Linux Commands Archive

If you ever need a quick reminder of the commands to check things on Linux systems, this post has you covered. Hope it helps you find what you’re looking for! I’ll keep this post updated — promise! The article covers commands mostly used in RHEL. – hope that does not matter. :/

Organized as:

1. Basic Commands

2. Essential File Management Commands

3. Text Management Commands

4. Essential System Management Commands

5. User and Group Management

6. Permissions Management

7. Configure Networking

8. Commands used to Manage Softwares

9. Managing Processes

10. Working with Systemd

11. Scheduling Tasks

12. Configuring Logging

13. Managing Storage

14. Managing Advanced Storage

15. Basic Kernal Management

16. Managing the Boot Process

17. Essential Troubleshooting Commands

18. Automation with Bash Shell Scripting

19. Configuring SSH

20. Managing Apache HTTP Services

21. Managing SELinux

22. Configuring Firewall

23. Accessing Network Storage

24. Configuring Time Services

25. Managing Containers

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1. Basic Commands

Command	Arguments	Use Case / Description	Important Notes
type	pwd	Finds out whether a command is a Bash internal command or executable file
which	ls	Finds out where the shell will get a command from
time	ls	Executes command and shows time information
/usr/bin/time	ls	Runs the external time command (different from internal)
echo	$PATH	Shows contents of PATH variable
ls	> /dev/null	Redirects STDOUT to null device
ls ilwehgi	2> /dev/null	Redirects STDERR to null device
ls ilwehgi /etc	2> /dev/null > output	Sends errors to /dev/null and output to file
echo hello	> output	Overwrites file contents
ls	>> output	Appends output to file
ls -R /	| less	Shows recursive directory listing in pager
ls	> /dev/tty1	Redirects output to device file (requires root)
history	(none)	Shows command history
\!number	(e.g., \!31)	Executes command with specific number from history
history	-d number	Deletes specific command from history
history	-c	Clears current history
history	-w	Writes current history to history file
vim	~/testfile	Opens file in vim editor
env	(none)	Shows current environment variables
echo	$LANG	Reads value of LANG variable
LANG=es_ES.UTF-8	(variable assignment)	Temporarily sets language to Spanish
man	man	Opens man page of man command
man	apropos	Opens man page of apropos
sudo	mandb	Updates mandb database as root	makewhatis command used in previous RHEL versions
pinfo	‘(coreutils) ls invocation’	Shows info page for ls command

Used Scenarios with Q/A:

Questions	Answers
Create a file named “practice.txt” and redirect the output of ls -l to it	ls -l > practice.txt
Append the current date to “practice.txt” without overwriting existing content	date >> practice.txt
Use a pipe to count how many lines are in /etc/passwd	cat /etc/passwd | wc -l
Create a shell variable named COLOR with value “blue” and display it	COLOR=”blue” && echo $COLOR
Use vim to create a file with 10 lines of text and save it as “testfile”	vim testfile (press i, type 10 lines, Esc, :wq)
In vim, delete lines 3-5 from “testfile”	In vim: :3,5d then :wq
In vim, replace all occurrences of “old” with “new” in the entire file	In vim: :%s/old/new/g then :wq
Use history to find and re-execute the last command that contained “ls”	Ctrl+R then type ls
Clear your entire command history	history -c && history -w
Use man to find which option for ls shows file sizes in human-readable format	man ls shows -h option
Search the man pages for commands related to “partition”	man -k partition or apropos partition
Use grep to find all lines in /etc/passwd that contain “root”	grep root /etc/passwd
Use grep to find lines in /etc/passwd that do NOT contain “nologin”	grep -v nologin /etc/passwd
Create an alias called “ll” that runs “ls -l” and make it permanent	alias ll=’ls -l’ and add to ~/.bashrc
Redirect both standard output and standard error of a command to /dev/null	command > /dev/null 2>&1
Use tab completion to quickly type a long filename	Type partial filename and press Tab
Display the first 5 lines of /etc/passwd	head -5 /etc/passwd
Display the last 3 lines of /etc/passwd	tail -3 /etc/passwd
Follow (monitor) new entries in /var/log/messages in real-time	tail -f /var/log/messages
Find which command would be executed when you type “ls”	which ls or type ls

2. Essential File Management Commands

Command	Arguments	Use Case / Description	Important Notes
mount	(none)	Shows all mounted devices
df	-hT	Shows disk space with human-readable format and filesystem type
cd	(none)	Changes to home directory
touch	file1	Creates empty file
cd	/	Changes to root directory
cd	/tmp	Changes to /tmp directory
mkdir	files	Creates directory (relative path)
mkdir	/home/$USER/files	Creates directory (absolute path)
rmdir	files	Removes empty directory
pwd	(none)	Shows current directory
ls	-l	Long listing with file properties
ls	-a	Shows all files including hidden
ls	-lrt	Shows files sorted by modification time (newest last)
ls	-d	Shows directory names, not contents
ls	-R	Recursively shows directory contents
touch	.hidden	Creates hidden file
cp	/etc/hosts /tmp	Copies file to directory
cp	-R /etc /tmp	Recursively copies directory
cp	-a ~ /tmp	Archives copy preserving all attributes
cp	-a /somedir/.\* /tmp	Copies hidden files (gives errors for directories)
cp	-a /somedir/ .	Copies entire directory to current directory
cp	-a /somedir/. .	Copies all files (regular and hidden) to current directory
mv	myfile /tmp	Moves file to directory
mkdir somefiles; mv somefiles /tmp	(compound command)	Creates then moves directory
mv	myfile mynewfile	Renames file
rm	-rf newfiles	Recursively and forcefully removes directory
ln	/etc/hosts .	Creates hard link to file
ln	-s /etc/hosts .	Creates symbolic link to file
ln	-s /home /tmp	Creates symbolic link to directory
\ls	-l	Shows ls output without alias (shows link properties)
tar	cvf etc.tar /etc	Creates tar archive
file	etc.tar	Analyzes file type
gzip	etc.tar	Compresses file
tar	tvf etc.tar.gz	Shows contents of compressed tar archive
tar	xvf etc.tar.gz etc/hosts	Extracts specific file from archive
gunzip	etc.tar.gz	Decompresses file
tar	xvf etc.tar -C /tmp etc/passwd	Extracts file to specific directory
tar	cjvf homes.tar /home	Creates bzip2 compressed archive
rm	-f \*gz \*tar	Removes all gz and tar files

