[RHCSA] 06 – Deploy, configure, and maintain systems

06.1 – Schedule tasks using at and corn

Overview

Scheduling tasks is a fundamental aspect of system administration. The at command is used for one-time task scheduling, while cron is used for recurring tasks. This guide covers how to use both tools to automate tasks.

Using `at` for One-Time Task Scheduling

Installing `at`

Ensure the at package is installed on your system.

sudo yum install at

Starting the `atd` Service

Start and enable the atd service to ensure it runs at boot.

sudo systemctl start atd

sudo systemctl enable atd

Scheduling a One-Time Task

Use the at command to schedule a one-time task.

Syntax

at <time>

Examples

Schedule a Task for a Specific Time:

echo "backup.sh" | at 02:00

Schedule a Task for a Specific Date and Time:

echo "backup.sh" | at 02:00 2023-10-01

Schedule a Task for a Relative Time:

echo "backup.sh" | at now + 1 hour

Viewing Scheduled `at` Jobs

Use the atq command to view the list of scheduled at jobs.

atq

Sample Output:

1   2023-10-01 02:00 a user

Removing Scheduled `at` Jobs

Use the atrm command to remove a scheduled at job.

atrm <job_id>

Example

atrm 1

Using `cron` for Recurring Task Scheduling

Understanding `cron`

cron is a time-based job scheduler that allows you to run commands or scripts at specified times and intervals.

Editing the Crontab

Use the crontab command to edit the cron table (crontab) for scheduling recurring tasks.

Syntax

crontab -e

Crontab Format

The crontab file consists of lines with the following format:

<minute> <hour> <day_of_month> <month> <day_of_week> <command>

<minute>: Minute (0-59)
<hour>: Hour (0-23)
<day_of_month>: Day of the month (1-31)
<month>: Month (1-12)
<day_of_week>: Day of the week (0-7, where 0 and 7 represent Sunday)
<command>: The command to execute

Examples

Run a Script Every Day at 2 AM:

0 2 * * * /path/to/backup.sh

Run a Script Every Monday at 3 PM:

0 15 * * 1 /path/to/weekly_report.sh

Run a Script Every 15 Minutes:

*/15 * * * * /path/to/check_status.sh

Run a Script on the First Day of Every Month at Midnight:

0 0 1 * * /path/to/monthly_cleanup.sh

Viewing Scheduled `cron` Jobs

Use the crontab -l command to list the current user’s scheduled cron jobs.

crontab -l

Removing Scheduled `cron` Jobs

Edit the crontab file and remove the line corresponding to the job you want to delete.

crontab -e

Practical Examples

Example 1: Schedule a One-Time Task Using `at`

Schedule a Task to Run a Script at 3 PM Today:

echo "/path/to/script.sh" | at 15:00

View Scheduled at Jobs:

atq

Remove a Scheduled at Job:

atrm 1

Example 2: Schedule a Recurring Task Using `cron`

Edit the Crontab:

crontab -e

Add a Job to Run a Script Every Day at 5 AM:

0 5 * * * /path/to/daily_task.sh

View Scheduled cron Jobs:

crontab -l

Remove a Scheduled cron Job:Edit the crontab file and delete the corresponding line.crontab -e

Additional Tips

Check cron Logs:View the cron logs to troubleshoot scheduled jobs.

sudo tail -f /var/log/cron

Use crontab for Specific Users:Edit the crontab for a specific user.

sudo crontab -u username -e

Use Environment Variables in Crontab:Set environment variables in the crontab file.

MAILTO=user@example.com

Run cron Jobs with Specific Shell:Specify the shell to use for cron jobs.

SHELL=/bin/bash

Conclusion

Scheduling tasks using at and cron allows for automation of one-time and recurring tasks, respectively. Mastery of these tools ensures efficient system administration and is a key competency for the RHCSA exam.

These notes should help you understand how to schedule tasks using at and cron for the RHCSA exam.

