Currently the ls subcommand always prints all sessions, both active and exited. I suggest adding a --alive flag that only shows sessions that are alive. The name should probably be bikeshedded. Some other options:

• --only-alive
• --active
• --only-active
• --hide-dead
• --hide-exited
• --show-dead/exited=false
• etc

This would add to the two existing flags: --no-formatting and --short.

My specific use case is that I've set up ssh to show a list of active sessions, but I want it to be nice and short and I really don't care about dead sessions. In general it's a nice option that I think a lot of people would expect to have.

I've looked through the code and it seems like a simple enough change. I would be more than glad to make a PR if it's something the maintainers agree with!