Used Scenarios with Q/A:

Questions	Answers
/home/student/test/dir1/dir2	mkdir -p /home/student/test/dir1/dir2
/etc/hosts to your home directory preserving all attributes	cp -a /etc/hosts ~/
/tmp	mv ~/hosts /tmp/
/etc/passwd in your home directory	ln /etc/passwd ~/passwd_link
/etc in your home directory	ln -s /etc ~/etc_link
all files in /etc that start with ‘pass’	ls -d /etc/pass*
all files in /etc that are exactly 3 characters long	find /etc -maxdepth 1 -name ‘???’
a tar archive of your home directory compressed with gzip	tar -czf home_backup.tar.gz ~/
the tar archive to /tmp	tar -xzf home_backup.tar.gz -C /tmp
the contents of the tar archive without extracting it	tar -tzf home_backup.tar.gz
the test directory and all its contents	rm -rf /home/student/test
all files in /var/log that were modified in the last 2 days	find /var/log -mtime -2
all .conf files from /etc to /tmp	cp /etc/*.conf /tmp/
disk usage of your home directory in human-readable format	du -h ~/
the filesystem type used for your root partition	df -T /
a file with spaces in the name and practice manipulating it	touch ‘file with spaces.txt’ and mv ‘file with spaces.txt’ /tmp/
relative pathnames to navigate the directory structure	cd ../somedir or cd ../../otherdir
absolute pathnames to perform operations from any location	cp /etc/hosts /tmp/ (from any location)
available disk space on all mounted filesystems	df -h
the inode number of a file	ls -i filename

3. Text Management Commands

Command	Arguments	Use Case / Description	Important Notes
less	/etc/passwd	Opens file in pager for easy reading
ps aux	| less	Sends command output to pager
cat	/etc/passwd	Dumps file contents to screen
tac	(mentioned)	Shows file contents in reverse order
head	-n 5 /etc/passwd	Shows first 5 lines of file
tail	-n 2 /etc/passwd	Shows last 2 lines of file
tail	-5 /etc/passwd	Shows last 5 lines (alternative syntax)
tail	-f /var/log/messages	Follows (monitors) file in real time
head	-n 11 /etc/passwd | tail -n 1	Shows only line 11 of file
cut	-d : -f 1 /etc/passwd	Filters first field using colon delimiter
sort	/etc/passwd	Sorts file contents
cut -f 1 -d : /etc/passwd	| sort	Sorts first column of file
cut -f 3 -d : /etc/passwd	| sort -n	Sorts third field numerically
du -h	| sort -rn	Sorts disk usage with biggest files first
sort	-k3 -t : /etc/passwd	Sorts third column using colon delimiter
ps aux	| sort -k 4 -n	Sorts processes by memory usage
ps aux	| wc	Counts lines, words, characters in output
grep	anna /etc/passwd	Searches for text in file
grep	^anna /etc/passwd	Searches for lines starting with text
grep	ash$ /etc/passwd	Searches for lines ending with text
grep	‘^anna’ /etc/passwd	Uses escaping for regular expressions
grep	-E ‘b.\+t’ regex.txt	Uses extended regular expressions
grep	‘ \#’ /etc/services	Searches for commented lines
grep	-v ‘^\#’ /etc/services	Shows lines NOT starting with #
grep	-v ‘^\#’ /etc/services -B 5	Shows matching lines plus 5 before
grep	-v -e ‘^\#’ -e ‘^$’ /etc/services	Excludes comments and blank lines
awk	-F : ‘{ print $4 }’ /etc/passwd	Prints fourth field
awk	-F : ‘/user/ { print $4 }’ /etc/passwd	Searches for text and prints field
sed	-n 5p /etc/passwd	Prints fifth line of file
sed	-i s/old-text/new-text/g ~/myfile	Replaces text in file
sed	-i -e ‘2d’ ~/myfile	Deletes second line from file
sed	-i -e ‘2d;20,25d’ ~/myfile	Deletes lines 2 and 20-25

Used Scenarios with Q/A:

Questions	Answers
Use grep to find all lines in /etc/services that contain “http”	grep http /etc/services
Use grep with regex to find lines starting with “ftp” in /etc/services	grep ‘^ftp’ /etc/services
Use cut to display only the usernames from /etc/passwd	cut -d: -f1 /etc/passwd
Use sort to sort /etc/passwd by the third field numerically	sort -t: -k3 -n /etc/passwd
Count how many user accounts can login with a shell in /etc/passwd	grep -c ‘/bin/bash\|/bin/sh’ /etc/passwd
Use awk to print the first and last fields of /etc/passwd	awk -F: ‘{print $1, $NF}’ /etc/passwd
Use sed to replace “root” with “admin” in a test file	sed -i ‘s/root/admin/g’ testfile
Display lines 10-20 of /etc/passwd using head and tail	head -20 /etc/passwd | tail -11
Find all files in /etc that contain the word “localhost”	grep -r localhost /etc/ 2>/dev/null
Use regular expression to find IP addresses in a file	grep -E ‘([0-9]{1,3}\.){3}[0-9]{1,3}’ file
Sort the output of ps aux by memory usage	ps aux –sort=-%mem
Filter ps aux to show only processes owned by root	ps aux | grep ‘^root’
Use wc to count words in /etc/hosts	wc -w /etc/hosts
Compare two files and show differences	diff file1 file2
Use less to search for text in a large file	less /var/log/messages then /searchterm
Extract specific columns from command output using cut	ps aux | cut -c 1-80 (adjust columns as needed)
Use awk to calculate the total memory used by all processes	ps aux | awk ‘{sum += $4} END {print sum}’
Use sed to delete empty lines from a file	sed -i ‘/^$/d’ file
Create a file with multiple columns and practice sorting by different columns	Create file with: echo -e “3 apple\n1 banana\n2 cherry” > test.txt then sort -k2 test.txt
Use grep with context options to show lines before and after a match	grep -A2 -B2 searchterm file