06.2 – Start and stop services and configure services to start automatically at boot

Overview

Managing services is a fundamental task in system administration. In RHEL, services are managed using systemd and the systemctl command. This guide covers how to start, stop, and configure services to start automatically at boot.

Starting and Stopping Services

Starting a Service

Use the systemctl start command to start a service.

Syntax

sudo systemctl start <service_name>

Example

Start the SSH service:

sudo systemctl start sshd

Stopping a Service

Use the systemctl stop command to stop a service.

Syntax

sudo systemctl stop <service_name>

Example

Stop the SSH service:

sudo systemctl stop sshd

Restarting a Service

Use the systemctl restart command to restart a service.

Syntax

sudo systemctl restart <service_name>

Example

Restart the SSH service:

sudo systemctl restart sshd

Reloading a Service

Use the systemctl reload command to reload a service’s configuration without stopping it.

Syntax

sudo systemctl reload <service_name>

Example

Reload the SSH service:

sudo systemctl reload sshd

Checking the Status of a Service

Use the systemctl status command to check the status of a service.

Syntax

systemctl status <service_name>

Example

Check the status of the SSH service:

systemctl status sshd

Sample Output:

● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2023-10-01 12:00:00 UTC; 1h 30min ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 1234 (sshd)
    Tasks: 1
   Memory: 5.0M
   CGroup: /system.slice/sshd.service
           └─1234 /usr/sbin/sshd -D

Enabling and Disabling Services at Boot

Enabling a Service

Use the systemctl enable command to configure a service to start automatically at boot.

Syntax

sudo systemctl enable <service_name>

Example

Enable the SSH service to start at boot:

sudo systemctl enable sshd

Disabling a Service

Use the systemctl disable command to prevent a service from starting automatically at boot.

Syntax

sudo systemctl disable <service_name>

Example

Disable the SSH service from starting at boot:

sudo systemctl disable sshd

Checking if a Service is Enabled

Use the systemctl is-enabled command to check if a service is enabled to start at boot.

Syntax

systemctl is-enabled <service_name>

Example

Check if the SSH service is enabled:

systemctl is-enabled sshd

Sample Output:

enabled

Practical Examples

Example 1: Start, Stop, and Restart the Apache HTTP Server

Start the Apache HTTP Server:

sudo systemctl start httpd

Check the Status of the Apache HTTP Server:

systemctl status httpd

Restart the Apache HTTP Server:

sudo systemctl restart httpd

Stop the Apache HTTP Server:

sudo systemctl stop httpd

Example 2: Enable and Disable the Firewall Service

Enable the Firewall Service to Start at Boot:

sudo systemctl enable firewalld

Check if the Firewall Service is Enabled:

systemctl is-enabled firewalld

Disable the Firewall Service from Starting at Boot:

sudo systemctl disable firewalld

Check if the Firewall Service is Disabled:

systemctl is-enabled firewalld

Additional Tips

List All Services:Use the systemctl list-units command to list all services.

systemctl list-units --type=service

List All Enabled Services:Use the systemctl list-unit-files command to list all enabled services.

systemctl list-unit-files --type=service | grep enabled

Mask a Service:Use the systemctl mask command to prevent a service from being started manually or automatically.

sudo systemctl mask <service_name>

Unmask a Service:Use the systemctl unmask command to remove the mask from a service.

sudo systemctl unmask <service_name>

Reload Systemd Configuration:Use the systemctl daemon-reload command to reload the systemd manager configuration.

sudo systemctl daemon-reload

Conclusion

Starting, stopping, and configuring services to start automatically at boot are essential tasks for system administration. Mastery of these tasks ensures that services are managed efficiently and reliably, which is crucial for the RHCSA exam.

These notes should help you understand how to start and stop services and configure services to start automatically at boot for the RHCSA exam.

06.3 – Configure systems to boot into a specific target automatically

Overview

In RHEL, systemd uses targets to define different states or modes of operation for the system. Targets are similar to runlevels in the older SysV init system. Configuring the system to boot into a specific target automatically is essential for controlling the system’s behavior at startup.

