Do I need to quote command substitutions?

Question

According to the Google Shell Style Guide, I should:

Always quote strings containing variables, command substitutions, spaces or shell meta characters

Answer 1

When not to quote a command substitution?

Answer: When you want the output of the command substitution to be treated as separate arguments.

Example:

We want to extract specific packets (packet numbers 1, 5, 10 and 20) from a .pcap file. The relevant command is the following:

editcap -r capture.pcap select.pcap 1 5 10 20

(Source to the command)

Now, we want to extract random packets from the file. To do that, we will use shuf from GNU Coreutils.

shuf -i 0-50 -n 4
8
24
20
31

The command above has generated 4 random numbers between 0 and 50.

Using it with editcap:

editcap -r source.pcap select.pcap "$(shuf -i 0-50 -n 4)"
editcap: The specified packet number "1
34
4
38" isn't a decimal number

As you can see, quoting the command substitution has resulted in the output string of the shuf to be treated as a single big string, including the newlines and whatnot as well. That is, putting quotes has resulted in the equivalent of the following command:

editcap -r source.pcap select.pcap "1
34
4
38"

Instead, we want Bash the chop the output of shuf and insert this chopped output as separate arguments to editcap. Omitting the quotes accomplishes that:

editcap -r source.pcap select.pcap $(shuf -i 0-50 -n 4)