Why doesn't set -x work within eval within a function in fish?

Question

I ran into an issue trying to dynamically set some variables based on output of a program, in a fish function.

I narrowed my issues down to a MWE:

function example
    eval (echo 'set -x FOO 1;')
end

calling:

>example
>echo $FOO

results in no output -- ie the FOO environment variable has not been set. How should I have done this?

Answer 1

The same thing happens in a simpler form:

function trythis
     set -x foo bar
end

If you now run trythis and echo $foo, it is not set either. That's because fish's -x by itself doesn't change the scope of the variable, which is by default local to the function unless it exists globally or universally already.

Try:

  eval (echo 'set -gx FOO 1;') 

Where the g is for global. This makes the variable work like a normal POSIX exported value. It's interesting that it works the same way with eval as it would with just plain set; if you use that line sans g straight on the command line, $FOO is set, so eval and process substitution () have not introduced a new scope or subshell, and when executed that way within a function, the scope of the function applies.

Answer 2

You will need to give a scope to your set command. Without it, the default scope is local to the function. See the fish manual section on Environment Variables for the default rules for variable scoping

Try set -gx FOO.