Bash – How to Run the Same Command Multiple Times in Background

background-processbashcommand line

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.

Best Answer

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.

Related Solutions

Bash – Combine two commands in .bash_profile

aliases do not support input parameters, and there's no need to wrap functions in aliases. Simply use a function:

pd() {
    pushd "$@"
    set_title_tab
}

pd ~/Documents

Shell Scripting – How to Run Commands in Parallel and Wait for Completion

The commands within each group run in parallel, and the groups run sequentially, each group of parallel commands waiting for the previous group to finish before starting execution.

The following is a working example:

Assume 3 groups of commands as in the code below. In each group the three commands are started in the background with &.

The 3 commands will be started almost at the same time and run in parallel while the script waits for them to finish.

After all three commands in the the third group exit, command 10 will execute.

$ cat command_groups.sh 
#!/bin/sh

command() {
    echo $1 start
    sleep $(( $1 & 03 ))      # keep the seconds value within 0-3
    echo $1 complete
}

echo First Group:
command 1 &
command 2 &
command 3 &
wait

echo Second Group:
command 4 &
command 5 &
command 6 &
wait

echo Third Group:
command 7 &
command 8 &
command 9 &
wait

echo Not really a group, no need for background/wait:
command 10

$ sh command_groups.sh 
First Group:
1 start
2 start
3 start
1 complete
2 complete
3 complete
Second Group:
4 start
5 start
6 start
4 complete
5 complete
6 complete
Third Group:
7 start
8 start
9 start
8 complete
9 complete
7 complete
Not really a group, no need for background/wait:
10 start
10 complete
$

Best Answer

Related Solutions

Bash – Combine two commands in .bash_profile

Shell Scripting – How to Run Commands in Parallel and Wait for Completion

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