What does a hyphen do next to the argument position in bash shell script? Like ${1-}

Question

I've come across this piece of code from here:

#!/bin/bash

# ...

if [[ "${1-}" =~ ^-*h(elp)?$ ]]; then
    echo "Usage..."
    exit
fi

I understand what this does is print the usage information when any argument like -h, --help, -help, -------help is passed.

But why "${1-}" and not just "${1}"? What does the hyphen do?

At first I thought it could mean any argument from the position 1 onward. I tried it and no, it wasn't that.

Then I thought it could be a typo and the author meant "${1-9}" to allow all number positions. Again, it didn't work, it isn't even valid code.

Answer 1

A hyphen in a parameter expansion allows a default value to be specified. So ${1-} means “the value of the first parameter if it is set, and the empty string otherwise”.

That doesn’t seem particularly useful as-is, because an unset variable expands to the empty string anyway. However, ${1} will cause an error if set -u is enabled and no parameter is given, whereas ${1-} won’t — so some shell programmers always use this form when referring to variables which may legitimately be unset.

In the context of the site you found this on, this fits in with “thing” number 5:

5. Prefer to use set -o nounset. You may have a good excuse to not do this, but, my opinion, it’s best to always set it.

   • This will make the script fail, when accessing an unset variable. Saves from horrible unintended consequences, with typos in variable names.
   • When you want to access a variable that may or may not have been set, use "${VARNAME-}" instead of "$VARNAME", and you’re good.