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Tutorial on bash completion scripts

Once mastered, bash completions scripts will greatly improve your terminal experience and make you more productive. This article tackles the technical details of implementing a bash completion script using pure bash. The main article about bash completion tweaking is: Faster workflow using bash completion scripts.

In this article, we focus on the process of creating a bash completion script.

goto

What is bash completion?

Bash completion is a functionality through which bash helps users type their commands faster and easier. It accomplishes that by presenting possible options when users press the tab key while typing a command.

$ git<tab><tab>
git                 git-receive-pack    git-upload-archive  
gitk                git-shell           git-upload-pack     
$ git-s<tab>
$ git-shell

How it works

The completion script is code that uses the builtin bash command complete to define which completion suggestions can be displayed for a given executable. The nature of the completion options vary from simple static to highly sophisticated.

Why bother

Code and comments

You can find the code of this tutorial on GitHub.

Hands on

Here's what we are going to do in this tutorial.

We will first create a dummy executable script called dothis. All it does is execute the command which resides on the number, that was passed as an argument, in user's history. For example, the following command will simply execute the ls -a command given that it exists in history with number 235:

dothis 235

Then we will create a bash completion script that will display commands along with their number from user's history and we will "bind" it to the dothis executable.

$ dothis <tab><tab>
215 ls
216 ls -la
217 cd ~
218 man history
219 git status
220 history | cut -c 8-

Bash completion image

Let the show begin.

Creating the executable script

Create a file named dothis in your working directory and add the following code:

if [ -z "$1" ]; then
  echo "No command number passed"
  exit 2
fi

exists=$(fc -l -1000 | grep ^$1 -- 2>/dev/null)

if [ -n "$exists" ]; then
  fc -s -- "$1"
else
  echo "Command with number $1 was not found in recent history"
  exit 2
fi

Notes: We first check if the script was called with an argument We then check if the specific number is included in the last 1000 commands * if it exists we execute the command using the fc functionality * otherwise we display an error message

Make the script executable with:

chmod +x ./dothis

We are going to execute this script many times in this tutorial so I suggest you place it in a folder that is included in your path so that we can access it from anywhere with just typing dothis.

I installed it in my home bin folder using:

install ./dothis ~/bin/dothis

You can do the same given that you have a ~/bin folder and that it is included in your PATH variable.

Try to see if it's working:

dothis

You should see this.

$ dothis
No command number passed

Done.

Creating the completion script

Create a file named dothis-completion.bash. From now on we will be referring to this file with the term completion script.

Once we add some code to it, we will source it to allow the completion to take effect. We must source this file every single time we change something in it.

We will discuss our options for registering this script whenever a bash shell opens later on.

Static completion

Suppose that the dothis program supported a list of commands, for example: now tomorrow * never

Let's use the complete command to register this list for completion. To use the proper terminology, we say we use the complete command to define a completion specification (compspec) for our program.

Add this to the completion script.

#/usr/bin/env bash
complete -W "now tomorrow never" dothis

What did we specify with the complete command above: we used the -W (wordlist) option to provide a list of words for completion. we defined to which "program" these completion words will be used (the dothis parameter)

Source the file:

source ./dothis-completion.bash

Now try pressing tab twice in the command line as shown below:

$ dothis <tab><tab>
never     now       tomorrow

Try again after typing the n:

$ dothis n<tab><tab>
never now

Magic! The completion options are automatically filtered to match only those starting with n.

Note: The options are not displayed in the order that we defined them in the word list, they are automatically sorted.

There are a lot of other options to be used instead of the -W that we used in this section. Most of them produce completions in a fixed manner, meaning that we don't intervene dynamically to filter their output.

For example, if we wanted to have directory names as completion words for the dothis program, we would change the complete command to the following:

complete -A directory dothis

Pressing tab after the dothis program, we would get a list of the directories in the current directory from which we execute the script:

$ dothis <tab><tab>
dir1/ dir2/ dir3/

Find the complete list of the available flags here.

