By default, when looking for command lines inside the input files, doctest.fish expects all the commands to be preceded by the following prompt: '> ' (greater-than sign followed by a single space).
You are free to use any other prompt identifier in your test files, just make sure you inform it via --prompt when calling doctest.fish.
For example, there's a docs/include/prompt-custom.md example file in this repository that uses prompt$ (followed by a space) as the custom prompt for the command lines. Here's its contents:
prompt$ echo foo
foo
When calling doctest.fish on that file without specifying a custom prompt, no test will be found:
> ./doctest.fish docs/include/prompt-custom.md
docs/include/prompt-custom.md: No tests found
The same happens if you specify a custom prompt that is not found in the test file:
> ./doctest.fish --prompt 404 docs/include/prompt-custom.md
docs/include/prompt-custom.md: No tests found
So just make sure you are informing the correct prompt string in --prompt and the tests will be found:
> ./doctest.fish --prompt 'prompt$ ' docs/include/prompt-custom.md
docs/include/prompt-custom.md: 1 tests PASSED
The trailing space in prompt identifiers is usual, but it is not required.
In docs/include/prompt-no-space.md there's an example of a prompt which is just a single >, with no trailing space.
> ./doctest.fish --prompt '>' docs/include/prompt-no-space.md
docs/include/prompt-no-space.md: 1 tests PASSED
If you to have multiple trailing spaces in your prompt identifier, this is also allowed. The example in docs/include/prompt-extra-spaces.md uses three spaces after >:
> ./doctest.fish --prompt '> ' docs/include/prompt-extra-spaces.md
docs/include/prompt-extra-spaces.md: 1 tests PASSED
Custom prompts are allowed, but you have to have something as a prompt identifier, otherwise doctest.fish cannot find the command lines to be tested.
Trying to inform an empty prompt is an error:
> ./doctest.fish --prompt '' docs/include/prompt-custom.md
doctest.fish: Error: The prompt string cannot be empty, set it via --prompt
Note that when searching for commands to be tested, doctest.fish tries to find a combination of prefix+prompt. Check the --prefix option to set a custom prefix.