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bashloopsbackground
WestCoastProjects
WestCoastProjects

Reputation: 63062

How to background tasks within a bash loop

Given the following bash loop:

for ((x=1; x<=$y; x++)); do echo $x; done
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How to "background" the individual tasks?

09:25:58/~ $for ((x=1; x<=$y; x++)); do echo $xi &; done
-sh: syntax error near unexpected token `;'

I tried "bash"ing the echo and that did not work either:

09:26:37/~ $for ((x=1; x<=$y; x++)); do bash -c "echo $x" &; done
-sh: syntax error near unexpected token `;'

Upvotes: 8

Views: 3842

Answers (3)

Etan Reisner
Etan Reisner

Reputation: 80931

Both & and ; are command terminators in the shell.

You only need to terminate each command once. So don't use both together:

for ((x=1; x<=$y; x++)); do echo $x & done

You would get the same error by using two ;s as well:

$ for ((x=1; x<=$y; x++)); do echo $x ; ; done
-bash: syntax error near unexpected token `;'

Note that trying to use ;; gets a different error because ;; is a special token to the shell (used in case statements):

-bash: syntax error near unexpected token `;;'

Shell grammar:

%start  complete_command
%%
complete_command : list separator
                 | list
                 ;
list             : list separator_op and_or
                 |                   and_or
                 ;
....
separator_op     : '&'
                 | ';'
                 ;
separator        : separator_op linebreak
                 | newline_list
                 ;

Upvotes: 17

betterworld
betterworld

Reputation: 254

This will work:

for ((x=1; x<=y; x++)); do echo $x & done

Note that there is no ; after &. However using line breaks will make it more readable:

for ((x=1; x<=y; x++)); do
  echo $x &
done

Upvotes: 2

jbm
jbm

Reputation: 3163

for ((x=1; x<=$y; x++)); do { echo $x & } ; done

Upvotes: 1

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