I am also running Ubuntu 12.04, which I've been using as a vanilla Minecraft server successfully. I wanted to start experimenting with mods, and so installing Forge seemed like the logical first step. I downloaded the installer, specifically:
forge-1.7.2-10.12.2.1147-installer.jar
I ran this successfully, and it generated the following file:
forge-1.7.2-10.12.2.1147-universal.jar
When I run that file, however, I get the aforementioned "missing one or more essential library files" error. Specifically, the output is:
We appear to be missing one or more essential library files.
You will need to add them to your server before FML and Forge will run successfully.java.lang.ClassNotFoundException: net.minecraft.launchwrapper.Launch
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:270)
at cpw.mods.fml.relauncher.ServerLaunchWrapper.run(ServerLaunchWrapper.java:25)
at cpw.mods.fml.relauncher.ServerLaunchWrapper.main(ServerLaunchWrapper.java:12)