Common Systemd Targets

graphical.target: Multi-user system with a graphical interface (equivalent to runlevel 5).
multi-user.target: Multi-user system without a graphical interface (equivalent to runlevel 3).
rescue.target: Single-user mode with basic system services (equivalent to runlevel 1).
emergency.target: Minimal environment for emergency maintenance.
reboot.target: Reboots the system.
poweroff.target: Shuts down the system.

Viewing the Current Default Target

Use the systemctl get-default command to view the current default target.

systemctl get-default

Sample Output:

graphical.target

Setting the Default Target

Use the systemctl set-default command to set the default target.

Syntax

sudo systemctl set-default <target>

Examples

Set the Default Target to Multi-User Mode:

sudo systemctl set-default multi-user.target

Set the Default Target to Graphical Mode:

sudo systemctl set-default graphical.target

Verify the Change

After setting the default target, verify the change using the systemctl get-default command.

systemctl get-default

Sample Output:

multi-user.target

Changing the Current Target

To change the current target without rebooting, use the systemctl isolate command.

Syntax

sudo systemctl isolate <target>

Examples

Switch to Multi-User Mode:

sudo systemctl isolate multi-user.target

Switch to Graphical Mode:

sudo systemctl isolate graphical.target

Verify the Change

Check the current target using the systemctl list-units --type=target command.

systemctl list-units --type=target

Sample Output:

UNIT                LOAD   ACTIVE SUB    DESCRIPTION
basic.target        loaded active active Basic System
graphical.target    loaded active active Graphical Interface
multi-user.target   loaded active active Multi-User System

Practical Examples

Example 1: Configure the System to Boot into Multi-User Mode

Set the Default Target to Multi-User Mode:

sudo systemctl set-default multi-user.target

Verify the Default Target:

systemctl get-default

Reboot the System:

sudo reboot

Verify the Current Target After Reboot:

systemctl list-units --type=target

Example 2: Configure the System to Boot into Graphical Mode

Set the Default Target to Graphical Mode:

sudo systemctl set-default graphical.target

Verify the Default Target:

systemctl get-default

Reboot the System:

sudo reboot

Verify the Current Target After Reboot:

systemctl list-units --type=target

Additional Tips

List All Available Targets:Use the systemctl list-unit-files --type=target command to list all available targets.

systemctl list-unit-files --type=target

Check Dependencies of a Target:Use the systemctl list-dependencies <target> command to check the dependencies of a target.

systemctl list-dependencies multi-user.target

Create Custom Targets:Custom targets can be created by defining new unit files in the /etc/systemd/system/ directory.
Emergency and Rescue Targets:Use emergency.target and rescue.target for troubleshooting and maintenance.

sudo systemctl isolate emergency.target
sudo systemctl isolate rescue.target

Conclusion

Configuring systems to boot into a specific target automatically is essential for controlling the system’s behavior at startup. Mastery of these tasks ensures that the system operates in the desired state, which is crucial for the RHCSA exam.

These notes should help you understand how to configure systems to boot into a specific target automatically for the RHCSA exam.

06.4 – Configure time service clients

Overview

Accurate timekeeping is crucial for system operations, logging, and security. In RHEL, chrony is the default NTP (Network Time Protocol) client and server used to synchronize the system clock with remote NTP servers. This guide covers how to configure chrony as a time service client.

Installing Chrony

Ensure the chrony package is installed on your system.

sudo yum install chrony

Configuring Chrony

Step 1: Edit the Chrony Configuration File

The main configuration file for chrony is /etc/chrony.conf.

sudo nano /etc/chrony.conf

Step 2: Configure NTP Servers

Add or modify the server lines to specify the NTP servers you want to use. You can use public NTP servers or your organization’s NTP servers.

