Unix: How to redirect stderr to stdout

Today I ran a Java application in a Unix console. It printed out some messages before it crashed with an exception and a loong stack trace. Too long to see the top of. So, I figured I’d just pipeline it into the lovely less like this:

$ java -jar foobar.jar | less

The result was not what I expected however. All I got to see was the messages that had been printed out. No exception or stack trace to be seen.

The reason for this is quite simply that a Unix process has three different streams: stdin(0), stdout(1) and stderr(2). When we pipe output from one process into another we sort of connect stdout of the first one to stdin of the second.

So, to be able to see my giant stack trace, we can for example redirect stderr to stdout. This way we will get both streams into stdin of the next process.

$ java -jar foobar.jar 2>&1 | less

Fancy, huh? We can also direct these streams into files, which might come in handy.

$ java -jar foobar.jar 1>out.log 2>err.log

If you want the output to be appended to the log files instead of replacing them, you can do adjust the command to this:

$ java -jar foobar.jar 1>>out.log 2>>err.log