting [Tab] after two dashes gives the possible parameters preceded by two or more dashes.
Now, just what is the point of having to jump through flaming hoops to enable command-line tab completion? It saves keystrokes.
Resources:
Project bash-completion in Debian.
Mitch Frazier's Linux Journal article, More on Using the Bash Complete Command.
See: https://www.linuxjournal.com/content/more-using-bash-complete-command
Steve Kemp's excellent article, An Introduction to Bash Completion
/etc/profile.d directory
or in /etc/bash_completion. These autoload on system
startup. So, after writing a useful completion script, you might wish
to move it (as root, of course) to one of these directories.