bash: start multiple chained commands in background

Mad_Ady_23780 0
Viewed 92354

I'm trying to run some commands in paralel, in background, using bash. Here's what I'm trying to do:

forloop {
  //this part is actually written in perl
  //call command sequence
  print `touch .file1.lock; cp bigfile1 /destination; rm .file1.lock;`;
}

The part between backticks (``) spawns a new shell and executes the commands in succession. The thing is, control to the original program returns only after the last command has been executed. I would like to execute the whole statement in background (I'm not expecting any output/return values) and I would like the loop to continue running.

The calling program (the one that has the loop) would not end until all the spawned shells finish.

I could use threads in perl to spawn different threads which call different shells, but it seems an overkill...

Can I start a shell, give it a set of commands and tell it to go to the background?

background command bash
15 Answers

I haven't tested this but how about

print `(touch .file1.lock; cp bigfile1 /destination; rm .file1.lock;) &`;

The parentheses mean execute in a subshell but that shouldn't hurt.

Hugh_Allen_150690

Thanks Hugh, that did it:

adrianp@frost:~$ (echo "started"; sleep 15; echo "stopped")
started
stopped
adrianp@frost:~$ (echo "started"; sleep 15; echo "stopped") &
started
[1] 7101
adrianp@frost:~$ stopped

[1]+  Done                    ( echo "started"; sleep 15; echo "stopped" )
adrianp@frost:~$

The other ideas don't work because they start each command in the background, and not the command sequence (which is important in my case!).

Thank you again!

Mad_Ady_237800 
for command in $commands
do
    "$command" &
done
wait

The ampersand at the end of the command runs it in the background, and the wait waits until the background task is completed.

GavinCattell_216440

Try to put commands in curly braces with &s, like this:

{command1 & ; command2 & ; command3 & ; }

This does not create a sub-shell, but executes the group of commands in the background.

HTH

user_116210

I don't know why nobody replied with the proper solution:

my @children;
for (...) {
    ...
    my $child = fork;
    exec "touch .file1.lock; cp bigfile1 /destination; rm .file1.lock;" if $child == 0;
    push @children, $child;
}
# and if you want to wait for them to finish,
waitpid($_) for @children;

This causes Perl to spawn children to run each command, and allows you to wait for all the children to complete before proceeding.

By the way,

print `some command`

and

system "some command"

output the same contents to stdout, but the first has a higher overhead, as Perl has to capture all of "some command"'s output

ephemient_207130

You can pass parameters to a command group (having sequential commands) and run them in background.

for hrNum in {00..11};
do
    oneHour=$((10#$hrNum + 0))
    secondHour=$((10#$hrNum + 12))
    { echo "$oneHour"; echo "$secondHour"; } &
    wait
done
user_83034070
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