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I was trying to re-attach to a long-running tmux session to check up on a python web-application. However tmux attach claims that there is no running session, and ps shows a tmux process (first line), but with a question mark instead of the pts number.

What does this mean---is this tmux session permanently lost, and what could have caused it? Is there still a way to look at the current state of the python process, spawned in the tmux session and running in pts/19 (second line)?

[mhermans@web314 ~]$ ps -ef | grep mhermans
mhermans 16709     1  0 Mar04 ?        00:26:32 tmux
mhermans  8526 16710  0 Mar04 pts/19   00:20:04 python2.7 webapp.py
root      9985  6671  0 10:18 ?        00:00:00 sshd: mhermans [priv]
mhermans 10028  9985  0 10:18 ?        00:00:00 sshd: mhermans@pts/16
mhermans 10030 10028  0 10:18 pts/16   00:00:00 -bash
mhermans 16247 10030  6 10:28 pts/16   00:00:00 ps -ef
mhermans 16276 10030  0 10:28 pts/16   00:00:00 grep mhermans
mhermans 16710 16709  0 Mar04 pts/19   00:00:00 -bash
mhermans 16777 16709  0 Mar04 pts/21   00:00:00 -bash
share|improve this question
What does tmux ls show? – jasonwryan Jun 16 '12 at 9:04
"failed to connect to server: Connection refused" – mhermans Jun 16 '12 at 10:31
up vote 16 down vote accepted

Solution courtesy of the Webfaction-support:

As the process was still running, the issue was a deleted socket, possibly caused by a purged tmp-directory.

According to the tmux mapage:

If the socket is accidentally removed, the SIGUSR1 signal may be sent to the tmux server process to recreate it.

So sending the signal and attaching works:

killall -s SIGUSR1 tmux
tmux attach
share|improve this answer
I am using byobu, which uses tmux as a backend, and this worked for me after I accidentally deleted the socket from /tmp/tmux-<pid>. Thanks! – Nicu Stiurca Aug 20 '15 at 3:44

Terminal absence is a sign of detached session. And all your tmux session names can be found thus:

ls $TMP/tmux-$(id -u) or ls /var/run/tmux/tmux-$(id -u)

— this is kinda distro-dependent. Almost distro-independent (and more hardcore) would be:

lsof -n -p 16709 | awk '{if($5 == "unix") print;}'

where 16709 is the PID of tmux in your listing.

share|improve this answer
id -u gives 532 and /tmp/tmux-532 contains a single file "default". How does this help me further? – mhermans Jun 16 '12 at 20:27
Try tmux -S /tmp/tmux-532/default at, but default socket-name should be just ok with tmux at. What does lsof-version say? – poige Jun 17 '12 at 2:31
The first command results in a "no sessions" response. lsof -v output. – mhermans Jun 17 '12 at 22:11
lsof as given in my answer, dude. :) – poige Jun 18 '12 at 2:31
@mhermans, I meant lsof-version of my answer, we don't need its software version. :) – poige Jun 18 '12 at 3:40

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