Category: bash

I love Bash tab completion, and I want it for the command I am writing, so it can automatically complete parts of the command line when I run my program.

Code

Here is the script (install-bash-completion) I wrote to set it up (no need to be root – it installs in ~/.local):

#!/bin/bash

set -u
set -e

DIR=${BASH_COMPLETION_USER_DIR:-${XDG_DATA_HOME:-$HOME/.local/share}/bash-completion}/completions

mkdir -p ${DIR}

cp _myprogram ${DIR}

The actual completion script (_myprogram) it installs looks like this:

_myprogram_commands()
{
    local cur prev opts
    COMPREPLY=()
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"
    opts=$(bash -c "./myprogram --commands")

    COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
    return 0
}
complete -F _myprogram_commands ./myprogram

Installing

To install it, run:

./install-bash-completion

Then log out and log in again.

Now when you type ./myprogram and press TAB a couple of times, you should see the possible completions listed.

Notes

The completion script must be named to match the program name, with a leading underscore.

If I wanted it to work when it was installed in my PATH, I would need to change ./myprogram to just myprogram in 2 places.

Notice the line opts=$(bash -c "./myprogram --commands") – it actually runs my program to get the list of completions. This means my program needs to accept a --commands option which prints the valid commands. Alternatively, I could have hard-coded it by replacing that line with just:

opts="cmd1 cmd2 --help --etc"

More info:

• Creating a bash completion script by Lazarus Lazaridis
• An introduction to bash completion from Debian Administration

Here’s a little script I wrote to avoid copy-pasting the ffmpeg command from superuser every time I needed it.

It converts a video to a GIF file by pre-calculating a good palette, then using that palette.

Usage:

./to_gif input.mp4 output.gif

The file to_gif (which should be executable):

#!/bin/bash

set -e
set -u

# Credit: https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality

INPUT="$1"
OUTPUT="$2"

PALETTE=$(mktemp --suffix=.png)

ffmpeg -y -i "${INPUT}" -vf palettegen "${PALETTE}"
ffmpeg -y -i "${INPUT}" -i "${PALETTE}" \
    -filter_complex "fps=15,paletteuse" "${OUTPUT}"

rm -f "${PALETTE}"

Note: you might want to modify the number after fps= to adjust how fast the video plays.

Update: changed to use mktemp instead of tempfile.

Update: and here is how I add it to the context menu in Caja:

Place the above file in $HOME/bin/to_gif.

Create a file ~/.config/caja/scripts/to_gif (don’t forget to make it executable):

#!/bin/bash

mate-terminal -x "${HOME}/bin/to_gif" "$1" "$1".gif