journalctl - Query the systemd journal

Table of Contents

journald is used to query the systemd journal contents.

Example 1
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Lets try and see the kubelet service logs in the systemd journal

Checking if the service exists

systemctl list-units --show-types|grep kubelet

on my system the output is as below.

kubelet.service                                                                                                   loaded active running   kubelet: The Kubernetes Node Agent

Time to get the log information

journalctl -u kubelet -f -o json

Please note the options

-u
- Identifies the unit kubelet in our case
-f
- To follow the journal
-o
- Output formation

In my case output is as under

{
   "__CURSOR":"s=4b762904ccb24ceeb1cfa518f498790c;i=cf33fa;b=5e07b79a1d1444ef9729bb834c352d1b;m=f1cfad0b04;t=5b18a9f763acf;x=db876470889c7586",
   "__REALTIME_TIMESTAMP":"1602583822482127",
   "__MONOTONIC_TIMESTAMP":"1038571342596",
   "_BOOT_ID":"5e07b79a1d1444ef9729bb834c352d1b",
   "_TRANSPORT":"stdout",
   "_STREAM_ID":"ffb8882786f14db1bcd227a42a088329",
   "PRIORITY":"6",
   "SYSLOG_FACILITY":"3",
   "SYSLOG_IDENTIFIER":"kubelet",
   "_PID":"1582",
   "_UID":"0",
   "_GID":"0",
   "_COMM":"kubelet",
   "_EXE":"/usr/bin/kubelet",
   "_CMDLINE":"/usr/bin/kubelet --bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf --kubeconfig=/etc/kubernetes/kubelet.conf --config=/var/lib/kubelet/config.yaml --cgroup-driver=cgroupfs --network-plugin=cni --pod-infra-container-image=k8s.gcr.io/pause:3.2",
   "_CAP_EFFECTIVE":"1fffffffff",
   "_SYSTEMD_CGROUP":"/system.slice/kubelet.service",
   "_SYSTEMD_UNIT":"kubelet.service",
   "_SYSTEMD_SLICE":"system.slice",
   "_MACHINE_ID":"05cb8c7b39fe0f70e3ce97e5beab809d",
   "_HOSTNAME":"mymachine.net",
   "MESSAGE":"E1013 10:10:22.481755    1582 kubelet.go:2183] node \"mymachine.net\" not found"
}

Let’s do a quick check using systemd-cat util.

From the official man page for systemd-cat

systemd-cat - Connect a pipeline or program’s output with the journal — from man pages

Let’s execute a dummy statement that has a timestamp added to the output which is connected to the journal using systemd-cat

echo "My date is: `date`" | systemd-cat

If you now see the journalctl using either -r or may be use grep to look for

journalctl |grep "My date is"

You will see an output as under

Oct 13 13:43:42 mymachine.net cat[244214]: My date is: Tue Oct 13 13:43:42 UTC 2020

Others
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ps is a useful utility to show the current active processes snapshot.
ps e and ps -e are distinct
Man page - https://man7.org/linux/man-pages/man1/ps.1.html

Author

Samarthya

Engineer, learner, tinkerer. This is my trophy shelf of everyday work and leisure.

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