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.

Answer 1

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.

Answer 2

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

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

Answer 3

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

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

Answer 4

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