I'm trying to construct a command dynamically, but I can't figure out how to deal with quoted multi-word arguments.
Some people attempt to do things like this:
# Non-working example
args="-s 'The subject' $address"
mail $args < $bodyThis fails because of word-splitting. When $args is evaluated, it becomes four words: 'The is the second word, and subject' is the third word.
What's needed is a way to maintain each word as a separate item, even if that word contains multiple spaces. Quotes won't do it, but an [#faq5 array] will.
# Working example
args=(-s "The subject" "$address")
mail "${args[@]}" < $bodyOften, this question arises when someone is trying to use dialog to construct a menu on the fly. For an example of how to do this properly, see [#faq40 FAQ #40].
Another reason people attempt to do this is because they want to echo "I am going to run this command: $command" before they actually run it. If that's all you want, then simply use the set -x command, or invoke your script with #!/bin/bash -x or bash -x ./myscript. Note that you can turn it off and back on inside the script with set +x and set -x.
It's worth noting that you cannot put a pipeline command into an array variable and then execute it using the "${array[@]}" technique. The only way to store a pipeline in a variable would be to add (carefully!) a layer of quotes if necessary, store it in a string variable, and then use eval or sh to run the variable. This is [#faq48 not recommended], for security reasons.