bash: start multiple chained commands in background

Question

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
          


        

Answer 1

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!

Answer 2

Run the command by using an at job:

# date
# jue sep 13 12:43:21 CEST 2012
# at 12:45
warning: commands will be executed using /bin/sh
at> command1
at> command2
at> ...
at> CTRL-d
at> <EOT>
job 20 at Thu Sep 13 12:45:00 2012

The result will be sent to your account by mail.

Answer 3

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.

Answer 4

run the commands in a subshell:

(command1 ; command2 ; command3) &

Answer 5

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

Answer 6

I stumbled upon this thread here and decided to put together a code snippet to spawn chained statements as background jobs. I tested this on BASH for Linux, KSH for IBM AIX and Busybox's ASH for Android, so I think it's safe to say it works on any Bourne-like shell.

processes=0;
for X in `seq 0 10`; do
   let processes+=1;
   { { echo Job $processes; sleep 3; echo End of job $processes; } & };
   if [[ $processes -eq 5 ]]; then
      wait;
      processes=0;
   fi;
done;

This code runs a number of background jobs up to a certain limit of concurrent jobs. You can use this, for example, to recompress a lot of gzipped files with xz without having a huge bunch of xz processes eat your entire memory and make your computer throw up: in this case, you use * as the for's list and the batch job would be gzip -cd "$X" | xz -9c > "${X%.gz}.xz".