Question

Is there any way to make a background process in Mathematica so that the external command will run but not hold up other evaluations?

startTime = AbsoluteTime[];
Run["sleep 2" (*, a magic option here perhaps? *)];
AbsoluteTime[] - startTime
(* Returns 2.016960 on my system *)

I want a version of / option for Run that will not hold up the evaluation, so that the last line in the above code will return something close to zero.

(The above will run in *nix and OSX; there doesn’t seem to be any universal/portable way to “sleep 2” in a DOS/Windows shell:
Sleeping in a batch file. Although, perhaps the easiest way to get the sleep command is to install the MS resource kit – see this answer to the Sleeping in a batch file question or the comments below.)

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At the moment, the reason I want this is so that my ALSASound command given in No sound in Mathematica under linux will not stop the fireworks. But it would also be a good improvement to ALSASound and a useful thing to know in general.

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Edit

The two solutions posted below rely on using the shell to spawn a new/child process using start cmd in Windows/DOS and cmd& in *nix. Is there a platform independent and/or Mathematica based solution? (Points will be awarded for the creation of a command that checks the OS using $OperatingSystem!)

Answer 1

As Simon points out, Switch is the correct tool for procedural conditions and is more appropriate than Piecewise. Here’s the same solution using Switch:

BackgroundProcess[proc_String] := 
  Switch[$OperatingSystem, 
    "MacOSX" | "Unix", Run["(" <> proc <> ")&"], 
    "Windows", Run["start /b " <> proc],
     _, $Failed]

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You could write a wrapper function like so that handles multiple OSes.

BackgroundProcess[proc_String] := 
 Module[{
  command = Piecewise[{
       {proc <> " &", $OperatingSystem == "MacOSX" || "Unix"}, 
       {"start /b " <> proc, $OperatingSystem == "Windows"}
     }]
    }, 
  Run[command]]

The /b after start runs the process in the background, without spawning a new command window and the output is printed to stdout. You can then build upon this to include error messages, some input sanitization (e.g., don’t allow rm and the like) if you want that, and you should be all set.

A predefined function from Mathematica (if it existed) is probably going to be along these lines. Background processes are inherently OS dependent, so there is no real “platform independent” way of doing it. Any implementation that claims to be so (like the os module in python) is basically a set of rules defined for each possible OS known to exist, so that you don’t really have to worry about the finer details.