Backgrounding

The great thing about Unix, and the Bourne shell, when it was introduced back in the day, was multitasking. It’s such an overused buzzword these days, but at the time, it was really a new thing. If you’ve only got one connection in to a machine, you can get it doing as much as you want.

The shell command to “do this in the background, then give me a new prompt to provide the next command” is the ampersand (“&”):

$  # I need to trawl the filesystem for files called "*dodgy*"
    #  (should have installed slocate
    #  (http://packages.debian.org/stable/source/slocate), 
    # but it's too late for that)
$ find / -name "*dodgy" -print
    (wait for a very very very long time)
/foo/bar/thisfileisnotdodgy.txt
/bar/foo/thisisadodgyfile.txt
$

Well, that’s a good hour of my life wasted.

Chuck it into a script, and run it in the background. If you want the outcome, direct it to a file:

$ cd /tmp
$ cat myfindscript.sh
find / -name "*dodgy*"
$ chmod u+rx myfindscript.sh
$ ./myfindscript.sh > /tmp/mysuspectfiles.txt &
[4402]
$
$ # wow, didja see that? It'll take ages, but I've got 
   # control back. "4402" is the Process ID (PID), 
   # so I can run "ps -fp 4402" to check on its
   # progress, but it's happening, in the backrgound.

You don’t get a lot of job control here; the “ps” mentioned above is about your lot, but you can spawn a child process and let it run, whilst you get on with the stuff you need.

This is known as “backgrounding” a task; if you know it will take a long time, just background it. Of course, if the next thing you need to do is to read the entire file, then you won’t get away with it, you’ll have to wait for it to finish. However, you could background it and then “tail -f /tmp/mysuspectfiles.txt” to check on the status.

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One Response to Backgrounding

  1. unixshell says:

    Just a quick note to add a mention of nohup… if you’re logging out, you can “nohup do_something_slow&” before you logout. Nohup means “No Hangup”, and it’ll keep running on your behalf, even though you are not logged in. Standard output goes to a file “nohup.out” by default.

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