"${1-}" vs "$1"

Question

The code for git bash completion, specifically the function __gitcomp, uses parameter expansions like "${1-}". This appears to be similar to "$1". What is the difference?

Also: where is this documented in the bash manual?

Answer 1

First, recall that ${foo-bar} expands to the value of foo, like $foo or ${foo}, except that if foo is unset, ${foo-bar} expands to bar ($foo expands to the empty string if foo is unset). There is a more often-used variant of this syntax, ${foo:-bar}, which expands to bar if foo is unset or empty. (This is explained in the manual if you look closely enough: search for :-, and note the sentence “Omitting the colon results in a test only for a parameter that is unset.” above.)

For a positional parameter $1, ${1-bar} expands to bar if $1 is unset, that is, if the number of positional parameters is less than 1. Unless the positional parameters have been changed with set or shift, this means that the current function, or if not applicable the current script, has no parameter.

Now when bar is empty, ${1-} looks like a useless complication: the expansion is that of $1, except that when $1 is unset, the expansion is empty, which it would be anyway. The point of using ${1-} is that under set -u (a.k.a. set -o nounset), a plain $1 would result in an error if the parameter was unset, whereas ${1-} always successfully expands to the empty string if $1 is unset.

Answer 2

echo "${foo-default}"

Prints $foo, if foo is defined, and 'default', if foo is undefined. So I conclude

"${1-}"

is empty, if the first argument to the script is not defined.