Dash double semicolon (;;) syntax

Question

I'm trying to find a way to run multiple commands in parallel in sh and wait for it completion.

I've found that following doesn't work (sh: 1: Syntax error: ";" unexpected):

sh -c '(sleep 3 && echo 1) & ; (sleep 3 && echo 2) & ;  wait'

But this syntax works as expected:

sh -c '(sleep 3 && echo 1) & ;; (sleep 3 && echo 2) & ;;  wait'

But I don't understand what is the difference.

What is the meaning of ;; and when it should be used?

neither command line worked here. `;;` is option separator for `case` statement. — Kent, Jun 03 '13 at 20:39
sorry, i was wrong, it actually gives error in bash, but works in dash — valodzka, Jun 03 '13 at 21:04

score 100 · Accepted Answer · answered Jun 03 '13 at 21:01

;; is only used in case constructs, to indicate the end of an alternative. (It's present where you have break in C.)

case $answer in
  yes) echo 'yay!';;
  no) echo 'boo!';;
esac

Syntactically, ; and & both mark the end of a command. A newline is equivalent to ;, in a first approximation. The difference between them is that ; or newline indicates that the command must be executed in the foreground, whereas & indicates that the command must be executed in the background.

So here you need & wait. & ; is a syntax error (you can't have an empty command). & ;; is also a syntax error; ash lets it go (as if you'd written just &), but bash complains. Evidently your sh is some ash variant (such as dash, which is /bin/sh on many Debian derivatives).

Quoting the important bit: Syntactically, `;` and `&` both mark the end of a command. So here you need `& wait`. — Serge Stroobandt, Feb 22 '14 at 11:17

score 9 · Answer 2 · answered Jun 03 '13 at 20:40

9

It should be used in a case statement, between cases. The issue you're having here is that both & and ; are command separators, and you should only be using one of them.

answered Jun 03 '13 at 20:40

Ignacio Vazquez-Abrams

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Thank you for clarification, `sh -c '(sleep 3 && echo 1) & (sleep 3 && echo 2) & wait'` is working as expected. It doesn't explains though what is meaning `;;` in this context. – valodzka Jun 03 '13 at 20:57
Are you sure that `sh` is `bash`? – Ignacio Vazquez-Abrams Jun 03 '13 at 20:57
Sorry, I was wrong, it isn't bash, it [dash](http://gondor.apana.org.au/~herbert/dash/) – valodzka Jun 03 '13 at 21:03

Dash double semicolon (;;) syntax

2 Answers2