How to make your Scala shell scripts run faster by pre-compiling them

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is a short recipe, Recipe 14.13, “How to make your Scala shell scripts run faster by pre-compiling them.”


You love using Scala as a scripting language, but you’d like to eliminate the lag time in starting up a script.


Use the -savecompiled argument of the Scala interpreter to save a compiled version of your script.

A basic Scala script like this:

exec scala "$0" "$@"

println("Hello, world!")
args foreach println

consistently runs with times like this on one of my computers:

real    0m1.573s
user    0m0.574s
sys     0m0.089s

Those times are pretty slow for a do-nothing script, so to improve this, add the -savecompiled argument to the Scala interpreter line:

exec scala -savecompiled "$0" "$@"

println("Hello, world!")
args foreach println

Then run the script once. This will still run slow, but it generates a compiled version of the script. After that, the script runs with a consistently lower real time (wall clock) on all subsequent runs:

real   0m0.458s
user   0m0.487s
sys    0m0.075s

Precompiling your script shaves a nice chunk of time off the runtime of your script, even for a simple example like this.


When you run your script the first time, Scala generates a JAR file that matches the name of your script. For instance, I named my script, and then ran it like this:

$ ./

After running the script, I looked at the directory contents and saw that Scala created a file named Scala creates this new file and also leaves your original script in place.

On subsequent runs, the Scala interpreter sees that there’s a JAR file associated with the script, and if the script hasn’t been modified since the JAR file was created, it runs the precompiled code from the JAR file instead of the source code in the script. This results in a faster runtime because the source code doesn’t need to be compiled.

As an interesting note about how this works, you can look at the contents of the JAR file using the jar command:

$ jar tvf

 43 Wed Jul 25 15:44:26 MDT 2012 META-INF/MANIFEST.MF
965 Wed Jul 25 15:44:26 MDT 2012 Main$$anon$1$$anonfun$1.class
725 Wed Jul 25 15:44:26 MDT 2012 Main$$anon$1.class
557 Wed Jul 25 15:44:26 MDT 2012 Main$.class
646 Wed Jul 25 15:44:26 MDT 2012 Main.class

In this example, I didn’t include a main method in an object or use the App trait with an object, so Scala assumed the name Main for the main/primary object that it created to run my script.

The Scala Cookbook

This tutorial is sponsored by the Scala Cookbook, which I wrote for O’Reilly:

You can find the Scala Cookbook at these locations:

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