Replacing shell scripts with Clojure+JamVM

We all hate shell scripting. Scripts are annoyingly hard to debug, test and verify. Would be lovely, to use some kind of lisp for scripting, right? To do interactive development with repl in your favorite editor. To write it in a nice predictable language that you also enjoy. But sometimes it’s impossible to add some external dependencies to the system. What if you have only JVM to your disposal, will you be able to pull it off only with JVM and clojure.jar?

Basic setup

First what we will need is to get clojure jar file:

wget -O /opt/clojure.jar 'https://central.maven.org/maven2/org/clojure/clojure/1.6.0/clojure-1.6.0.jar'

Next lets create executable /usr/bin/clojure that will live in /usr/bin (or /opt/bin or /home/youruser/bin):

#!/bin/sh

exec java -jar /opt/clojure.jar "$@"

And now it’s time for our hello world script /opt/test.clj:

#!/usr/bin/clojure

(println "hello world")

Make it executable:

chmod +x /opt/test.clj

And run it:

$ /opt/test.clj
hello world

Yay! But it feels kind of slow:

time /opt/test.clj
hello world

real  0m2.684s
user  0m2.239s
sys   0m0.186s

2 seconds startup time, not really suitable for scripting, right? Can we improve that? What if there would be JVM with fast startup and low memory usage.

Introducing JamVM.

“But… but you told us that there is only JVM available on production system without ability to add external dependencies.”

I lied, sorry.

Compiling JamVM with OpenJDK support:

# Fetching required dependencies and source
apt-get -y install openjdk-7-jdk openjdk-7-jre build-essential zlib1g-dev
cd /opt
wget -O jamvm-2.0.0.tar.gz 'https://downloads.sourceforge.net/project/jamvm/jamvm/JamVM%202.0.0/jamvm-2.0.0.tar.gz'
tar -xvzf jamvm-2.0.0.tar.gz

# Building
cd /opt/jamvm-2.0.0
./configure --with-java-runtime-library=openjdk7 && make check && make && make install

# Installing in to the openjdk installation
mkdir /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/jamvm
cp /usr/local/jamvm/lib/libjvm.so /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/jamvm/libjvm.so

# Trying it out
java -jamvm -version

JamVM will be installed as separate vm in openjdk, so it will not mess with existing installation. You will need to use -jamvm option to java command to run it with small overhead vm.

Let’s update our clojure executable /usr/bin/clojure:

#!/bin/sh

exec java -jamvm -jar /opt/clojure.jar "$@"

Let’s try it out:

time /opt/test.clj
hello world

real  0m0.866s
user  0m0.764s
sys   0m0.076s

Better, right?

How slow is JamVM? Some benchmarks:

Clojure 1.6

JamVM:

(factorial 5000) Avg: 248.65890986500017
(fib 20)         Avg: 35.33471996000001
(sort-seq)       Avg: 405.7438969800002

OpenJDK:

(factorial 5000) Avg: 25.016900630000006
(fib 20)         Avg: 0.69957772
(sort-seq)       Avg: 11.553695560000001

Much slower, but if you think about it shell scripting most of the time is about executing external commands, IO and data filtering. Might be as well not so bad. Also memory usage of JamVM makes it perfect for embedded systems.

Why not use something like lein exec?

Lein exec is nice. But it adds overhead. If you need external dependencies you can solve it (in theory) with classpath manipulations in java command (java -cp dep.jar:dep2.jar:.). Still you can plug lein exec to JamVM if you want.

Update

I just noticed that in Ubuntu 14:04 repos there is already JamVM package, so you can just run apt-get -y install icedtea-7-jre-jamvm to install latest build.