Writing your own prompt

Fish ships a number of prompts that you can view with the fish_config command, and many users have shared their prompts online.

However, you can also write your own, or adjust an existing prompt. This is a good way to get used to fish’s scripting language.

Unlike other shells, fish’s prompt is built by running a function - fish_prompt. Or, more specifically, three functions:

These functions are run, and whatever they print is displayed as the prompt (minus one trailing newline).

Here, we will just be writing a simple fish_prompt.

Our first prompt

Let’s look at a very simple example:

function fish_prompt
    echo $PWD '>'
end

This prints the current working directory (PWD) and a > symbol to show where the prompt ends. The > is quoted because otherwise it would signify a redirection.

Because we’ve used echo, it adds spaces between the two so it ends up looking like (assuming _ is your cursor):

/home/tutorial >_

Formatting

echo adds spaces between its arguments. If you don’t want those, you can use string join like this:

function fish_prompt
    string join '' -- $PWD '>'
end

The -- indicates to string that no options can come after it, in case we extend this with something that can start with a -.

There are other ways to remove the space, including echo -s and printf.

Adding colo(u)r

This prompt is functional, but a bit boring. We could add some color.

Fortunately, fish offers the set_color command, so you can do:

echo (set_color red)foo

set_color can also handle RGB colors like set_color 23b455, and other formatting options including bold and italics.

So, taking our previous prompt and adding some color:

function fish_prompt
    string join '' -- (set_color green) $PWD (set_color normal) '>'
end

A “normal” color tells the terminal to go back to its normal formatting options.

What set_color does internally is to print an escape sequence that tells the terminal to change color. So if you see something like:

echo \e\[31mfoo

that could just be set_color red.

Shortening the working directory

This is fine, but our PWD can be a bit long, and we are typically only interested in the last few directories. We can shorten this with the prompt_pwd helper that will give us a shortened working directory:

function fish_prompt
    string join '' -- (set_color green) (prompt_pwd) (set_color normal) '>'
end

prompt_pwd takes options to control how much to shorten. For instance, if we want to display the last two directories, we’d use prompt_pwd --full-length-dirs 2:

function fish_prompt
    string join '' -- (set_color green) (prompt_pwd --full-length-dirs 2) (set_color normal) '>'
end

With a current directory of “/home/tutorial/Music/Lena Raine/Oneknowing”, this would print

~/M/Lena Raine/Oneknowing>_

Status

One important bit of information that every command returns is the status. This is a whole number from 0 to 255, and usually it is used as an error code - 0 if the command returned successfully, or a number from 1 to 255 if not.

It’s useful to display this in your prompt, but showing it when it’s 0 seems kind of wasteful.

First of all, since every command (except for set) changes the status, you need to store it for later use as the first thing in your prompt. Use a local variable so it will be confined to your prompt function:

set -l last_status $status

And after that, you can set a string if it not zero:

# Prompt status only if it's not 0
set -l stat
if test $last_status -ne 0
    set stat (set_color red)"[$last_status]"(set_color normal)
end

And to print it, we add it to our string join:

string join '' -- (set_color green) (prompt_pwd) (set_color normal) $stat '>'

If $last_status was 0, $stat is empty, and so it will simply disappear.

So our entire prompt is now:

function fish_prompt
    set -l last_status $status
    # Prompt status only if it's not 0
    set -l stat
    if test $last_status -ne 0
        set stat (set_color red)"[$last_status]"(set_color normal)
    end

    string join '' -- (set_color green) (prompt_pwd) (set_color normal) $stat '>'
end

And it looks like:

~/M/L/Oneknowing[1]>_

after we run false (which returns 1).

Where to go from here?

We have now built a simple but working and usable prompt, but of course more can be done.

  • Fish offers more helper functions: - prompt_login to describe the user/hostname/container or prompt_hostname to describe just the host - fish_is_root_user to help with changing the symbol for root. - fish_vcs_prompt to show version control information (or fish_git_prompt / fish_hg_prompt / fish_svn_prompt to limit it to specific systems)

  • You can add a right prompt by changing fish_right_prompt or a vi-mode prompt by changing fish_mode_prompt.

  • Some prompts have interesting or advanced features - Add the time when the prompt was printed - Show various integrations like python’s venv - Color the parts differently.

You can look at fish’s sample prompts for inspiration. Open up fish_config, find one you like and pick it. For example:

fish_config prompt show # <- shows all the sample prompts
fish_config prompt choose disco # <- this picks the "disco" prompt for this session
funced fish_prompt # <- opens fish_prompt in your editor, and reloads it once the editor exits