Variable with colon dash ${VAR:-}

Question

How does "${2:-}" work in an 'if-then-else example' below? Somehow I cannot help but think that being ${2:-} it means the second argument, but I am curious what the colon(:) and dash(-) after the digit 2 mean?

  if [ "${2:-}" = "Y" ]; then
     prompt="Y/n"
     default=Y
  elif [ "${2:-}" = "N" ]; then
     prompt="y/N"
     default=N
  else
     prompt="y/n"
     default=
  fi

  read -p "$1 [$prompt] " REPLY </dev/tty

DopeGhoti · Answer 1 · 2018-07-30T14:59:57.147

16

The syntax ${VAR:-default} evaluates to the value of VAR or, if it is unset or null, it evaluates to the text after the hyphen (in this case, default); the syntax ${VAR- default} is similar shortened of the a similar function only for when the variable is unset. $2 is a positional parameter, so your statement is testing the value of the second argument, and if it is not set, using an empty value as a default.

Why use an empty default, since that would have the same effect as a plain $2? Because under set -u (equivalent to set -o nounset), substituting an unset variable causes an error: if there are less than 2 parameters, $2 errors out. But ${2:-} won't error out, because it explicitly substitutes the empty string if the parameter is unset or null.

edited Jul 30 '18 at 14:59

answered May 12 '16 at 16:16

DopeGhoti

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[bash reference](https://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion) – Jeff Schaller May 12 '16 at 16:59
1

which begs the question, how `${2:-}` differs from `${2}` which also has an empty expansion if it's empty or unset. – Gilles 'SO- stop being evil' May 12 '16 at 22:24
2

I suppose it depends on whether the context of this section of the script has `set -u` in place (: – DopeGhoti May 12 '16 at 22:30
It has a 'set -o nounset' at the very beginning of the script, would that be the same as 'set -u'? – nocturnalmonkey May 13 '16 at 15:20
Yes, the latter is shorthand for the former. – DopeGhoti Jul 30 '18 at 15:00

Variable with colon dash ${VAR:-}

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