Run the same command multiple times in background

Question

How is it possible to run multiple commands and background them using bash?

For example:

$ for i in {1..10}; do wait file$i &; done

where wait is a custom binary.

Right now I get an error:

syntax error near unexpected token `;'

when running the above command.

Once backgrounded the commands should run in parallel.

Am ashuming your not referring to: nohup allowing you to execute a command in the background — Dave Hamilton, Mar 23 '16 at 16:03
The error you're seeing is due to & and ; are both "command terminators". You don't need to use both: for ...; do wait $arg & done will work. — glenn jackman, Mar 23 '16 at 16:07
@glenn jackman. Yes, I actually tried it after posting the question. There is no need for both ; and & — Sebi, Mar 23 '16 at 16:08
For future readers, a link to the documentation: "A list is a sequence of one or more pipelines separated by one of the operators ‘;’, ‘&’, ‘&&’, or ‘||’, and optionally terminated by one of ‘;’, ‘&’, or a newline." (emphasis mine) — glenn jackman, Mar 23 '16 at 16:12

score 34 · Accepted Answer · edited Apr 13 '17 at 12:36

The &, just like ; is a list terminator operator. They have the same syntax and can be used interchangeably (depending on what you want to do). This means that you don't want, or need, command1 &; command2, all you need is command1 & command2.

So, in your example, you could just write:

for i in {1..10}; do wait file$i & done

and each wait command will be launched in the background and the loop will immediately move on to the next.

marc · Answer 2 · 2016-03-23T15:52:08.360

4

For the sake of compatibility use the posix form instead of expansion:

for i in $(seq 1 10); do (./wait file$i &); done

edited Mar 23 '16 at 15:52

answered Mar 23 '16 at 15:42

marc

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Note that seq is not a POSIX command and is generally only found on GNU systems. The behaviour or $(...) (and $i) depends on the current value of $IFS. Also note that by doing (cmd &), cmd will be a child of a subshell, so you won't be able to wait for and get its exit status for instance. – Stéphane Chazelas Mar 23 '16 at 20:32

score 2 · Answer 3 · answered Mar 23 '16 at 15:19

2

You can group the commands and put the grouped commands in background. Like :

$ for i in {1..10}; do ((wait file$i)&); done

answered Mar 23 '16 at 15:19

magor

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There is no need to use the inner parentheses... – marc Mar 23 '16 at 15:44
only need to use the inner parentheses if you want to put in the background multiple commands as asked in the question. Like ((sleep 1; wait file$i)&); – magor Mar 23 '16 at 16:17

score 2 · Answer 4 · answered Mar 23 '16 at 15:25

Is your binary really named wait? I don't recommend to do so, because wait is a shell builtin.

I believe bash doesn't parse well a one-line loop that launches background processes. I suggest you to change the code to:

$ for i in {1..10}; do ./wait file$i & echo "Running 'wait' using PID=$!..."; done

Run the same command multiple times in background

4 Answers4

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