Optimal way to trace back TTY session that locks a resource?

Question

Say you have plenty of interactive shells being run under same user id, like lots of screen or tmux (or both) "tabs". You tried unmount'ing a device and realised you couldn't since one of those shell sessions had its current directory changed there. You can easily find TTY-name and list of processes associated with (just another bash or zsh), but assuming you don't want just kill it until you're sure it's safe to do that, how are you going to find corresponding screen/tmux "tab"?

Answer 1

Not sure if this is the optimal way, but here is "a" way.

First, identify which "screen" or "tmux" session has the device open:

lsof -R $mountpoint

The few things you need from this output is the PID of the process, and its PPID (assume you assign these to $PID and $PPID respectively).

Next, check what child processes are running under this PID, this should help you out in case you sshed into another machine from this shell, or you have an editor or something else running at the moment. If something is running, it should be easy enough for you to find the tab that you need based on the child commands.

pstree -p $PID

Assuming the above command produces no output, you should now check what is the parent of that shell.

ps -f $PPID

If this is anything but SCREEN or tmux you should be able to easily figure it out.

In case it is SCREEN and you have multiple ones, you should be able to figure which one by looking at the child processes of the SCREEN you need.

pstree -p $PPID

If there are still multiple ones, you can open up a new tab in each SCREEN and keep rerunning pstree -p $PPID until you find out which one it is. After that, you have to check each tab and somehow figure out the correct one. In the shell, you can check the PID of the shell if it matches the $PID of the problematic one, or one of its children as produced by pstree -p $PID above.

If it is tmux, it is not so easy as all the shells are under a single tmux. However, something you can do with tmux is automatically send keypressess to all open panes. These could do something unexpected if you have an editor running in there, but you probably know best if it is okay to do this or not.

for i in $(tmux list-panes -a | awk -F': ' '{print $1}'); do
  tmux send-keys -t $i "[[ \$\$ == $PID ]] && logout" Enter
done