4. Essential System Management Commands

Command	Arguments	Use Case / Description	Important Notes
sudo	-i	Opens root shell (for authorized users)
su	–	Opens root shell (requires root password)
w	(none)	Shows logged in users and their terminals
chvt	4	Switches to virtual terminal 4
systemctl	reboot	Reboots system properly
systemctl	halt	Halts system
systemctl	poweroff	Powers off system
echo b	> /proc/sysrq-trigger	Emergency reset (use only as last resort)
systemctl	status sshd	Checks if SSH service is running
ip a	| grep ‘inet ‘	Shows IP addresses
ssh	root@192.168.4.220	Connects to remote server as root
ssh	-Y linda@server2	Connects with X11 forwarding for GUI apps
ssh	-v	Verbose mode for debugging connections
ssh	-p port user@host	SSH connection on non-default port
scp	/etc/hosts server2:/tmp	Securely copies file to remote server
scp	root@server2:/etc/passwd ~	Copies file from remote server
scp	-r server2:/etc/ /tmp	Recursively copies directory
scp	-P port file user@host:/path	SCP on non-default port (uppercase P)
sftp	student@server2	Opens secure FTP session
rsync	-r	Synchronizes recursively
rsync	-l	Copies symbolic links as links
rsync	-p	Preserves permissions
rsync	-n	Dry run (no actual sync)
rsync	-a	Archive mode (preserves all)
rsync	-A	Archive mode plus ACLs
rsync	-X	Synchronizes SELinux context
ssh-keygen	(none)	Generates SSH key pair
ssh-copy-id	server2	Copies public key to remote server
sed	-i -e ’25d’ ~/.ssh/known_hosts	Removes line 25 from known_hosts file
tail	-f /var/log/secure	Monitors security logs in real-time

Used Scenarios with Q/A:

Questions	Answers
Generate SSH key pair without passphrase	ssh-keygen -t rsa -b 2048 -N “” -f ~/.ssh/id_rsa
Copy your public key to another server	ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote-server
Connect to another server using SSH without password	ssh -i ~/.ssh/id_rsa user@remote-server
Use scp to copy a file to a remote server	scp file.txt user@remote-server:/path/
Use scp to copy a directory recursively to a remote server	scp -r directory/ user@remote-server:/path/
Use sftp to transfer files interactively	sftp user@remote-server then use commands: put localfile, get remotefile, ls, exit
Configure SSH to use a different port	Edit /etc/ssh/sshd_config: change #Port 22 to Port 2222, then systemctl restart sshd
Restrict SSH root login in SSH configuration	Edit /etc/ssh/sshd_config: change PermitRootLogin yes to PermitRootLogin no
Use rsync to synchronize directories between two servers	rsync -av /local/dir/ user@remote-server:/remote/dir/
Set up SSH X11 forwarding for graphical applications	ssh -X user@remote-server then run graphical commands
Switch between virtual terminals using keyboard shortcuts	Use Ctrl+Alt+F2 through Ctrl+Alt+F6 for terminals, Ctrl+Alt+F1 for GUI
Reboot the system properly using systemctl	systemctl reboot or reboot
Shut down the system properly using systemctl	systemctl poweroff or poweroff
Monitor who is logged into the system and what they’re doing	w or who
Force kill a frozen SSH session	~. (tilde followed by period) in SSH session or find PID with ps aux | grep ssh then kill PID
Configure SSH to use key authentication only (disable passwords)	Edit /etc/ssh/sshd_config: set PasswordAuthentication no and PubkeyAuthentication yes
Set up an SSH tunnel for port forwarding	ssh -L 8080:localhost:80 user@remote-server
Use w to see current system users and their activities	Edit ~/.ssh/config: Host myserver Hostname server.example.com User myuser Port 2222 IdentityFile ~/.ssh/id_rsa
Configure SSH client options in ~/.ssh/config	ssh -v user@remote-server
Troubleshoot SSH connection issues	Check: systemctl status sshd, ss -tlnp | grep :22, firewall-cmd –list-all

5. User and Group Management

Command	Arguments	Use Case / Description	Important Notes
useradd	username	Creates a new user account
useradd	-m -c “User Comment” username	Creates user with home directory and comment
useradd	-s /bin/bash username	Creates user with specified shell
useradd	-m -u UID -G groups username	Creates user with home dir, UID, and groups
usermod	-aG groupname username	Adds user to supplementary group
usermod	-L username	Locks a user account
usermod	-U username	Unlocks a user account
userdel	-r username	Deletes user and their home directory
groupadd	groupname	Creates a new group
groupmod	-n newname oldname	Renames a group
groupdel	groupname	Deletes a group
passwd	username	Sets or changes user password
passwd	-l username	Locks user’s password
passwd	-u username	Unlocks user’s password
passwd	-n min -w warn -x max username	Sets password aging policies
chage	-l username	Lists password aging information
chage	-M 90 username	Sets maximum password age to 90 days
chage	-E YYYY-MM-DD username	Sets account expiration date
id	username	Displays user and group information
su	– username	Switches to another user’s environment
sudo	command	Executes command as another user (typically root)
visudo	(none)	Edits sudoers file safely
vipw	(none)	Safely edit /etc/passwd file
vigr	(none)	Safely edit /etc/group file
groups	username	Shows user’s group memberships
newgrp	groupname	Changes effective primary group temporarily
groupmems	-g sales -l	Lists members of a specific group

Used Scenarios with Q/A:

Questions	Answers
Create a new user account with home directory	useradd -m -c “Test User” testuser
Create a new group called “developers”	groupadd developers
Add a user to the “developers” group	usermod -aG developers testuser
Set password aging policies for a user account	chage -M 90 -m 7 -W 14 testuser
Create a user without a home directory	useradd -M -s /sbin/nologin systemuser
Delete a user account and their home directory	userdel -r testuser
Modify a user’s login shell	usermod -s /bin/csh username
Change a user’s primary group	usermod -g developers username
Lock and unlock a user account	passwd -l username and passwd -u username
Configure sudo access for a user to run all commands	visudo add: username ALL=(ALL) ALL
Configure sudo to allow a user to run specific commands only	visudo add: username ALL=(ALL) NOPASSWD: /bin/systemctl
Set up passwordless sudo for specific commands	visudo add: username ALL=(ALL) NOPASSWD: /usr/bin/dnf
Check which groups a user belongs to	id username or groups username
Set default user creation parameters	useradd -D to view, edit /etc/default/useradd
Modify user account expiration date	chage -E 2024-12-31 username
Create a system account (no login, no home directory)	useradd -r -s /sbin/nologin systemaccount
Change user’s UID	usermod -u 1500 username
Change group’s GID	groupmod -g 1500 groupname
Force user to change password on next login	chage -d 0 username
Backup and restore user home directories	tar -czf user_backup.tar.gz /home/username and tar -xzf user_backup.tar.gz -C /

