How I switch colours in Alacritty

published:

categories: open-source

tags: linux, terminal

My current terminal emulator is Alacritty. One thing I would like is a way to change the colours while the terminal is running. The only way to do so is the edit the configuration file, but doing so by hand can be annoying. There are some scripts out there that let you do it automatically, but all that I have found rely on bloated stuff like having Node.js installed. We can do better by using just what we already have on Unix out of the box.

The end goal is a script alacritty-theme which will present a menu of predefined themes and apply the selection. We need the following prerequisite knowledge and tools:

  • Unix shell scripting

  • Some selection program like fzf, dmenu or rofi

  • ed, the original Unix text editor

That's right, where we're going we won't need any fancy GUIs and TUIs. It's 1970s teletypes all over again!

The plan

Alacritty is configured through a YAML file named alacritty.yml. YAML by itself has no “include” mechanism that would allow us to splice in the content of another YAML file, but Alacritty implements such feature through the import key. The value is a list of file paths to files whose contents will be read in.

This lets us move the theme into a separate file like theme/solarized-dark.yml which we then reference in the main configuration file. Here is an example configuration file:

import:
  - '~/.config/alacritty/themes/solarized-dark.yaml'

window:
  padding:
    x: 1
    y: 1
  dynamic_padding: true
  opacity: 0.9

font:
  size: 9
  normal:
    family: monospace

The plan is as follows:

  • Find that line

  • Replace the file name portion with the name of theme

  • Save the changes

The execution

Editing the config file

We will start with the hardest and most important part: applying the theme. Let us for now pretend that we already have the name of the theme. We have to execute the following steps:

  1. Open the config file

  2. Find the line which sets the theme

  3. Substitute the name of the theme

  4. Save the changes

First we have to open the file. We use ed as our editor, so we give the file name as the argument to the command.

ed alacritty.yml

This will put us in the interactive ed prompt. Ed is a command-based text editor, we type an instruction on what to do and ed carries it out. It is like editing a text file blindly, which is perfect for later when we want to automate the entire process.

Now let us find the line of interest. To find a line we can use the / command followed by a regular expression.

/^  - '.*\/alacritty\/themes\/.*[.]yaml'$

Note that we are including the leading whitespace and the dash as well. We also assume that there are no trailing comments or trailing whitespace. This reduces the likelihood of a false positive match. Now we have to apply the substitution using the s command.

s/[^/]*[.]yaml'$/solarized-light.yaml'/

The s command's syntax is s/regex/substitute/ where regex is a regular expression to match and substitute is the new text. The separator / can be any other character in case we want to use / inside the regular expression.

All that is left now is to save the changes and exit the editor using the wq command. Insert joke about exiting ed here.

wq

We can automate the entire process. Ed reads commands from the standard input, so we provide them line by line as a here-document.

# The $theme variable holds the name of the theme
ed alacritty.yml <<- EOF
    /  - '.*\/alacritty\/themes\/.*[.]yaml'
    s/[^/]*[.]yaml'$/${theme}.yaml'/
    wq
EOF

Selecting a theme

All themes are YAML files inside the themes directory of the Alacritty configuration directory. Here is how to list them:

# The configuration directory depends on the environment
config_dir="${XDG_CONFIG_HOME:-${HOME}/.config}/alacritty"

# Get all files, pick only those with 'yaml' as extension, strip extension
ls ${config_dir}/themes | grep '\.yaml$' | sed 's/\.yaml$//'

This is slightly fragile because a file name might contain newline characters, so please don't do that. At this point we have many options for how to select a theme. I like to use a fuzzy finder, so I will pipe the list into rofi and store the result in a variable.

theme=$(ls ${config_dir}/themes | grep '\.yaml$' | sed 's/\.yaml$//' | rofi -dmenu)

Putting it all together

Here is a complete shell script:

#!/bin/sh
set -e

config_dir="${XDG_CONFIG_HOME:-${HOME}/.config}/alacritty"
theme=$(ls ${config_dir}/themes | grep '\.yaml$' | sed 's/\.yaml$//' | rofi -dmenu)
ed  "${config_dir}/alacritty.yml" <<- EOF
    /  - '.*\/alacritty\/themes\/.*[.]yaml'
    s/[^/]*[.]yaml'$/${theme}.yaml'/
    wq
EOF

Here are a couple of ideas for improvement:

  • If the script is called with an argument use that argument as the name of the theme

  • Display an error message if the theme does not exist and exit with an error

  • Handle the argument -h or --help to display a help message

  • Make the fuzzy finder configurable

  • Fall back to a different selection mechanism if the fuzzy finder is not available

  • Send the standard output of ed to /dev/null to silence the script

Conclusion

A small shell script gets the job done, but there a few issues. For starters, editing a structured file blindly through regular expressions can be fragile. With a proper YAML parser we could understand the structure of the file and make precise edits. It does not really matter in something this simple, but it is worth keeping in mind for the next time we try to automate text editing with ed. In the opening paragraph I called those other scripts bloated and I stand by what I said, but only because this particular case does not warrant the effort.

A better solution

The other issue is that we are editing a configuration file. If the configuration is under version control it will now be marked as edited. There is nothing that can be done, this is simply a limitation of Alacritty. The closest to a solution is to have an extra file theme.yaml in-between and have version control ignore it. The config file first imports the default theme and then imports the theme.yaml file:

import:
  - '~/.config/alacritty/themes/solarized-dark.yaml'
  - '~/.config/alacritty/theme.yaml'

The theme.yaml file then imports the actual theme we want to use:

import:
  - '~/.config/alacritty/themes/solarized-light.yaml'

Alacritty will ignore missing files on import, so if the theme file does not exist nothing happens. Thus instead of editing the main config file we can simply write a new theme file from scratch with the name of theme spliced in. Then all we have to do is use touch to update the modification time of the main config file (without any actual changes) so Alacritty knows to source it again. Alacritty only tracks the main config file, but none of the imported files. Here is the complete script:

#!/bin/sh
set -e

config_dir="${XDG_CONFIG_HOME:-${HOME}/.config}/alacritty"
theme=$(ls ${config_dir}/themes | grep '\.yaml$' | sed 's/\.yaml$//' | rofi -dmenu)

cat << EOF > "${config_dir}/theme.yaml"
import:
  - '${config_dir}/themes/${theme}.yaml'
EOF
touch "${config_dir}/alacritty.yml"

Actually, we did not need ed after all. Oops. Well, I guess we all learned something in the process at least.