Suppose that you want to know what processes on your system are ignoring certain signals, such as the SIGTERM that systemd uses to try to get lingering processes to quit . How do you find this out? As with many process-related things in Linux, the answer is that you look in /proc .

Specifically, you look in /proc/<pid>/status and get the SigIgn: field. This is in hex, and may look something like this:

SigIgn: 0000000000004007

(There is also SigCgt: for the signals that the process has installed signal handlers for.)

This is a bitmap of signals. You can see the mapping between signal names and numbers with ' kill -L ' (which reports them in decimal), and then use your favorite decimal+hex calculator to work out what bit this corresponds to. Suppose we want SIGTERM , signal 15. I'm a Python person, so:

$ python -c 'print "%x" % (1<<(15-1))'
4000

(We subtract one because signals are numbered starting from 1 instead of 0.)

In this case, the question about our SigIgn: above is very easy; this process is ignoring SIGTERM , but not any other signals around it.

If we want to look at a mass of processes, we can abuse gawk :

cd /proc
gawk '/^SigIgn:/ && (and(strtonum("0x" $2), 0x4000) > 0) {print FILENAME}' */status

Somewhat to my surprise, it turns out that there are quite a few programs on my Linux machine that are ignoring SIGTERM . But how many are ignoring both SIGTERM and SIGHUP (signal 1, and thus it has the mask 0x01)?

cd /proc
gawk '/^SigIgn:/ && (and(strtonum("0x" $2), 0x4001) == 0x4001) {print FILENAME}' */status

Once again there were more than I expected. However, I think I don't notice most of them because they exit if their connection to the X server goes away. The exception is my non-friend kio_http_cache_cleaner , which is apparently so disconnected from everything that it doesn't notice my X session disappearing.

(Perhaps it would if I was running a proper KDE session, or even a Gnome session. Also, now I wonder if I am somehow indirectly starting it in such a way that it ignores more signals than usual, since a number of things in my X session turn out to have the same set of ignored signals as it does.)

Sidebar: seeing what signals a Linux process is ignoring

On some systems, such as Solaris and its descendents, you have a handy psig program that will report all sorts of information about what signals a process is ignoring or catching and what it's doing with them and so on. On Linux, as far as I know, even a basic version is not part of eg procps-ng or similar packages. If you need this information, see eg Erik Weathers' psig , which I found via this popular Stackexchange question and its answers .

I'm probably going to wind up keeping a copy of psig around, as using it on various processes here has already given me things to think about and look into.