6. Permissions Management

Command	Arguments	Use Case / Description	Important Notes
chmod	755 file	Sets rwx for owner, rx for group and others
chmod	u+x file	Adds execute permission for owner
chmod	g-w file	Removes write permission for group
chmod	o=r file	Sets others’ permissions to read only
chmod	-R 755 directory	Recursively sets permissions on directory
chmod	g+w,o-r file	Multiple permission changes in one command
chmod	-R a+X dir	Recursively adds execute for directories only
chmod	u+s file	Sets SUID bit (run as owner)
chmod	g+s dir	Sets SGID bit on directory (inherit group)
chmod	+t dir	Sets sticky bit (only owner can delete)
chown	user:group file	Changes owner and group of file
chown	-R user:group directory	Recursively changes owner and group
chown	user file	Changes file owner only
chown	:group file	Changes file group only
chgrp	group file	Changes group of file
umask	022	Sets default file creation mask
umask	(none)	Shows current umask setting
setfacl	-m u:user:rw file	Adds ACL for user on file
setfacl	-m g:group:r file	Adds ACL for group on file
setfacl	-x u:user file	Removes ACL for user from file
getfacl	file	Displays ACL of file
chattr	+i file	Makes file immutable
chattr	-i file	Removes immutable attribute
lsattr	file	Lists file attributes
find	/ -user linda	Finds all files owned by specific user
find	/ -group users	Finds all files owned by specific group

Used Scenarios with Q/A:

Question	Answer
Set read, write, execute permissions for owner on a file using symbolic notation	chmod u+rwx filename
Set permissions using octal notation (755, 644, etc.)	chmod 755 filename (owner: rwx, group: r-x, others: r-x)
Change file ownership to a different user	chown newuser filename
Change file group ownership	chgrp newgroup filename
Set the setuid bit on an executable file	chmod u+s filename
Set the setgid bit on a directory	chmod g+s directory
Set the sticky bit on a directory	chmod +t directory
Set default file creation permissions using umask	umask 022 (files: 644, directories: 755)
Set ACL to give specific user read access to a file	setfacl -m u:username:r filename
Set ACL to give specific group write access to a directory	setfacl -m g:groupname:w directory
Remove specific ACL entries from a file	setfacl -x u:username filename
Set default ACL on a directory	setfacl -d -m g:groupname:rw directory
Check current permissions and ownership of files	ls -la filename
Troubleshoot permission denied errors	Check: ls -la, getfacl filename, verify user/group membership with id username
Recursively change permissions on a directory and its contents	chmod -R 755 directory/
Find all setuid files on the system	find / -perm -4000 2>/dev/null
Find all world-writable files	find / -perm -o=w 2>/dev/null
Configure permissions to allow group collaboration on files	chmod g+rw shared_file and chgrp developers shared_file
Reset permissions to default values	chmod 644 file and chmod 755 directory (common defaults)
Verify and fix SELinux context on files	restorecon -R /path/ for SELinux context

7. Configure Networking

Command	Arguments	Use Case / Description	Important Notes
ip	addr show	Shows IP addresses and network interfaces
ip	addr add 192.168.1.100/24 dev eth0	Adds IP address to interface	Every change you do using ip is not persistent therefore, use nmtui or nmcli
ip	route show	Shows routing table
ip	route add default via 192.168.1.1	Adds default route
ip	link show	Shows network link status
ip	-s link show	Shows link statistics
nmcli	con show	Shows NetworkManager connections
nmcli	con show ens33	Shows all properties of the ens33 connection
nmcli	con add con-name “MyConn” type ethernet ifname eth0	Adds new connection	Example 1 (dhcp) : nmcli con add con-name testconn type ethernet ifname ens33 ipv4.method auto Example 2 (static) : nmcli con add con-name testconn1 ifname ens33 autoconnect no type ethern et ipv4.addresses 192.168.199.199/24 gw4 192.168.199.2 ipv4.method manual
nmcli	con mod “MyConn” ipv4.addresses 192.168.1.100/24	Modifies connection’s IP address	Example 1 (Change Autoconnect) : nmcli con mod testconn connection.autoconnect no Example 2 (Change DNS) : nmcli con mod testconn ipv4.dns 1.1.1.1 Example 3 (Add Second Item) : nmcli con mod testconn +ipv4.dns 8.8.4.4 nmcli con mod testconn1 +ipv4.addresses 192.168.199.200/24
nmcli	con up “MyConn”	Activates a connection
nmcli	dev status	Shows network device status
nmcli	dev show devicename	Shows settings for specific device
nmcli	general permissions	Shows current network configuration permissions
nmcli	connection reload en33	Reload connection files from disk related to en33
nmtui	(none)	Text-based UI for NetworkManager
nm-connection-editor	(none)	Opens the GUI network connection editor
ss	-tuln	Shows listening TCP and UDP ports
ss	-lt	Shows listening TCP ports
netstat	(none)	Shows active connections	Replaced by ss command, and this command is mostly obsolete
ping	-c 4 192.168.1.1	Sends 4 ICMP echo requests to host
ping	8.8.8.8	Tests network connectivity
hostnamectl	set-hostname server1.example.com	Sets system hostname	Configuration file location: /etc/hostname
hostnamectl	status	Shows hostname and system information
getent	hosts servername	Verifies hostname resolution
cat	/etc/NetworkManager/system-connections/ ens160.nmconnection	Views NetworkManager Connection File	In previous RHEL versions thy was under: /etc/sysconfig/network-scripts
systemctl	status NetworkManager	Shows NetworkManager service status
man	5 nm-settings	Shows man pages of how to do configuration using nmcli
man	nmcli-examples	Shows examples of nmcli	You do not need to remember the arguemtns to configure network settings – use this man pages

