Why does ZSH local variable assignment fail?

Question

I was reading this question and answer: zsh - variable assignment isn't working. The post explains how to fix it, but not why. Why does the local variable assignment fail in that case ?

There are also some other examples of similar behaviour like this:

myfun()
{
local x=$(echo "a b"); echo ${x} 
}

myfun2()
{
local x 
x=$(echo "a b")
echo ${x}
}

myfun3()
{
local x=$(echo "a b")
echo ${x}
}

This results in the following output:

a
a b
a

If I put quotes around the 'x="$(echo "a b")" in all cases I get the correct variable assignment.

If I run the same thing in bash, I get 'a b' for all functions.

Can someone explain why bash and zsh are acting differently here ?

Answer 1

If you run your script with -x (both bash and zsh) you will see that the $(...) code gets interpreted/parsed differently. This is because, as stated in the FAQ, word splitting in zsh is backwards/bash/sh incompatible.

You can use SH_WORD_SPLIT to address this difference.

Ok, so it is not a word split problem as we have a command substitution, quoting the docs again:

...If the substitution is not enclosed in double quotes, the output is broken into words using the IFS parameter...

So your alternative is to local IFS="" :/