How to see error message in journald

Question

I've created a program that intentionally has a divide by zero error.

If I run it in the command line it returns: "Floating point exception"

But if I run this as a systemd service I can not see this error message.

In my systemd script I have added:

StandardError=journal

But the error message is nowhere to be seen when using journalctl. How can this error message be added to the log seen with journalctl?

Answer 1

To get all errors for running services using journalctl:

$ journalctl -p 3 -xb

where -p 3 means priority err, -x provides extra message information, and -b means since last boot

Answer 2

By default stdout and stderr of a systemd unit are sent to syslog. So you can access them in the file /var/log/messages or /var/log/dmesg.

To change that you can use this command:

sudo journalctl -u [yourunit]

Where [yourunit] is the systemd .service name. Eg, to see messages from yourapp.service,

sudo journalctl --unit=yourapp

then restart the journalctl using the command

sudo systemctl restart systemd-journald

if this doesn't work for you add StandardOutput=syslog+console and StandardError=syslog+console to your systemd script and restart systemctl. After that you can access the output from your unit using journalctl.

Answer 3

As JdeBP is trying to hint, when you run your program from a shell and it is killed by the signal SIGFPE, the specific message "Floating point exception" is printed by your shell. Technically your program could catch the signal and print an error if you wrote it that way - but that wouldn't work for SIGKILL.

You need some parent process which uses wait() or similar, to read the exit status of your program and log about it. The systemd service manager is such a parent. Its message will not look identical, but it should have something equivalent when you look at the service's logs in the journal. It might mention SIGFPE.

Additionally, if you have systemd-coredump enabled, it should hopefully log a crash message for SIGFPE, since the default action of this signal is to generate a core dump. These crash messages should be tagged such that they show up when querying your service's messages. I like systemd-coredump and make sure to enable it on my Debian systems, although currently installing it will remove Apport, the crash reporter on Ubuntu. When developing, some people might prefer to deal with core files directly and not use systemd-coredump, although it's sort of nice that the core dumps won't clutter your directory, plus they will be cleaned up after a while (freeing the disk space).