Configuration File Path	Use Case	Important Notes
/etc/NetworkManager/system-connections/	NetworkManager Connection File – contains IP information
/etc/sysconfig/network-scripts	Contains IP information – Used in old RHEL versions	NetworkManager uses the following directory: /etc/NetworkManager/system-connections/
/etc/hostname	Contains hostname information
/etc/hosts	Contains DNS information
/etc/nsswitch.conf	Controls how and in what order system databases (like hosts, passwd, group) are looked up	Do not edit this file manually
/etc/resolv.conf	DNS server configuration file

Used Scenarios with Q/A:

Questions	Answers
Configure static IP address using nmcli	nmcli con add type ethernet con-name static-eth ifname eth0 ip4 192.168.1.100/24 gw4 192.168.1.1
Configure DHCP using nmcli	nmcli con add type ethernet con-name dhcp-eth ifname eth0
Set system hostname	hostnamectl set-hostname server1.example.com
Configure DNS servers	nmcli con mod static-eth ipv4.dns ‘8.8.8.8,8.8.4.4’
Add static host entries to /etc/hosts	echo ‘192.168.1.50 server50’ >> /etc/hosts
Troubleshoot network connectivity using ping	ping -c 4 8.8.8.8 then traceroute 8.8.8.8
Check network interface configuration	ip addr show or nmcli con show
View routing table	ip route show or route -n
Test port connectivity using telnet or nc	nc -zv hostname port or telnet hostname port
Configure network bonding/teaming	nmcli con add type bond con-name bond0 ifname bond0 bond.options mode=active-backup ip4 192.168.1.100/24 gw4 192.168.1.1
Restart network service	systemctl restart NetworkManager
Configure network interface to start at boot	nmcli con mod con-name connection.autoconnect yes
Change MTU size on network interface	nmcli con mod con-name 802-3-ethernet.mtu 1500
Configure multiple IP addresses on one interface	nmcli con mod con-name +ipv4.addresses ‘192.168.1.101/24’
Set up network routes	ip route add 10.0.0.0/8 via 192.168.1.1
Monitor network traffic and connections	ss -tuln or netstat -tuln
Configure firewall rules for specific services	firewall-cmd –add-service=http –permanent && firewall-cmd –reload
Troubleshoot DNS resolution issues	Check: ping 8.8.8.8, nslookup google.com, cat /etc/resolv.conf
Configure network interface using nmtui (text UI)	nmtui (interactive text UI)
Backup and restore network configuration	cp -r /etc/NetworkManager/ ~/network-backup/

8. Commands used to Manage Softwares

Command	Arguments	Use Case / Description	Important Notes
dnf	install package	Installs a package
dnf	remove package	Removes a package
dnf	update	Updates all packages
dnf	update package	Updates specific package
dnf	search keyword	Searches for package by keyword
dnf	search all keyword	Deep search in package descriptions
dnf	provides */file	Finds package containing specific file
dnf	info package	Shows detailed package information
dnf	list installed	Lists installed packages
dnf	list available	Lists available packages
dnf	list package	Shows installed and available versions
dnf	repolist	Shows configured repositories
dnf	group list	Lists package groups
dnf	groupinstall “Group Name”	Installs package group
dnf	group info “Group Name”	Shows package group contents
dnf	history	Shows transaction history
dnf	history undo ID	Undoes specific transaction
dnf	clean all	Clears cached repository data
dnf	config-manager –add-repo=URL	Adds new repository
dnf	module list	Lists available modules
dnf	module info modulename	Shows module information
dnf	module install name:stream/profile	Installs module with specific stream/profile
rpm	-qi package	Queries information about installed package
rpm	-ql package	Lists files in package
rpm	-qf /path/to/file	Queries which package owns file
rpm	-qc package	Shows package configuration files
rpm	-qd package	Shows package documentation
rpm	-q –scripts package	Shows scripts in package
rpm	-qp –scripts package.rpm	Shows scripts in package file (not installed)
rpm	-qR package	Shows package dependencies
rpm	-V package	Verifies package integrity
rpm	-Va	Verifies all installed packages
rpm	-qa	Lists all installed packages
subscription-manager	register	Registers system with Red Hat
subscription-manager	list –available	Lists available subscriptions
subscription-manager	attach –auto	Attaches available subscription
subscription-manager	unregister	Unregisters system

Used Scenarios with Q/A:

Questions	Answers
Install a software package using dnf	dnf install httpd
Remove a software package using dnf	dnf remove httpd
Update all system packages using dnf	dnf update
Search for available packages	dnf search python3
Show information about an installed package	dnf info httpd
List all installed packages	dnf list installed or rpm -qa
Clean dnf cache	dnf clean all
Install a package group	dnf groupinstall “Development Tools”
Configure additional software repository	Create /etc/yum.repos.d/myrepo.repo: [myrepo] name=My Repository baseurl=file:///path/to/repo enabled=1 gpgcheck=0
Install local RPM package	dnf install /path/to/package.rpm
Verify integrity of installed packages	rpm -V package-name
Check what package provides a specific file	dnf provides /usr/bin/python3
List files installed by a package	rpm -ql package-name
Check for available updates	dnf check-update
Downgrade a package to previous version	dnf downgrade package-name
Manage dnf history and undo transactions	dnf history then dnf history undo 3
Configure dnf to exclude certain packages from updates	echo “exclude=kernel*” >> /etc/dnf/dnf.conf
Set up automatic updates	dnf install dnf-automatic and configure /etc/dnf/automatic.conf
Work with module streams	dnf module list then dnf module enable nodejs:16
Troubleshoot package dependency issues	dnf deplist package-name to see dependencies

9. Managing Processes

Command	Arguments	Use Case / Description	Important Notes
ps	aux	Shows all processes with detailed info
ps	-ef	Shows all processes in full format
ps	fax	Shows process hierarchy tree
top	(none)	Interactive process viewer
htop	(none)	Enhanced interactive process viewer
kill	PID	Sends TERM signal to process (graceful)
kill	-9 PID	Sends SIGKILL to process (force stop)
killall	processname	Kills processes by name
pkill	processname	Kills processes by name pattern
nice	-n 10 command	Starts command with nice value of 10
renice	-n 5 -p PID	Changes nice value of running process
renice	value PID	Changes nice value (alternative syntax)
jobs	(none)	Lists background jobs in current shell
bg	%1	Resumes background job
fg	%1	Brings background job to foreground
tuned-adm	profile profile	Sets system performance profile
tuned-adm	list	Lists available tuned profiles
tuned-adm	active	Shows currently active tuned profile

