fish-shell/doc_src/prompt.rst

Writing your own prompt
=======================

.. only:: builder_man

   .. warning::
      This document uses formatting to show what a prompt would look like. If you are viewing this in the man page,
      you probably want to switch to looking at the html version instead. Run ``help custom-prompt`` to view it in a web browser.

Fish ships a number of prompts that you can view with the :doc:`fish_config <cmds/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 :doc:`scripting language <language>`.

Unlike other shells, fish's prompt is built by running a function - :doc:`fish_prompt <cmds/fish_prompt>`. Or, more specifically, three functions:

- :doc:`fish_prompt <cmds/fish_prompt>`, which is the main prompt function
- :doc:`fish_right_prompt <cmds/fish_right_prompt>`, which is shown on the right side of the terminal.
- :doc:`fish_mode_prompt <cmds/fish_mode_prompt>`, which is shown if :ref:`vi mode <vi-mode>` is used.

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 (:envvar:`PWD`) and a ``>`` symbol to show where the prompt ends. The ``>`` is :ref:`quoted <quotes>` because otherwise it would signify a :ref:`redirection <redirects>`.

Because we've used :doc:`echo <cmds/echo>`, it adds spaces between the two so it ends up looking like (assuming ``_`` is your cursor):

.. role:: white
.. parsed-literal::
    :class: highlight

    :white:`/home/tutorial >`\ _

Formatting
----------

``echo`` adds spaces between its arguments. If you don't want those, you can use :doc:`string join <cmds/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 :doc:`printf <cmds/printf>`.

Adding color
------------

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

Fortunately, fish offers the :doc:`set_color <cmds/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.

``set_color`` works by producing an escape sequence, which is a special piece of text that terminals
interpret as instructions - for example, to change color. So ``set_color red`` produces the same
effect as::

  echo \e\[31m

Although you can write your own escape sequences by hand, it's much easier to use ``set_color``.

Shortening the working directory
--------------------------------

This is fine, but our :envvar:`PWD` can be a bit long, and we are typically only interested in the last few directories. We can shorten this with the :doc:`prompt_pwd <cmds/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

.. role:: green
.. parsed-literal::
    :class: highlight

    :green:`~/M/Lena Raine/Oneknowing`>_

Status
------

One important bit of information that every command returns is the :ref:`status <variables-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 :doc:`set <cmds/set>`) changes the status, you need to store it for later use as the first thing in your prompt. Use a :ref:`local variable <variables-scope>` so it will be confined to your prompt function::

  set -l last_status $status
  
And after that, you can set a string if it is 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:

.. role:: green
.. role:: red
.. parsed-literal::
    :class: highlight

    :green:`~/M/L/Oneknowing`\ :red:`[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 :doc:`fish_right_prompt <cmds/fish_right_prompt>` or a vi mode prompt by changing :doc:`fish_mode_prompt <cmds/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 :doc:`fish_config <cmds/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