Logging & Troubleshooting

1. How to view all messages generated by the system since the last reboot on RHEL7/CentOS 7?

   • journalctl
   • journalctl | grep ssh

   The boot process, kernel, and all systemd services put messages into the systemd journal.

2. What are 7 different ways of showing the kernel messages?

   • dmesg
   • journalctl -k
   • (Additional options:)
   • cat /var/log/kern.log
   • tail -f /var/log/syslog
   • less /var/log/messages
   • grep kernel /var/log/messages
   • sudo find /var/log -name 'kern*'

3. How can you continuously monitor logs as they come in?

   • journalctl
   • tail -f /var/log/messages

   You can use tail -f /var/log/messages to continuously monitor the messages log file.

4. Where can you find messages related to the installation of Linux?

   • anaconda.log
   • /var/log/anaconda/anaconda.log

   Messages related to the installation of Linux can be found in the anaconda.log file or /var/log/anaconda/anaconda.log.

5. Where are most of the log files located?

   • /var/log

   Most of the log files are located in the /var/log directory.

6. How to check system load without the top command?

   • w
   • uptime

7. By default, load average is shown in how many intervals?

   • 1, 5, and 15-minute intervals.
   • glances shows load average by time.

8. How can you get the physical and virtual memory statistics?

   • free -m or -g
   • vmstat -a (active and inactive memory), d (disk stats)
   • vmstat -a 2 4 (every 2 seconds with 4 intervals)

9. How to check CPU utilization and other statistics?

   • Use sar, which is part of the sysstat package.
   • sar -u shows CPU utilization stats for the current day.
   • sar -u 2 3 shows real-time CPU stats every 2 seconds with 3 intervals.
   • sar -r for memory utilization.
   • sar -S for swap space.

10. How to find the process ID of a process and kill it immediately?

    • Ex. Start cat command and then:
    • pidof cat to find the process ID.
    • kill -9 <PID> to kill the process. -9 is the priority.

11. How to list all open files by a specified user?

    • lsof -u <user>

12. How to list all files opened by a particular command?

    • lsof -c <command>

13. How can you list all network connections by port 22?

    • lsof -i :22

14. How to check scheduled jobs?

    • crontab -l

15. If your cron job didn't work, how would you check?

    1. First check your system time,
    2. then crontab entry,
    3. then check /var/log/messages

16. How to check open port on Linux system?

    • netstat putan | grep :22

17. How to check the IP of the VM

    • hostname -I