Cookbooks
Fish Shell
Chapter 9. Testing and Debugging
Printing debug output

Fish Shell:
Printing debug output

How to:

Get cozy with echo - the Swiss Army knife for output in Fish. Here’s how to sprinkle some debug prints into your shell scripts.

function greet
    set name $argv[1]
    echo "Hey, $name! Let's debug."
    echo "Running the greet function" >&2
end

greet "Ada"

Sample output:

Hey, Ada! Let's debug.
Running the greet function

Standard out (stdout) is your script’s main stage, but for debug chatter, use standard error (stderr) with >&2.

Deep Dive

Back when monitors were as deep as they were wide, output was precious. Standard out (stdout) became the pure, user-facing channel, while standard error (stderr) turned into the back-alley for programmer-only gossip, like debug info.

In Fish, standard commands for output are echo, printf, and print. The echo is straightforward and mostly used for simple messages and inline debug.

You’re not stuck with just echo, though. Prefer printf for formatted strings, or use redirection (> or >>) to dump debug info into a file for later.

As for implementation, using stderr for debug output is a convention from the Unix world, helping to separate the wheat (actual output) from the chaff (debug noise). This means users can still pipe your script’s real output without getting debug garble mixed in.

See Also

Using an interactive shell (REPL)