BASH: not able to run command in loop as a job

Question

seems I am missing something but I am getting desperate with my script.. from testing I came to conclusion that this simple script is not working:

for i in 1 2; do sleep 2 &; done

When I put "; done" in new line with back slash it is just working fine. Is there any logical explanation why I cannot run job from one line script? Does it mean that "&" expect nothing behind it?

for i in 1 2; do sleep 2 &\
> done
[1]     18862
[2]     18863

Thank you for an advise

Answer 1

You've run into an odd corner case of the shell grammar. What you really want is

for i in 1 2; do sleep 2 & done

with no ; after the &. You can put an extra newline here (as you've discovered), but its not necessary.

As to why you can't have a semicolon here, that's an odd artifact of the way command lists are defined -- they are one or more pipelines separated by the tokens ;, &, &&, or ||. However, within the list, you can't have any empty commands, so when you have & ; with nothing between, that's an empty command in the middle of a list.

Note that you can have empty commands in other places. In addition, you can have one or more newlines after any of the above separators, or instead of a ;. So it ends up seeming like you can have empty commands anywhere, but it turns out there are a few odd places they won't work (and will cause syntax errors)