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I'm having a bit of trouble with GNU's screen. What I want to do is this (from cron):

If a screen session named cron exists, add a new window to the session and launch a process in it (but do not attach to the session). Otherwise, create a new session called cron and launch the process in it (again, do not attach). If possible, all of this should be done using just one "screen" command.

I've already tried a bit but haven't come to a solution that satisfies all of my needs:

screen -S cron -dRRm screen ping
screen -S cron -X screen ping

Is there something I have missed?

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I don't think that you can do this in one command, but this should do what you want:

screen -ls cron | grep -q '(\w*tached)' >&- || screen -dmS cron
screen -S cron -X screen ping
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