Key Combinations

Key Combo	Use Case / Description
Ctrl+Z	Suspends current foreground job
Ctrl+C	Terminates current job
Ctrl+D	Sends EOF (End Of File) signal
& (at end of command)	Starts command in background
Alt+F1-F6	Switches between virtual terminals
Ctrl+Alt+F1-F6	Switches from GUI to text terminals

Used Scenarios with Q/A:

Questions	Answers
List all processes running on the system	ps aux or ps -ef
Find processes using most CPU	ps aux –sort=-%cpu | head -10
Find processes using most memory	ps aux –sort=-%mem | head -10
Kill a process by PID	kill 1234
Kill a process by name	pkill process-name or killall process-name
Change process priority using nice	nice -n 10 /path/to/command
Change priority of running process using renice	renice -n 10 -p 1234
Run process in background	long-running-command &
Move process between foreground and background	fg %1 (bring to foreground), bg %1 (send to background)
List background jobs	jobs
Use kill to send different signals to processes	kill -1 1234 (HUP), kill -9 1234 (KILL), kill -15 1234 (TERM)
Monitor processes in real-time	top or htop
Find parent-child process relationships	ps -ef –forest or pstree
Identify zombie processes	ps aux | grep defunct or ps aux | awk ‘$8==”Z” {print $2}’
Set process CPU affinity	taskset -p 1234 (view), taskset -cp 0,1 1234 (set to CPUs 0,1)
Monitor system resource usage	vmstat 1, iostat 1, mpstat 1
Configure process limits for users	ulimit -a (view), edit /etc/security/limits.conf
Use pstree to view process hierarchy	pstree -p
Find which process is using a specific port	ss -tulnp | grep :80 or lsof -i :80
Manage services as processes	systemctl status servicename

10. Working with Systemd

Command	Arguments / Options	Use Case / Description	Important Notes
systemctl	start <unit>	Starts a service/unit.
systemctl	stop <unit>	Stops a service/unit.
systemctl	restart <unit>	Restarts a service/unit.
systemctl	reload <unit>	Reloads a service’s configuration.
systemctl	enable <unit>	Enables a unit to start automatically at boot.
systemctl	disable <unit>	Disables a unit from starting automatically at boot.
systemctl	status <unit>	Shows the current status of a unit.
systemctl	status -l <unit>	Shows detailed status information.
systemctl	list-units -t service	Lists all active service units.
systemctl	list-units -t service –all	Lists all service units, including inactive ones.
systemctl	–failed -t service	Shows all services that have failed.
systemctl	-t help	Shows all available Systemd unit types.
systemctl	list-dependencies <unit>	Shows the dependencies of a specific unit.
systemctl	list-dependencies –reverse <unit>	Shows which units depend on a specific unit.
systemctl	show <unit>	Shows all available configuration options for a unit.
systemctl	edit <unit>	Creates an override file for a unit in /etc/systemd/system.
systemctl	cat <unit>	Displays the current, combined configuration of a unit.
systemctl	mask <unit>	Completely disables a unit, preventing manual and automatic start.
systemctl	unmask <unit>	Reverses the effect of mask.
systemctl	daemon-reload	Reloads Systemd manager configuration after changing unit files.
journalctl	-u <unit>	Shows logs for a specific Systemd unit.
journalctl	-f	Follows (tails) the journal.
systemd-analyze	(none)	Shows boot performance statistics.

Used Scenarios with Q/A:

Questions	Answers
Start a system service	systemctl start httpd
Stop a system service	systemctl stop httpd
Restart a system service	systemctl restart httpd
Check status of a service	systemctl status httpd
Enable service to start at boot	systemctl enable httpd
Disable service from starting at boot	systemctl disable httpd
Mask a service to prevent starting	systemctl mask httpd
Unmask a service	systemctl unmask httpd
View system boot time and performance	systemd-analyze (boot time), systemd-analyze blame (service times)
Change default system target (runlevel)	systemctl set-default multi-user.target
List all running services	systemctl list-units –type=service –state=running
List all failed services	systemctl –failed
View service dependencies	systemctl list-dependencies httpd
Analyze why a service failed to start	journalctl -u httpd and systemctl status httpd -l
Create a custom systemd service	Create /etc/systemd/system/myservice.service: [Unit] Description=My Custom Service [Service] ExecStart=/path/to/command [Install] WantedBy=multi-user.target
Configure service resource limits	Add to service file: LimitNOFILE=65536
View service logs using journalctl	journalctl -u servicename -f
Set service to restart on failure	Add to service file: Restart=on-failure
Work with systemd timers	Create /etc/systemd/system/mytimer.timer and .service files
Troubleshoot systemd service issues	journalctl -u servicename –since “1 hour ago”

11. Scheduling Tasks

Command	Arguments / Options	Use Case / Description	Important Notes
crontab	-e	Edits the current user’s crontab file.
crontab	-l	Lists the current user’s crontab entries.
crontab	-r	Deletes the current user’s crontab.
crontab	-u <username> -e	Edits the crontab for a specific user (requires root).
at	HH:MM	Schedules a one-time job for a specific time. Opens an interactive shell.
atq	(none)	Lists pending at jobs.
atrm	<job_id>	Removes a scheduled at job.
systemctl	status atd	Checks the status of the at daemon.
systemctl	enable crond	Enables the cron daemon to start at boot.
systemctl	start crond	Starts the cron daemon.
batch	(none)	Schedules a job to run when system load is low.