Example Configuration

# Use public servers from the pool.ntp.org project.
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst

# Allow NTP client access from local network.
allow 192.168.0.0/16

# Ignore stratum in source selection.
stratumweight 0

# Record the rate at which the system clock gains/losses time.
driftfile /var/lib/chrony/drift

# Enable kernel RTC synchronization.
rtcsync

server <hostname> iburst: Specifies an NTP server to synchronize with. The iburst option speeds up the initial synchronization.
allow <subnet>: Allows NTP client access from the specified subnet.
driftfile: Specifies the location of the drift file.
rtcsync: Enables synchronization of the system clock with the hardware clock.

Step 3: Start and Enable the Chrony Service

Start the chronyd service and enable it to start at boot.

sudo systemctl start chronyd
sudo systemctl enable chronyd

Step 4: Verify the Chrony Service

Check the status of the chronyd service to ensure it is running.

sudo systemctl status chronyd

Sample Output:

● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2023-10-01 12:00:00 UTC; 1h 30min ago
     Docs: man:chronyd(8)
           man:chrony.conf(5)
 Main PID: 1234 (chronyd)
    Tasks: 1
   Memory: 1.5M
   CGroup: /system.slice/chronyd.service
           └─1234 /usr/sbin/chronyd

Monitoring and Managing Chrony

Check Synchronization Status

Use the chronyc tracking command to check the synchronization status.

chronyc tracking

Sample Output:

Reference ID    : 203.0.113.1 (ntp.example.com)
Stratum         : 3
Ref time (UTC)  : Mon Oct  1 13:30:00 2023
System time     : 0.000000123 seconds fast of NTP time
Last offset     : +0.000001234 seconds
RMS offset      : 0.000001234 seconds
Frequency       : 0.123 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.123 ppm
Root delay      : 0.012345 seconds
Root dispersion : 0.012345 seconds
Update interval : 64.0 seconds
Leap status     : Normal

Check NTP Sources

Use the chronyc sources command to list the NTP sources.

chronyc sources

Sample Output:

210 Number of sources = 4
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* ntp1.example.com              2   6    17    64   +0ns[  +0ns] +/-  10ms
^+ ntp2.example.com              2   6    17    64   +0ns[  +0ns] +/-  10ms
^- ntp3.example.com              3   6    17    64   +0ns[  +0ns] +/-  10ms
^- ntp4.example.com              3   6    17    64   +0ns[  +0ns] +/-  10ms

Force a Manual Synchronization

Use the chronyc makestep command to force a manual synchronization.

sudo chronyc makestep

Practical Examples

Example 1: Configure Chrony to Use Public NTP Servers

Edit the Chrony Configuration File:

sudo nano /etc/chrony.conf

Add Public NTP Servers:

server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst

Start and Enable the Chrony Service:

sudo systemctl start chronydsudo systemctl enable chronyd

Verify the Chrony Service:

sudo systemctl status chronyd

Check Synchronization Status:

chronyc tracking

Example 2: Configure Chrony to Use Internal NTP Servers

Edit the Chrony Configuration File:

sudo nano /etc/chrony.conf

Add Internal NTP Servers:

server ntp1.internal.example.com iburst
server ntp2.internal.example.com iburst

Allow NTP Client Access from the Local Network:

allow 192.168.0.0/16

Start and Enable the Chrony Service:

sudo systemctl start chronyd
sudo systemctl enable chronyd

Verify the Chrony Service:

sudo systemctl status chronyd

Check NTP Sources:

chronyc sources

Additional Tips

Synchronize the Hardware Clock:Use the hwclock command to synchronize the hardware clock with the system clock.

sudo hwclock --systohc

Check Chrony Logs:View the chrony logs for troubleshooting.

sudo journalctl -u chronyd

Configure Time Zone:Use the timedatectl command to configure the system time zone.

sudo timedatectl set-timezone America/New_York

Restart Chrony Service:Restart the chronyd service after making configuration changes.

sudo systemctl restart chronyd

Conclusion

Configuring time service clients using chrony ensures accurate timekeeping, which is essential for system operations, logging, and security. Mastery of these tasks is crucial for the RHCSA exam.

