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Recently, I had to close down Skype and try to log into it again, but Skype wouldn't let me:
"Another skype instance may exist", it kept telling me.
This is a common problem. Most of the advice on the Internet is to delete the ~/.Skype/shared.lck and ~/.Skype/USERNAME/config.lck files. This didn't work for me.
However, lsof | grep .Skype revealed that files in the Skype folder were being used. ps aux told me that they were being used by Chromium, because I had clicked on a link in a Skype chat window. It's crazy that a Chromium process spawned by Skype should lock the Skype directory but there you have it. I was able to kill the Chromium and log into Skype.
This is why lsof is an essential command.