Customizing the output of various jj commands with templates

Just like revsets are a functional language that allow you to describe a set of commits, templates are a typed functional language that allows you to customize the output of commands in jj.

A bunch of commands support -T or --template to allow you to customize their output. For example, right now @ is on an empty commit, so let's refresh ourselves on what

$ jj log -r @-
◉  yzlysuwt steve@steveklabnik.com 2024-02-29 00:35:12.000 -06:00 main f80a73c1
│  Fill out a table of contents

Let's give it a template instead:

$  jj log -r @- -T 'separate(" ", change_id, description.first_line())'
◉  yzlysuwtylswszxknppsyqxqoktqpqpz Fill out a table of contents
│

That's quite different! We've got a few things going on here:

• separate() is a function that takes a separator, a bunch of other templates, and produces the contents of those templates separated by the separator.
• change_id and description are keywords that resolve to what you'd expect.
• first_line() is a method on strings that returns the first line.

What if we wanted a nicer change ID? We can do that:

$  jj log -r @- -T 'separate(" ", change_id.shortest(8), description.first_line())'
◉  yzlysuwt Fill out a table of contents
│

The shortest method returns the shortest prefix of an ID, and the 8 we pass to it says "show at least eight letters even if the shortest is shorter."

Templates are powerful, and let you do a lot of interesting things. I would suggest reading the documentation on templates to learn all of the details.