Cookbooks
Fish Shell
Chapter 9. Testing and Debugging
Using a debugger

Fish Shell:
Using a debugger

How to:

Fish doesn’t have a built-in debugger like some other shells, but you can use external tools like gdb for debugging compiled programs or fish -d for running fish with debug output at different levels. Let’s roll with fish -d:

# Run fish shell with debug level 2
fish -d2

# In the fish shell, let's test a simple function with a potential bug
function test_func
    set val 42
    echo "The value is $val"
    if test $val -eq 42
        echo "All is well."
    else
        echo "Something's fishy."
    end
end

# Call the function and observe the debug output
test_func

You’d see extra debug output before and after the function executes, helping you pinpoint issues.

Deep Dive

Historically, debugging in Unix-like environments has been a province of specialized tools like gdb for C/C++ or pdb for Python. In Fish, you’re usually reliant on external utilities or built-in features like functions -v for verbose output of functions and set -x to track variable changes.

Some folks choose alternative shells like Bash because of features like set -x for debugging scripts. However, Fish has its charm with a focus on user-friendliness and interactivity, which can reduce the need for hardcore debugging in many cases.

When it comes to implementation, debugging a script often involves running it with verbose output and tracing down where variables get set, unset, or mutated in unexpected ways. With Fish’s color-coded output and user-friendly approach, you can often avoid the nitty-gritty of debugging – but when you’re stuck, remember that verbosity and clarity are your best tools.

See Also

Here are some trusty lifelines for when you’re up to your fins in code:

Using an interactive shell (REPL)Writing tests