Recover a tmux session that tmux insists isn't running

Question

I have had a tmux session running for a while, and I can see it's running using pstree:

systemd─┬─accounts-daemon───2*[{accounts-daemon}]
        ├─2*[agetty]
        ├─containerd───11*[{containerd}]
        ├─cron
        ├─dbus-daemon
        ├─4*[dd]
        ├─dockerd───12*[{dockerd}]
        ├─fail2ban-server───2*[{fail2ban-server}]
        ├─irqbalance───{irqbalance}
        ├─networkd-dispat
        ├─polkitd───2*[{polkitd}]
        ├─qemu-ga
        ├─rsyslogd───3*[{rsyslogd}]
        ├─ssh-agent
        ├─sshd───sshd───sshd───bash───pstree
        ├─systemd───(sd-pam)
        ├─systemd-journal
        ├─systemd-logind
        ├─systemd-network
        ├─systemd-resolve
        ├─systemd-timesyn───{systemd-timesyn}
        ├─systemd-udevd
        ├─tmux: server─┬─bash───find_cli
        │              └─3*[bash]
        └─unattended-upgr───{unattended-upgr}

(it's down the bottom there)

However, tmux itself insists there are no sessions:

$ tmux attach
no sessions
$ tmux ls
no server running on /tmp/tmux-1000/default

Is there any way to recover a tmux session in this situation?

Answer 1

If the socket has been removed you should be able to make tmux recreate it with pkill -USR1 tmux.

Answer 2

If ps or other process information commands show that tmux is still running but doesn't respond properly to the tmux attach command, you might be able to make tmux interactive again by sending a SIGUSR1 signal to the tmux server.

This can be done, as mentioned in the answer above, by using the pkill -USR1 tmux command. You can then run the tmux attach command again to see if you can now interact with tmux.

An alternative to the machine-gun method of using pkill is to use a specific PID number with kill. The command template would be quite similar to the pkill method save for the use of a PID number rather than a program name pattern. Thus your command might look like the following:

$ kill -USR1 NNNN

You can get the NNNN by carefully reading the output of ps ax | grep tmux. This assumes you have several tmux instances running. Otherwise (i.e. if you're running only one tmux instance), it's indeed easier to pkill tmux.

As a side note, I find both pkill and killall to be pretty dangerous, especially when used as root.