Dynamic completion

We will be producing the completions of the dothis executable with the following logic:

Let's start by defining a function that will execute each time user requests completion on a dothis command. Change the completion script to this:

#/usr/bin/env bash
_dothis_completions()
{
  COMPREPLY+=("now")
  COMPREPLY+=("tomorrow")
  COMPREPLY+=("never")
}

complete -F _dothis_completions dothis

Note the following: we used the -F flag in the complete command defining that the _dothis_completions is the function that will provide the completions of the dothis executable COMPREPLY is an array variable used to store the completions - the completion mechanism uses this variable to display its contents as completions

Now source the script and go for completion:

$ dothis <tab><tab>
never now tomorrow

Perfect. We produce the same completions as in the previous section with the word list. Or not? Try this:

$ dothis nev<tab><tab>
never     now       tomorrow

As you can see, even though we type nev and then we request for completion, the available options are always the same and nothing gets completed automatically. Why is this happening? The contents of the COMPREPLY variable are always displayed. The function is responsible to add/remove entries from there now. If the COMPREPLY variable had only one element then that word would be automatically completed in the command. Since current implementation always return the same three words, this will not happen.

Enter compgen: a builtin command that generates completions supporting most of the options of the complete command (ex. -W for word list, -d for directories) and filtering them based on what already has been typed by the user.

Don't worry if you feel confused, everything will become clear later on.

Type the following in the console to better understand what compgen does:

$ compgen -W "now tomorrow never"
now
tomorrow
never
$ compgen -W "now tomorrow never" n
now
never
$ compgen -W "now tomorrow never" t
tomorrow

So now we can use it but we need to find a way to know what has been typed after the dothis command. We already have the way. The bash completion facilities provide bash variables related to the completion taking place. Here are the more important ones:

To access the word just after the dothis word, we can use the value of COMP_WORDS[1]

Change the completion script again:

#/usr/bin/env bash
_dothis_completions()
{
  COMPREPLY=($(compgen -W "now tomorrow never" "${COMP_WORDS[1]}"))
}

complete -F _dothis_completions dothis

Source and there you are: bash $ dothis never now tomorrow $ dothis n never now

Now, instead of the now, tomorrow, never words, we would like to see actual numbers from the command history.

The fc -l command followed by a negative number -n displays the last n commands. So, we will use:

fc -l -50

which lists the last 50 executed commands along with their numbers. The only manipulation we need to do is replace tabs with spaces in order to be displayed properly from the completion mechanism. sed to the rescue.

Change the completion script as follows:

#/usr/bin/env bash
_dothis_completions()
{
  COMPREPLY=($(compgen -W "$(fc -l -50 | sed 's/\t//')" -- "${COMP_WORDS[1]}"))
}

complete -F _dothis_completions dothis

Source and test in the console:

$ dothis <tab><tab>
649 source dothis-completion.bash   666 cat ~/.bash_profile
650 clear                           667 cat ~/.bashrc
651 source dothis-completion.bash   668 clear
652 source dothis-completion.bash   669 install ./dothis ~/bin/dothis
653 source dothis-completion.bash   670 dothis
654 clear                           671 dothis 6546545646
655 dothis 654                      672 clear
656 dothis 631                      673 dothis
657 dothis 150                      674 dothis 651
658 dothis                          675 source dothis-completion.bash
659 clear                           676 dothis 651
660 dothis                          677 dothis 659
661 install ./dothis ~/bin/dothis   678 clear
662 dothis                          679 dothis 665
663 install ./dothis ~/bin/dothis   680 clear
664 dothis                          681 clear
665 cat ~/.bashrc

Not bad.

We do have a problem though. Try typing a number as you see it in your completion list and the press the key again.

$ dothis 623<tab>
$ dothis 623  ls 623  ls -la
...
$ dothis 623  ls 623  ls 623  ls 623  ls 623  ls -la

This is happening because in our completion script, we used the ${COMP_WORDS[1]} to always check the first typed word after the dothis command (the number 623 in the above snippet). Hence the completion continues suggesting the same completion again and again while pressing the tab key.

To fix this, we will not allow any kind of completion to take place if at least one argument has already been typed. We will add a condition in our function checking the size of the aforementioned COMP_WORDS array.

#/usr/bin/env bash
_dothis_completions()
{
  if [ "${#COMP_WORDS[@]}" != "2" ]; then
    return
  fi

  COMPREPLY=($(compgen -W "$(fc -l -50 | sed 's/\t//')" -- "${COMP_WORDS[1]}"))
}

complete -F _dothis_completions dothis

Source and retry.

$ dothis 623<tab>
$ dothis 623 ls -la<tab> # SUCCESS: nothing happens here

There is another thing we don't like though. We do want to display the numbers along with the corresponding commands to help users decide which one is the desired but when there is only one completion suggestion and gets automatically picked by the completion mechanism, we shouldn't append the command literal too.

In other words, our dothis executable accepts only a number and we haven't added any functionality checking or expecting other arguments. When our completion function gives only one result, we should trim the command literal and respond only with the command number.

