Here's a blog about one of the commen problems when installing WebSphere Application Server 6.1 on Ubuntu
Read it here
Thankfully a quick Google turned up this article. I was completely unaware that /bin/sh was linked to dash instead of bash – what on earth possessed them. I didn't really like the suggested solution of running the installer, letting it fail and then changing all the scripts in the installed directory. Instead, I just relinked /bin/sh with a quick sudo unlink /bin/sh followed by sudo ln -s /bin/bash /bin/sh. After that, the installer ran like a dream and I was up and running with a WAS install in about 15 minutes.