Nested quotes in subshells

Question

Say I have to use quotes to encapsulate subshell output like:

DATA="$(cat file.hex | xxd -r)"

But I need to nest this kind of stuff like:

DATA="$(cat file.hex | xxd -r | tr -d \"$(cat trim.txt)\")"

I can't use single quotes because those do not expand variables that are inside of them. Escaping quotes doesn't work because they are just treated as passive text.

How do I handle this?

Solution 1

You don't need to escape the quotes inside a subshell, since the current shell doesn't interpret them (it doesn't interpret anything from $( to ), actually), and the subshell doesn't know about any quotes that are above.

Quoting a subshell at variable assignment is unnecessary too, for more info see man bash.

Solution 2

You don't need to escape the nested quotes inside. They get parsed properly, surprisingly!

DATA="$(cat file.hex | xxd -r | tr -d "$(cat trim.txt)")"

Comments

[cuonglm]

Why don't you use DATA="$(cat file.hex | xxd -r | tr -d "$(cat trim.txt)")"?

[LPCRoy]

It depends on what you're trying to do, but it's usually a best practice. See github.com/koalaman/shellcheck/wiki/Sc2086

[jmrah]

Why is quoting a subshell at variable assignment unnecessary?

[Pedro A]

So if I want to have a ) in the subcommand, should I escape the )?

[palswim]

My man bash says "If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results." (in EXPANSION > Command Substitution). That seems to imply that we would need to quote subshell invocations.