To accomplish this, we will keep the response of the compgen command in an array variable and if its size is 1 we will trim the one and only element to keep just the number. Otherwise, we'll let the array as is.

Change the completion script to this:

#/usr/bin/env bash
_dothis_completions()
{
  if [ "${#COMP_WORDS[@]}" != "2" ]; then
    return
  fi

  # keep the suggestions in a local variable
  local suggestions=($(compgen -W "$(fc -l -50 | sed 's/\t/ /')" -- "${COMP_WORDS[1]}"))

  if [ "${#suggestions[@]}" == "1" ]; then
    # if there's only one match, we remove the command literal
    # to proceed with the automatic completion of the number
    local number=$(echo ${suggestions[0]/%\ */})
    COMPREPLY=("$number")
  else
    # more than one suggestions resolved,
    # respond with the suggestions intact
    COMPREPLY=("${suggestions[@]}")
  fi
}

complete -F _dothis_completions dothis

Registering the completion script

If you want to enable the completion just for you on your machine, all you have to do is add a line in your .bashrc file sourcing the script:

source <path-to-your-script>/dothis-completion.bash

If you want to enable the completion for all users, you can just copy the script under /etc/bash_completion.d/ and it will automatically be loaded by Bash.

Fine tuning the completion script

Some extra steps for better results :)

Displaying each entry in a new line

Well, in the bash completion script I was working on, I too had to present suggestions consisting of two parts. I wanted to display the first part with the default color and the second part with a gray color to point out that it was just help text. In this tutorial's example, it would be nice to present the numbers in the default color but the command literal in another one, less fancy.

Unfortunately, this is not possible at least for the time being because the completions are displayed as plain text and color directives are not processed (for example: \e[34mBlue).

What we can do though to improve user experience (or not :D) is to display each entry in a new line. This solution is not that obvious since again we can't just append a new line character in each COMPREPLY entry. We will follow a rather hackish way and pad suggestion literals to such a width that fills the terminal's width.

Enter printf. If you want to display each suggestion on each own line, change the completion script to the following:

#/usr/bin/env bash
_dothis_completions()
{
  if [ "${#COMP_WORDS[@]}" != "2" ]; then
    return
  fi

  local IFS=$'\n'
  local suggestions=($(compgen -W "$(fc -l -50 | sed 's/\t//')" -- "${COMP_WORDS[1]}"))

  if [ "${#suggestions[@]}" == "1" ]; then
    local number="${suggestions[0]/%\ */}"
    COMPREPLY=("$number")
  else
    for i in "${!suggestions[@]}"; do
      suggestions[$i]="$(printf '%*s' "-$COLUMNS"  "${suggestions[$i]}")"
    done

    COMPREPLY=("${suggestions[@]}")
  fi
}

complete -F _dothis_completions dothis

Source and test:

dothis <tab><tab>
...
499 source dothis-completion.bash                   
500 clear
...       
503 dothis 500

Customizable behavior

In our case, we hard-coded to display the last 50 commands for completion. This is not a good practice. We should first respect what each user might prefer and if he/she hasn't made any preference then we should default to 50.

To accomplish that we will check if an environment variable DOTHIS_COMPLETION_COMMANDS_NUMBER has been set.

Change the completion script one last time:

#/usr/bin/env bash
_dothis_completions()
{
  if [ "${#COMP_WORDS[@]}" != "2" ]; then
    return
  fi

  local commands_number=${DOTHIS_COMPLETION_COMMANDS_NUMBER:-50}
  local IFS=$'\n'
  local suggestions=($(compgen -W "$(fc -l -$commands_number | sed 's/\t//')" -- "${COMP_WORDS[1]}"))

  if [ "${#suggestions[@]}" == "1" ]; then
    local number="${suggestions[0]/%\ */}"
    COMPREPLY=("$number")
  else
    for i in "${!suggestions[@]}"; do
      suggestions[$i]="$(printf '%*s' "-$COLUMNS"  "${suggestions[$i]}")"
    done

    COMPREPLY=("${suggestions[@]}")
  fi
}

complete -F _dothis_completions dothis

Source and test:

export DOTHIS_COMPLETION_COMMANDS_NUMBER=5
$ dothis <tab><tab>
505 clear
506 source ./dothis-completion.bash
507 dothis clear
508 clear
509 export DOTHIS_COMPLETION_COMMANDS_NUMBER=5

Useful links

Based on an original by Lazarus Lazaridis. Reposted with permission. Edited for style and conciseness.