Used Scenarios with Q/A:

Task	Command
Create a user cron job that runs daily	crontab -e add: 0 2 * * * /home/user/backup.sh
Create a system cron job	sudo vim /etc/crontab add: 0 2 * * * root /root/backup.sh
Schedule a one-time task using at	echo “tar -czf /backup/backup.tar.gz /home” | at 02:00 tomorrow
List pending at jobs	atq
Remove a scheduled at job	atrm 1
Configure cron to run every 5 minutes	*/5 * * * * /path/to/script.sh
Configure cron to run on specific days of week	0 2 * * 1 /path/to/script.sh
Redirect cron job output to a file	0 2 * * * /path/to/script.sh > /var/log/script.log 2>&1
Set up cron job that runs during specific hours only	0 9-17 * * * /path/to/script.sh
Configure email notification for cron jobs	Add MAILTO=user@example.com at top of crontab
Restrict cron access for users	echo “username” >> /etc/cron.deny
Backup cron configuration	crontab -l > cron-backup.txt
Troubleshoot why cron job didn’t run	Check: grep CRON /var/log/cron, verify script permissions, PATH
Set up anacron for missed jobs	dnf install anacron, configure /etc/anacrontab
Create cron job that runs on first day of month	0 0 1 * * /path/to/script.sh
Schedule system maintenance tasks	0 2 * * * /usr/bin/updatedb (update locate database)
Monitor cron job execution	tail -f /var/log/cron
Configure environment for cron jobs	Add to crontab: SHELL=/bin/bash, PATH=/usr/bin:/bin
Use systemd timers as cron alternative	Create systemd timer as shown in Chapter 11
Set up log rotation for cron output	Configure in /etc/logrotate.d/ for the log file

12. Configuring Logging

Command	Arguments / Options	Use Case / Description	Important Notes
journalctl	-f	Follows (tails) the journal in real-time.
journalctl	-u <unit>	Shows logs for a specific Systemd unit.
journalctl	–since “YYYY-MM-DD HH:MM:SS”	Shows logs since a specific time.
journalctl	–until “YYYY-MM-DD HH:MM:SS”	Shows logs until a specific time.
journalctl	-p <priority>	Filters logs by priority (e.g., err, info, debug).
journalctl	-b	Shows logs from the current boot only.
journalctl	-x	Adds explanation messages to the log output.
journalctl	-o verbose	Shows log entries with all available fields.
journalctl	_SYSTEMD_UNIT=<unit>	Filters logs using specific fields (e.g., by unit).
tail	-f /var/log/file	Monitors a log file in real-time.
logger	“message”	Writes a message to the syslog (and thus the journal).
logger	-p <facility.priority> “message”	Writes a message with a specific facility and priority.
Command	Arguments / Options	Use Case / Description
journalctl	-f	Follows (tails) the journal in real-time.
journalctl	-u <unit>	Shows logs for a specific Systemd unit.
journalctl	–since “YYYY-MM-DD HH:MM:SS”	Shows logs since a specific time.
journalctl	–until “YYYY-MM-DD HH:MM:SS”	Shows logs until a specific time.
journalctl	-p <priority>	Filters logs by priority (e.g., err, info, debug).
journalctl	-b	Shows logs from the current boot only.
journalctl	-x	Adds explanation messages to the log output.
journalctl	-o verbose	Shows log entries with all available fields.
journalctl	_SYSTEMD_UNIT=<unit>	Filters logs using specific fields (e.g., by unit).
tail	-f /var/log/file	Monitors a log file in real-time.
logger	“message”	Writes a message to the syslog (and thus the journal).
logger	-p <facility.priority> “message”	Writes a message with a specific facility and priority.
rsyslogd	-v	Shows the version of rsyslog.
logrotate	-f /etc/logrotate.conf	Forces log rotation to happen immediately.

Used Scenarios with Q/A:

Questions	Answers
Configure rsyslog to log to a specific file	vim /etc/rsyslog.conf add: *.info /var/log/all.log
Set up log rotation for custom application logs	Create /etc/logrotate.d/myapp: /var/log/myapp.log { daily rotate 7 compress missingok }
Use journalctl to view system logs	journalctl
Filter journal logs by time period	journalctl –since “2024-01-01” –until “2024-01-02”
Filter journal logs by service/unit	journalctl -u httpd
Configure persistent journal storage	mkdir -p /var/log/journal && systemctl restart systemd-journald
Clear journal logs	journalctl –vacuum-time=1d
Set log retention policy	Edit /etc/systemd/journald.conf: SystemMaxUse=1G
Forward logs to remote server	vim /etc/rsyslog.conf add: *.* @@remote-server:514
Search for specific error messages in logs	journalctl -p err or grep -i error /var/log/messages
Monitor logs in real-time	journalctl -f
Configure log level for specific services	Edit service file or use journalctl -p debug
Create custom log files	vim /etc/rsyslog.conf add: local0.* /var/log/myapp.log
Analyze log file permissions and ownership	ls -la /var/log/ and getfacl /var/log/messages
Troubleshoot logging issues	Check: systemctl status rsyslog, disk space, permissions
Use logger to add custom entries to logs	logger -p local0.info “Test message”
Configure log rate limiting	Edit /etc/rsyslog.conf: $SystemLogRateLimitInterval 0 (disable)
Archive old log files	find /var/log -name “*.gz” -o -name “*.1”
Set up log monitoring alerts	grep -i “error\|fail” /var/log/messages | mail -s “Errors” admin@example.com
Correlate logs from different services	journalctl -u httpd -u mariadb –since “1 hour ago”

13. Managing Storage

Command	Arguments / Options	Use Case / Description	Important Notes
fdisk	/dev/sdX	Partitions a disk using the MBR partition table scheme.
gdisk	/dev/sdX	Partitions a disk using the GPT partition table scheme.
parted	/dev/sdX	A versatile partitioning tool.
parted	/dev/sdX mklabel gpt	Creates a GPT partition table on a disk.
lsblk	(none)	Lists all block devices in a tree format.
blkid	(none)	Shows UUIDs, labels, and types of block devices.
mkfs	-t xfs /dev/sdX1	Creates an XFS filesystem on a partition.
mkfs.xfs	/dev/sdX1	Creates an XFS filesystem (the RHEL 9 default).
mkfs	-t ext4 /dev/sdX1	Creates an ext4 filesystem.
mkfs.ext4	/dev/sdX1	Creates an ext4 filesystem.
mount	/dev/sdX1 /mnt	Temporarily mounts a filesystem.
mount	-a	Mounts all filesystems defined in /etc/fstab.
umount	/mnt or /dev/sdX1	Unmounts a filesystem.
df	-h	Shows disk space usage of mounted filesystems in human-readable format.
du	-sh /path	Shows disk usage of a specific directory.
tune2fs	-l /dev/sdX1	Shows properties and information of an ext2/3/4 filesystem.
xfs_admin	-L <label> /dev/sdX1	Sets a label on an XFS filesystem.
mkswap	/dev/sdX1	Formats a partition as swap space.
swapon	/dev/sdX1	Activates a swap partition.
swapoff	/dev/sdX1	Deactivates a swap partition.
findmnt	–verify	Verifies the correctness of /etc/fstab before rebooting.

