(RHCSA) Operate Running Systems

This is part of the independent and unofficial RHCSA Exam Study Guide series providing free 🤑 resources to prepare for the exam.

This post covers the objectives under the section:

“OPERATE RUNNING SYSTEMS”

It contains two main parts:

• Resources: with links to learn and practice for each objective.
• Cheatsheet: containing some examples of commands and actions performed in each objective (when applied).

📚 Resources:

BOOT, REBOOT, AND SHUT DOWN A SYSTEM NORMALLY

• 📌 RHCSA v8 Practice Session: Boot, reboot, and shut down a system normally

• 📌 Chapter 16. Shutting down, suspending, and hibernating the system

• 📌 How Boot, Reboot, and Shut Down the System Normally on CentOS 7 / RHEL 7

BOOT SYSTEMS INTO DIFFERENT TARGETS MANUALLY

• 📌 RHCSA v8 Practice Session: Boot systems into different targets manually

• 📌 Chapter 15. Working with systemd targets

INTERRUPT THE BOOT PROCESS IN ORDER TO GAIN ACCESS TO A SYSTEM

• 📌 Reset the Root Password in FreePBX and RHEL-based Linux distros

• 📌 25.3. Resetting the root password on boot

IDENTIFY CPU/MEMORY INTENSIVE PROCESSES AND KILL PROCESSES

• 📌 RHCSA v8 Practice Session: Identify CPU/memory intensive processes and kill processes

• 📌 Linux Process States and Signals

• 📌 How To Use ps, kill, and nice to Manage Processes in Linux

• 📌 Linux Command Basics: 7 commands for process management

ADJUST PROCESS SCHEDULING

• 📌 Learning Linux Commands: nice & renice

• 📌 How to Set Linux Process Priority Using nice and renice Commands

• 📌 Linux commands: How to manipulate process priority

• 📌 Automate your Linux system tasks with cron

• 📌 24.1. Scheduling a Recurring Job Using Cron

MANAGE TUNING PROFILES

• 📌 RHCSA v8 Practice Session: Manage tuning profiles

• 📌 2. Getting started with Tuned (skip items: 2.4, 2.8 and 2.9)

LOCATE AND INTERPRET SYSTEM LOG FILES AND JOURNALS

• 📌 RHCSA v8 Practice Session: Locate and interpret system log files and journals

• 📌 Beginners Guide to CentOS/RHEL logging (systemd-journald and rsyslog) architecture)

• 📌 Overview of Syslog Priorities, rsyslog rules, log rotation and basic troubleshooting using syslog

• 📌 How to use journalctl to view log messages in journal for system troubleshooting

• 📌 Chapter 10. Troubleshooting problems using log files

PRESERVE SYSTEM JOURNALS

• 📌 RHCSA v8 Practice Session: Preserve system journals

• 📌 How to configure the system journal to preserve the record of events when a server is rebooted (CentOS/RHEL)

• 📌 23.10.5. Enabling Persistent Storage

START, STOP, AND CHECK THE STATUS OF NETWORK SERVICES

• 📌 RHCSA RHEL 8 - Start, stop, and check the status of network services

• 📌 Chapter 14. Managing system services with systemctl (skip items 14.2. and 14.5)

SECURELY TRANSFER FILES BETWEEN SYSTEMS

• 📌 RHCSA v8 Practice Session: Securely transfer files between systems

• 📌 10 Practical Examples of Using scp Command

• 📌 How to securely copy files between Linux hosts using SCP and SFTP

📑 Cheatsheet:

BOOT, REBOOT, AND SHUT DOWN A SYSTEM NORMALLY

• systemctl reboot ➡ shut down and reboot the system.

• systemctl halt ➡ shut down and halt the system (stops CPU but keeps it powered).

• systemctl poweroff ➡ shut down and poweroff the system (stops CPU and turns off the power).

BOOT SYSTEMS INTO DIFFERENT TARGETS MANUALLY

• systemctl get-default ➡ get the default boot target.

• systemctl set-default multi-user ➡ set the default boot target.

• systemctl --type target --all ➡ list all target units (loaded or not).

• Interrupt boot process to temporarily boot from another target (i.e.: multi.user.target), without changing the default target:

  • append systemd.unit=multi-user.target ➡ to the end of the boot cmd, when editing the grub menu entry, and start it with Ctrl+x.

