Different results when running in $() or not in it

Question

This is one of the weirdest things I've come across.

➜  ~ cat word.txt
mere doctrine
➜  ~ tr ' ' '\n' < ~/word.txt
mere
doctrine
➜  ~ echo $(tr ' ' '\n' < ~/word.txt)
mere doctrine

I don't understand what is the difference between executing inside $() environment and executing directly.

I am using zsh on manjaro linux.

Answer 1

When you write, for example, echo $(ls), word splitting occurs.

The best practice is to add double quotes to prevent word splitting:

➜  ~ echo "$(tr ' ' '\n' < word.txt)"
mere
doctrine