These notes should help you understand how to configure time service clients for the RHCSA exam.

06.5 – Install and update software packages from Red Hat Network, a remote repository, or from the local file system

Overview

Managing software packages is a fundamental task in system administration. In RHEL, the yum and dnf package managers are used to install, update, and remove software packages. This guide covers how to install and update packages from Red Hat Network (RHN), remote repositories, and the local file system.

Using `yum` and `dnf`

Installing Packages

From Red Hat Network (RHN) or Remote Repository

Install a Package:

sudo yum install <package_name>
# or
sudo dnf install <package_name>

Example:

sudo yum install httpd

Install Multiple Packages:

sudo yum install <package1> <package2>
# or
sudo dnf install <package1> <package2>

Example:

sudo yum install httpd mariadb-server

From the Local File System

Install a Package from a Local RPM File:

sudo yum install /path/to/package.rpm
# or
sudo dnf install /path/to/package.rpm

Example:

sudo yum install /tmp/vim-enhanced-8.0.1763-15.el8.x86_64.rpm

Updating Packages

Update a Specific Package:

sudo yum update <package_name>
#or
sudo dnf update <package_name>

Example:

sudo yum update httpd

Update All Packages:

sudo yum update
# or
sudo dnf update

Removing Packages

Remove a Package:

sudo yum remove <package_name>
# or
sudo dnf remove <package_name>

Example:

sudo yum remove httpd

Configuring Repositories

Viewing Enabled Repositories

Use the yum repolist or dnf repolist command to view the list of enabled repositories.

sudo yum repolist

sudo dnf repolist

Sample Output:

repo id                   repo name
rhel-8-for-x86_64-baseos  Red Hat Enterprise Linux 8 for x86_64 - BaseOS
rhel-8-for-x86_64-appstream  Red Hat Enterprise Linux 8 for x86_64 - AppStream

Adding a Remote Repository

Create a Repository Configuration File:

sudo nano /etc/yum.repos.d/myrepo.repo

Add Repository Information:

[myrepo]
name=My Custom Repository
baseurl=http://myrepo.example.com/repo
enabled=1
gpgcheck=1
gpgkey=http://myrepo.example.com/repo/RPM-GPG-KEY-myrepo

Save and Exit.

Adding a Local Repository

Create a Repository Configuration File:

sudo nano /etc/yum.repos.d/localrepo.repo

Add Repository Information:

[localrepo]
name=Local Repository
baseurl=file:///mnt/localrepo
enabled=1
gpgcheck=0

Save and Exit.

Enabling and Disabling Repositories

Enable a Repository:

sudo yum-config-manager --enable <repo_id>

sudo dnf config-manager --set-enabled <repo_id>

Example:

sudo yum-config-manager --enable rhel-8-for-x86_64-appstream

Disable a Repository:

sudo yum-config-manager --disable <repo_id>

sudo dnf config-manager --set-disabled <repo_id>

Example:

sudo yum-config-manager --disable rhel-8-for-x86_64-appstream

Practical Examples

Example 1: Install and Update Packages from RHN

Install the Apache HTTP Server:

sudo yum install httpd

Update the Apache HTTP Server:

sudo yum update httpd

Remove the Apache HTTP Server:

sudo yum remove httpd

Example 2: Install a Package from a Local RPM File

Download the RPM File:

wget http://example.com/packages/vim-enhanced-8.0.1763-15.el8.x86_64.rpm -P /tmp

Install the RPM File:

sudo yum install /tmp/vim-enhanced-8.0.1763-15.el8.x86_64.rpm

Example 3: Configure a Remote Repository

Create a Repository Configuration File:

sudo nano /etc/yum.repos.d/myrepo.repo

Add Repository Information:
Save and Exit.