INTERRUPT THE BOOT PROCESS IN ORDER TO GAIN ACCESS TO A SYSTEM

• Reset root pwd in 7 steps:

  • append rd.break ➡ to the end of the boot cmd, when editing the grub menu entry, and start it with Ctrl+x.

  • chroot /sysroot ➡ make the mounted (read-only) root file system (/sysroot) appear as mounted on /.

  • mount -o remount,rw / ➡ remount root file system in read/write mode for the passwd cmd to be able to modify the shadow file with a new pwd.

  • passwd ➡ change password for the root user.

  • touch .autorelabel ➡ create .autorelabel file to instruct the OS to run SELinux relabeling on all files on the next reboot (including the shadow that was updated).

  • exit ➡ the chroot shell.

  • reboot ➡ the system.

  • Extra Step: mandb ➡ after restart, rebuild manual pages db.

• Install most recent kernel, without removing/updating previous kernel (7 files):

  • linux-firmware
  • kernel
  • kernel-core
  • kernel-headers
  • kernel-modules
  • kernel-tools
  • kernel-tools-libs

• dnf list installed kernel* ➡ list installed kernels.

• cat /boot/grub2/grubenv ➡ check if new installed kernel is default boot option.

IDENTIFY CPU/MEMORY INTENSIVE PROCESSES AND KILL PROCESSES

• tar -czf home.tar.gz . & ➡ start tar as background process in the current shell.

• jobs ➡ list backgrund processes in the current shell.

• ps -aux ➡ list every process in the system.

• ps -eo pid,nice,cmd | grep top ➡ show pid and nice for the top ➡ running process.

• kill -sigstop 3251 ➡ send stop signal to process.

ADJUST PROCESS SCHEDULING

• nice -n +8 top ➡ run top cmd with a lower priority (by 8).

• renice -n -10 -p 3261 ➡ increase the priority of the 3261 running proccess to -10.

• crontab -e ➡ edit user crontab.

• 23 8 * * * echo "Hello World" > /dev/pts/0 ➡ in a crontab file, this entry will make the terminal /dev/pts/0 print the message everyday at 8:23.

• cat /var/log/cron ➡ cron execution logs.

MANAGE TUNING PROFILES

• tuned-adm profile, tuned-adm list➡ list all available system tuning profiles.

• tuned-adm active ➡ show current system tuning profile.

• tuned-adm recommend ➡ list recommended system tuning profile.

• tuned-adm profile balanced ➡ set system profile to balanced.

LOCATE AND INTERPRET SYSTEM LOG FILES AND JOURNALS

• cat /etc/rsyslog.conf ➡ see directories defined to save the log files generated by rsyslog.

• tail -f /var/log/secure ➡ live view of the secure log file.

• tail /var/log/audit/audit.log ➡ view SELinux related logs.

• journalctl --since 10:00 ➡ view journal logs from 10:00 until now.

• journalctl --unit sshd.service -o verbose ➡ view journal logs related to sshd.service displaying additional parameters.

• journalctl --priority crit ➡ view critial priority logs.

• logger "$LOGNAME is delivering this message at $(date)" ➡ write custom log message to /var/log/messages.

PRESERVE SYSTEM JOURNALS

• Preserve system journals:

  • mkdir -p /var/log/journal
  • systemctl restart systemd-journald

• journalctl -b -1 ➡ view log messages from previous boot.

• item ➡ description.

START, STOP, AND CHECK THE STATUS OF NETWORK SERVICES

• systemctl status sshd.service ➡ verify if sshd general status.

• systemctl enable sshd.service ➡ make sshd start at system startup.

• systemctl disable sshd.service ➡ make sshd do not start at system startup.

• systemctl start sshd.service ➡ make sshd start temporarily (this system do not persist reboots).

• systemctl disable sshd.service ➡ make sshd stop temporarily (this setting do not persist reboots).

SECURELY TRANSFER FILES BETWEEN SYSTEMS

• scp srv2:/etc/chrony.conf /tmp ➡ get file from srv2.

• scp -r /etc/sysconfig srv2:/tmp ➡ transfer entire directory to srv2.

• scp -p file1.txt srv2:/tmp ➡ transfer file to srv2 /tmp directory preserving file attributes (permissions, etc).

Next:

(RHCSA) Configure Local Storage

…or back to Red Hat Certified System Administrator (RHCSA) Exam Study Guide

Useful links & references:

• Red Hat Enterprise Linux Documentation

• Enable Sysadmin (a blog from Red Hat)

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