Execute command multiple times with curly brackets arguments list

Question

When I needed to run a command multiple times with a different argument I used this approach (without understanding it fully):

touch {a,b,c}

Which is equivalent of:

touch a
touch b
touch c

I think the same could be accomplished with the following loop:

for file in {a,b,c}; do touch $file; done

However I've stumbled across a case where this does not work:

pear channel-discover {"pear.phpunit.de","pear.symfony-project.com"}

I have a few questions:

What is the name of what is happening in the first example and what exactly is happening?
Is it better to use this approach for simple things rather than for-in loops?
Why the pear command is not working like that? Should the command script implement some techniques to handle such arguments or is it the shell responsible for that?

score 14 · Accepted Answer · answered Dec 20 '12 at 12:19

14

That's called Brace Expansion, which expands to a space-separated list of the given strings.

So touch {a,b,c} would be equivalent to

touch a b c

While touch {a,b,c}x would be equivalent to:

touch ax bx cx

You pear command would essentially be run as:

pear channel-discover pear.phpunit.de pear.symfony-project.com

which may not be what you expected. If you want the command to be run once for each string, use a for loop (which answers your 2nd question), or use a combination of brace expansion and xargs.

answered Dec 20 '12 at 12:19

Shawn Chin

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@Shawin Chin, you are right. I was expecting multiple executions of the command and not just expanding the arguments list. The link you've provided for Brace Expansion is very helpful. – Haralan Dobrev Dec 20 '12 at 12:21
1

I made the same mistake before, so I'm glad I could help you out. – Shawn Chin Dec 20 '12 at 12:23

score 6 · Answer 2 · answered Dec 20 '12 at 12:19

6

The problem is that contrary to your expectation, the brace expansion of

touch {a,b,c}

is equivalent to

touch a b c   # NOT 3 separate invocations.

(Use echo {a,b,c} to verify). It appears that pear channel-discover does not accept two args. You will probably see the same error with

pear channel-discover pear.phpunit.de pear.symfony-project.com

answered Dec 20 '12 at 12:19

Jens

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is there a bash notation that does perform multiple invocations? – Chris Snow Jan 01 '14 at 12:03
1

No; that "notation" would be a loop like `for i in a b c; do ...; done` – Jens Jan 01 '14 at 16:26

score 0 · Answer 3 · edited Mar 06 '15 at 18:22

0

Well, you have two options:

for i in "pear.phpunit.de" "pear.symfony-project.com"
do
  pear channel-discover "$i"
done

Or a one-liner (but calling xargs instead of using bash internals):

echo "pear.phpunit.de" "pear.symfony-project.com" | xargs -n1 pear channel-discover

The former is certainly easier to read by the human, the time efficiency will be basically the same.

edited Mar 06 '15 at 18:22

Jens

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answered Sep 09 '14 at 14:29

yo'

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Execute command multiple times with curly brackets arguments list

3 Answers3