[myrepo]
name=My Custom Repository
baseurl=http://myrepo.example.com/repo
enabled=1
gpgcheck=1
gpgkey=http://myrepo.example.com/repo/RPM-GPG-KEY-myrepo

Install a Package from the Custom Repository:

sudo yum install mypackage

Example 4: Configure a Local Repository

Mount the Local Repository:

sudo mount /dev/sr0 /mnt/localrepo

Create a Repository Configuration File:

sudo nano /etc/yum.repos.d/localrepo.repo

Add Repository Information:

[localrepo]
name=Local Repository
baseurl=file:///mnt/localrepo
enabled=1
gpgcheck=0

Save and Exit.
Install a Package from the Local Repository:

sudo yum install mylocalpackage

Additional Tips

Clean the Yum/DNF Cache:Use the yum clean all or dnf clean all command to clean the package cache.

sudo yum clean all
# or
sudo dnf clean all

List Installed Packages:Use the yum list installed or dnf list installed command to list all installed packages.

yum list installed
# or
dnf list installed

Search for Packages:Use the yum search <keyword> or dnf search <keyword> command to search for packages.

yum search httpd
# or
dnf search httpd

Get Package Information:Use the yum info <package_name> or dnf info <package_name> command to get detailed information about a package.

yum info httpd
# or
dnf info httpd

Conclusion

Installing and updating software packages from Red Hat Network, a remote repository, or the local file system is essential for managing software on RHEL systems. Mastery of these tasks ensures that systems are kept up-to-date and secure, which is crucial for the RHCSA exam.

These notes should help you understand how to install and update software packages from Red Hat Network, a remote repository, or from the local file system for the RHCSA exam.

06.6 – Modify the system bootloader

Overview

The system bootloader is responsible for loading the operating system kernel and initializing the system. In RHEL, GRUB2 (GRand Unified Bootloader version 2) is the default bootloader. This guide covers how to modify GRUB2 settings, including changing the default boot entry, setting kernel parameters, and managing the GRUB2 configuration.

GRUB2 Configuration Files

Main Configuration File: /etc/default/grub
Custom Configuration Files: /etc/grub.d/
GRUB2 Environment Block: /boot/grub2/grubenv

Viewing the Current GRUB2 Configuration

To view the current GRUB2 configuration, you can use the grub2-editenv command:

sudo grub2-editenv list

Sample Output:

saved_entry=0

Modifying GRUB2 Settings

Step 1: Edit the Main Configuration File

The main configuration file for GRUB2 is /etc/default/grub. Open this file in a text editor to make changes.

sudo nano /etc/default/grub

Step 2: Common Modifications

Change the Default Boot Entry

To change the default boot entry, modify the GRUB_DEFAULT parameter. You can set it to a specific menu entry (starting from 0) or to the name of the menu entry.

Example: Set the default boot entry to the second menu entry (index 1):

GRUB_DEFAULT=1

Example: Set the default boot entry to a specific kernel version:

GRUB_DEFAULT="Advanced options for RHEL>RHEL, with Linux 4.18.0-240.el8.x86_64"

Set the GRUB Timeout

To set the timeout for the GRUB menu, modify the GRUB_TIMEOUT parameter. This value is in seconds.

Example: Set the GRUB timeout to 5 seconds:

GRUB_TIMEOUT=5

Add Kernel Parameters

To add kernel parameters, modify the GRUB_CMDLINE_LINUX parameter.

Example: Add the rhgb and quiet parameters:

GRUB_CMDLINE_LINUX="rhgb quiet"

Step 3: Generate the GRUB2 Configuration

After making changes to /etc/default/grub, generate the GRUB2 configuration file using the grub2-mkconfig command.

