javac no source files found

From your comment above, it looks like you tried:

javac -cp .;lib.jar a.java on your Ubuntu system. The CLASSPATH separator is : on Unix systems and ; on Windows.

Ubuntu considered the command up to the ;, java -cp . and thus gave the message.

javac -cp .:lib.jar a.java should compile fine.

For anyone who is using powersehll on windows use CLASSPATH separator : instead of ;