How does the colon sign and dollar question mark combine in sh?

Question

The "colon sign" and the "dollar question mark" can be combined to check if a sh script does have an argument and assigns it directly to a variable of interest:

cmd=${1:? "Usage: $0 {build|upload|log}"}

Can someone explain step for step how this works, and where I can find the details on its implementation? For example I would like to be able to call a function instead of printing to stderr.

help() {
  echo $"Usage: $0 {build|upload|log}"
  exit 1
}
cmd=${1:? help}

Why is this not possible?

Answer 1

This is covered in Variable expansion:

${parameter:?word}

If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

Therefore if there is no argument with the execution of the script, then the variable cmd becomes help and is written to standard out and back to the shell.

However, it is just read in and not executed. If you surround it with backticks then it will be executed and run the Usage funtion:

help() {
  echo "Usage: $0 {build|upload|log}"
}
cmd=${1:? `help`}

Unfortunately, this will still present stderr, as the variable expansion is designed to.

You could alternatively, do the following:

cmd=${1:-help}
$cmd