For BIOS Systems

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

For UEFI Systems

sudo grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

Step 4: Reboot the System

Reboot the system to apply the changes.

sudo reboot

Practical Examples

Example 1: Change the Default Boot Entry

Edit the Main Configuration File:

sudo nano /etc/default/grub

Set the Default Boot Entry to the Second Menu Entry:

GRUB_DEFAULT=1

Generate the GRUB2 Configuration:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot the System:

sudo reboot

Example 2: Set the GRUB Timeout

Edit the Main Configuration File:

sudo nano /etc/default/grub

Set the GRUB Timeout to 10 Seconds:

GRUB_TIMEOUT=10

Generate the GRUB2 Configuration:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot the System:

sudo reboot

Example 3: Add Kernel Parameters

Edit the Main Configuration File:

sudo nano /etc/default/grub

Add the rhgb and quiet Kernel Parameters:

GRUB_CMDLINE_LINUX="rhgb quiet"

Generate the GRUB2 Configuration:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot the System:

sudo reboot

Additional Tips

List Available Boot Entries:Use the awk command to list available boot entries from the GRUB configuration file.

awk -F\' '/menuentry / {print $2}' /boot/grub2/grub.cfg

Set a Persistent Default Boot Entry:Use the grub2-set-default command to set a persistent default boot entry.

sudo grub2-set-default 1

Boot into Rescue Mode:Add the systemd.unit=rescue.target parameter to the kernel command line.

GRUB_CMDLINE_LINUX="systemd.unit=rescue.target"

Boot into Emergency Mode:Add the systemd.unit=emergency.target parameter to the kernel command line.

GRUB_CMDLINE_LINUX="systemd.unit=emergency.target"

Rebuild the GRUB2 Environment Block:Use the grub2-editenv command to rebuild the GRUB2 environment block.

sudo grub2-editenv /boot/grub2/grubenv create

Conclusion

Modifying the system bootloader is essential for controlling the boot process and configuring system behavior at startup. Mastery of these tasks ensures that the system boots correctly and efficiently, which is crucial for the RHCSA exam.

These notes should help you understand how to modify the system bootloader for the RHCSA exam.

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06.1 – Schedule tasks using at and corn

Overview

Using at for One-Time Task Scheduling

Installing at

Starting the atd Service

Scheduling a One-Time Task

Syntax

Examples

Viewing Scheduled at Jobs

Removing Scheduled at Jobs

Example

Using cron for Recurring Task Scheduling

Understanding cron

Editing the Crontab

Syntax

Crontab Format

Examples

Viewing Scheduled cron Jobs

Removing Scheduled cron Jobs

Practical Examples

Example 1: Schedule a One-Time Task Using at

Example 2: Schedule a Recurring Task Using cron

Additional Tips

Conclusion

06.2 – Start and stop services and configure services to start automatically at boot

Overview

Starting and Stopping Services

Starting a Service

Syntax

Example

Stopping a Service

Syntax

Example

Restarting a Service

Syntax

Example

Reloading a Service

Syntax

Example

Checking the Status of a Service

Syntax

Example

Enabling and Disabling Services at Boot

Enabling a Service

Syntax

Example

Disabling a Service

Syntax

Example

Checking if a Service is Enabled

Syntax

Example

Practical Examples

Example 1: Start, Stop, and Restart the Apache HTTP Server

Example 2: Enable and Disable the Firewall Service

Additional Tips

Conclusion

06.3 – Configure systems to boot into a specific target automatically

Overview

Common Systemd Targets

Viewing the Current Default Target

Setting the Default Target

Syntax

Examples

Verify the Change

Changing the Current Target

Syntax

Examples

Verify the Change

Practical Examples

Example 1: Configure the System to Boot into Multi-User Mode

Example 2: Configure the System to Boot into Graphical Mode

Additional Tips

Conclusion

06.4 – Configure time service clients

Overview

Installing Chrony

Configuring Chrony

Step 1: Edit the Chrony Configuration File

Step 2: Configure NTP Servers

Using `at` for One-Time Task Scheduling

Installing `at`

Starting the `atd` Service

Viewing Scheduled `at` Jobs

Removing Scheduled `at` Jobs

Using `cron` for Recurring Task Scheduling

Understanding `cron`

Viewing Scheduled `cron` Jobs

Removing Scheduled `cron` Jobs

Example 1: Schedule a One-Time Task Using `at`

Example 2: Schedule a Recurring Task Using `cron`

Using `yum` and `dnf`