Bash parameter substitution with multi line error message

Question

I'm using parameter substitution with an error message, such as ${var1:?'some message'}. I've incorporated a multi-line error message. Presently, it only functions properly when enclosed within single quotes, with line breaks inserted using the Enter key. Is there a clever way to allow acceptance of multiple lines, saved separately?

I'm just curious about exploring this. Please avoid suggesting alternative syntax involving if statements unless it's directly relevant.

Cases:

works: single quote

needed=${1:?'first error line
second error line
third error line'}

not working: call another string variable

usage_message='Please use this script by providing the following arguments:
1: <e.g user name>
2: <e.g run script>
3: <e.g something else>'
username=${1:?$usage_message}
run_script_path=${2:?$usage_message}
where_to_save=${3:?$usage_message}

not working: call a function

function get_message {
       echo -e "first line \nsecond line\nthird line"
       # or
       # printf "first line \nsecond line\nthird line"
}
needed=${1:? $(get_message)}

Other discussion about parameter substitution: https://stackoverflow.com/a/77772942/13413319

Answer 1

Although the POSIX standard for parameter expansion is not clear on the matter, and the RHS of an assignment is one of the few places in which bash parameter expansion does not undergo word splitting in general¹, it appears that you need to double quote the multiline variable expansion to prevent word splitting² in this context. So whereas

(unquoted)

$ username=${1:?$usage_message}
bash: 1: Please use this script by providing the following arguments: 1: <e.g user name> 2: <e.g run script> 3: <e.g something else>

(quoted)

$ username=${1:?"$usage_message"}
bash: 1: Please use this script by providing the following arguments:
1: <e.g user name>
2: <e.g run script>
3: <e.g something else>

See also When is double-quoting necessary?.

---

1. for example

    $ var=foobar
    $ username=${var/bar/$usage_message}
    $ declare -p username
    declare -- username="fooPlease use this script by providing the following arguments:
    1: <e.g user name>
    2: <e.g run script>
    3: <e.g something else>"
   

2. although apparently not globbing, for example

    $ word=*
    $ username=${1:?$word}
    bash: 1: *