Don't hang up on me

Can you tell me when a shell sends a “HANGUP” signal to a process? What happens if there is a pipeline? What if you prefix this pipeline with the “nohup” command?

In my previous post, I posted a command to log via syslog:

nohup myapp | nohup logger -i -t myapp &

It took quite a bit of work to get to that. This post is about that journey.

The beginning

I first started off with this:

myapp | logger -i -t myapp &

Tried it on my bash prompt, logged out, logged back in. All good.

OK, I put it in a script that is executed over ssh, and guess what? It failed. Myapp is nowhere to be found!

Hmmm, sounds like a case of SIGHUP. I knew the shell would send a hangup signal to myapp, so I had to ignore it. Easy:

nohup myapp | logger -i -t myapp &

But no! Myapp was still gone.

What’s going on here? And why did it work when I tried it in the shell myself?

When to hang up

Turns out bash does not send a SIGHUP by default, if it is an interactive shell. This can be verified by:

$ shopt | grep huponexit
huponexit	off

This was a surprise to me.

OK, I enabled huponexit. When I ran the same command line, I saw that myapp had exited. At least I could reproduce the problem quicker this way.

Reading further in the manual, I found that I could do disown -h to prevent SIGHUP from being sent to the processes. This is what my command sequence looked like:

$ shopt -s huponexit
$ myapp | logger -i -t myapp &
[1] 6584
$ disown -h
$ exit

This worked. I found that myapp was running.

I copied the lines to the ssh’ing script:

myapp | logger -i -t myapp &
disown -h

But surprise! Again myapp was gone. This was becoming a pain.

Trace …

I had to find out what was causing this problem. So I added a bit of delay:

myapp | logger -i -t myapp &
disown -h
sleep 40

While this slept, on another terminal I found the PID and tried ktrace on it.

$ pgrep myapp
$ ktrace -f myapp.trace -p 7513
$ pgrep logger
$ ktrace -f logger.trace -p 7514

After some time, when the program disappeared as usual, I looked at the trace:

$ kdump -f myapp.trace
7513 myapp	PSIG  SIGHUP SIG_DFL code=0x10001

Looks like I’m getting SIGHUP, even though I disowned it!

I went back and thought about this. It hit me that disown makes sense only on an interactive terminal where you have jobs and job control. It does not make sense on a non-interactive terminal. Maybe I had to do nohup instead, like I was doing before.

So now I did:

nohup myapp | logger -i -t myapp &

But no, again the application disappeared.

… And trace again

Another ktrace, and I saw that the program was not exiting due to SIGHUP, but something very different:

85932 myapp CALL  write(0x1,0xc820076dc0,0x89)
85932 myapp RET   write -1 errno 32 Broken pipe
85932 myapp PSIG  SIGPIPE caught handler=0x45c680 mask=0x0 code=0x10006

Looks like we tried to write to the pipe but there was nobody listening! Which means, logger is gone. Why did it disappear, since I had already prepended a nohup?

By this time I was already quite tired and this one took a long time. I looked at nohup source code. At its core, it was very simple. Just two lines to say ‘Set the program to ignore SIGHUP, run the program’.

         (void)signal(SIGHUP, SIG_IGN);
         execvp(*argv, argv);

I looked at bash source code where it would send SIGHUP. In jobs.c, it would simply call killpg() on the process group. Nothing special there either.

But then, it occurred to me: nohup will never see the full pipeline! That would have already been broken down by the shell! Maybe I should prepend nohup for logger as well!

The end

OK, that’s what I did:

nohup myapp | nohup logger -i -t myapp &

This, finally, is what worked, and where we started this story. Of course, I won’t mention the bits about writing a simple C program to narrow down testing, or using commands like sleep 40 | wc for experimentation, or trying out in vain a variety of syntactic suggestions offered at!

Update: Sep. 30 2016

A colleague pointed out that you can just put the pipeline in a separate script by itself, and then call nohup on the script. I don’t know why I never tried it, but it did work. Oh well.

See also