Creating functions in Fish

2 minutes read

Creating functions in Fish

Variable declarations

Variables in Fish might be a bit counter intuitive for some one with a programming background. It is done with the set buildin. The plain set galaxy_destroyers does one of two things depending on the global state of the shell.

  1. If there is already a global variable with the name galaxy_destroyers invokation of set galaxy_destroyers 42 changes that variable to 42.
  2. If there is no such variable invokation creates a local variable with said value.

To avoid this ambiguity it is wise to use set with the -l flag in your functions. This forces the variable to be local (with a twist…).

When following this best practice you’d might be temped to do:

function galaxy_destroyer
   if test 1 -eq 1 # always true
     set -l destroyer 234
   end
   echo $destroyer
 end

$ sheep
> 234

This works!

However:

function last_destroyer
  set -l destroyers dalek cylon borg
  for i in $destroyers
    set -l last $i
  end
  echo $last
end

$ herd
>

This returns nothing! That is because the if clause does not introduce a new scope but the for clause does. Using the -l flag makes the variable local to the scope.

To avoid these subtle errors you often see the variables declared in a function like so:

function herd
  set -l destroyers
  ...
end

This concludes part 1. In the next part I’ll elucidate return types in Fish.