Command substitution, assignment, tests and spaces

Question

I know the whole language is confusing, but I am having trouble understanding assignments with command substitutions:

x=Example sentence                  #1 fails due to space
x='Example sentence'                #2 ok 
echo $x                             #3 ok
[ -n $(echo $x) ]  && echo foo      #4 fails due to space
[ -n "$(echo $x)" ]  && echo foo    #5 ok
y=$x                                #6 ok
y=$(echo $x)                        #7 ok, not like 1?
[ -n $y ]                           #8 fails due to space

With practice, I have learned that command substitutions in tests should always be surrounded by quotes (e.g. [ -n "$(cmd)" ]), and that quotes can be used inside the substitution without interfering with outside quotes. Quoting a substitution seems to capture stdout, but not stderr (unless redirected).

Q1. But what happens when the substitution is not quoted (cases 4 and 7)?

Specifically, I am confused with the apparent difference in behaviour between assignments x=$(cmd), which seem to work fine with spaces, versus tests [ -n $(cmd) ], which seem to always be truthy.

Q2. Is it bad practice to assign command substitutions without quotes x=$(cmd)? Should I always quote them x="$(cmd)"?

This is very well explained at When is double-quoting necessary? — Inian, Jul 18 '18 at 13:15
@Inian Thanks for the link, and sorry for the redundant question. Please feel free to vote to close if you consider this to be a duplicate. — Jonathan H, Jul 18 '18 at 13:17
TL;DR Quotes generally prevent split+glob expansions. Variable assignment is a special case where quotes are not needed. Tests with double brackets [[ ]] is another special case, but single brackets [ ] require quotes. ---- Q1: assignments are just special. Q2: it's fine not to quote. — Jonathan H, Jul 18 '18 at 13:24

Command substitution, assignment, tests and spaces

0 Answers0