Why is there a number in the zsh parameter expansion ${1-$PWD}

Question

I have this script I'm basing my current script off of. I just don't understand why he has typeset result part dir=${1-$PWD} in there.

I get the same result if I just write dir=$PWD. With typeset is ${1-$PWD} changing how dir is set vs $PWD?

There's a question for every possible variation of this particular form of parameter expansion... What does this line in bash do? Parameter-||scriptname, or Bourne shell: trailing - operator in parameter substitution, or Using “${a:-b}” for variable assignment in scripts etc... — don_crissti, Oct 27 '17 at 20:53
I'll just link the relevant section of the standard here, too. There's a nice table describing the ${var*word} expansions (for different values of *): http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02 — ilkkachu, Oct 28 '17 at 09:13

score 8 · Answer 1 · answered Oct 27 '17 at 16:22

That's not brace expansion, that's a standard parameter expansion operator (dates back to the Bourne shell in the 70s).

${1-$PWD}

Expands to the value of $1 (the first positional parameter) if it is set (if $# is strictly greater than 0) even to the empty string, or to the content of the $PWD variable otherwise.

Run:

info zsh 'Parameter Expansion'

for details.

typeset is not Bourne nor POSIX, but it's not zsh-specific either. It comes from the Korn shell (from the early 80s) and is used to limit the scope of a variable to the current function. It's also found in bash and yash.

Run:

info zsh typeset

for details.

score 6 · Accepted Answer · edited Oct 27 '17 at 16:30

${1-$PWD} is a shell parameter expansion pattern.

It is used to expand to a default value based on another -- whatever on the right of -. Here, in your case:

If $1 is unset, then the expansion of $PWD would be substituted
Otherwise i.e. if $1 is set to any value (including null), its value would be used as the result of expansion

Example:

% echo "Foo${1-$PWD}"   
Foo/home/bar

% set -- Spam

% echo "Foo${1-$PWD}"
FooSpam

score 4 · Answer 3 · answered Oct 27 '17 at 16:08

4

It tests $1 for a value, using that before $PWD.

answered Oct 27 '17 at 16:08

Ignacio Vazquez-Abrams

45,725

Why is there a number in the zsh parameter expansion ${1-$PWD}

3 Answers3