Used Scenarios with Q/A:

Questions	Answers
Create a new disk partition using fdisk	fdisk /dev/sdb → n → p → 1 → Enter → Enter → w
Create a new disk partition using parted	parted /dev/sdb mklabel gpt mkpart primary 0% 100%
Create XFS filesystem on a partition	mkfs.xfs /dev/sdb1
Create ext4 filesystem on a partition	mkfs.ext4 /dev/sdb1
Mount filesystem manually	mount /dev/sdb1 /mnt
Configure persistent mounts in /etc/fstab	echo “/dev/sdb1 /mnt xfs defaults 0 0” >> /etc/fstab
Use filesystem UUID in fstab	blkid /dev/sdb1 then use: UUID=xxx /mnt xfs defaults 0 0
Use filesystem label in fstab	xfs_admin -L mylabel /dev/sdb1 then: LABEL=mylabel /mnt xfs defaults 0 0
Add swap space using a partition	mkswap /dev/sdc1 && swapon /dev/sdc1 add to fstab: /dev/sdc1 swap swap defaults 0 0
Add swap space using a file	dd if=/dev/zero of=/swapfile bs=1M count=1024 && mkswap /swapfile && swapon /swapfile add to fstab: /swapfile swap swap defaults 0 0
Check filesystem integrity	xfs_repair /dev/sdb1 or fsck.ext4 /dev/sdb1
Repair damaged filesystem	xfs_repair -L /dev/sdb1 (force repair, may lose data)
Monitor disk space usage	df -h and du -sh /path
Extend existing filesystem (for non-LVM)	For XFS: xfs_growfs /mountpoint, for ext4: resize2fs /dev/partition
Create encrypted filesystem	cryptsetup luksFormat /dev/sdb1 then cryptsetup luksOpen /dev/sdb1 encrypted_vol
Backup and restore filesystem	xfsdump -l 0 -f backup.xfsdump /dev/sdb1 and xfsrestore -f backup.xfsdump /mnt
Check disk health with SMART	smartctl -a /dev/sda
Mount network filesystems	mount -t nfs server:/export /mnt
Set filesystem mount options	mount -o noatime,nodiratime /dev/sdb1 /mnt
Troubleshoot mount issues	Check: dmesg | grep sdb, mount, /etc/fstab syntax

14. Managing Advanced Storage

Command	Arguments / Options	Use Case / Description	Important Notes
pvcreate	/dev/sdX1	Initializes a partition or disk as an LVM Physical Volume (PV).
pvs	(none)	Provides a summary of Physical Volumes.
pvdisplay	(none)	Provides detailed information about Physical Volumes.
vgcreate	vg_name /dev/sdX1	Creates a Volume Group (VG) named vg_name with an initial PV.
vgextend	vg_name /dev/sdY1	Adds a new PV to an existing VG.
vgreduce	vg_name /dev/sdY1	Removes a PV from a VG (must be unused or data moved first).
vgs	(none)	Provides a summary of Volume Groups.
vgdisplay	(none)	Provides detailed information about Volume Groups.
lvcreate	-L 10G -n lv_name vg_name	Creates a Logical Volume (LV) of 10GB.
lvcreate	-l 50%FREE -n lv_name vg_name	Creates an LV using 50% of the free space in the VG.
lvextend	-L +5G /dev/vg_name/lv_name	Extends an LV by 5GB.
lvextend / lvresize	-r -L +5G /dev/vg_name/lv_name	Extends an LV and resizes the filesystem within it.
lvs	(none)	Provides a summary of Logical Volumes.
lvdisplay	(none)	Provides detailed information about Logical Volumes.
xfs_growfs	/mount/point	Grows an XFS filesystem to the size of its underlying LV.
resize2fs	/dev/vg_name/lv_name	Resizes an ext4 filesystem to the size of its underlying LV.
pvmove	/dev/sdY1 /dev/sdX1	Moves data from one PV to another within the same VG.
stratis	pool create pool_name /dev/sdX	Creates a Stratis pool on a block device.
stratis	fs create pool_name fs_name	Creates a filesystem in a Stratis pool.
stratis	fs list	Lists all Stratis filesystems.
stratis	filesystem snapshot pool_name origin_fs snapshot_fs	Creates a snapshot of a Stratis filesystem.

Used Scenarios with Q/A:

Questions	Answers
Create physical volume	pvcreate /dev/sdb
Create volume group	vgcreate myvg /dev/sdb
Create logical volume	lvcreate -n mylv -L 10G myvg
Extend logical volume	lvextend -L +5G /dev/myvg/mylv then xfs_growfs /dev/myvg/mylv
Reduce logical volume	lvreduce -L -2G /dev/myvg/mylv (unmount and resize filesystem first)
Extend volume group	vgextend myvg /dev/sdc
Reduce volume group	vgreduce myvg /dev/sdc (move data first with pvmove)
Create snapshot of logical volume	lvcreate –snapshot –name mysnap –size 1G /dev/myvg/mylv
Restore from snapshot	lvconvert –merge /dev/myvg/mysnap
Move physical volume between volume groups	pvmove /dev/sdb /dev/sdc then vgreduce myvg /dev/sdb
Monitor LVM status	vgs, pvs, lvs
Configure LVM thin provisioning	lvcreate –type thin-pool -L 10G –name thin_pool myvg
Set up Stratis storage pool	dnf install stratisd stratis-cli, systemctl enable –now stratisd, stratis pool create mypool /dev/sdb
Create Stratis filesystem	stratis filesystem create mypool myfs
Extend Stratis filesystem	stratis filesystem mypool/myfs then stratis pool add-data mypool /dev/sdc
Monitor Stratis storage	stratis pool list, stratis filesystem list
Configure LVM caching	lvcreate –type cache -L 1G -n lvcache myvg /dev/slowlv /dev/fastpv
Backup LVM configuration	vgcfgbackup myvg
Troubleshoot LVM issues	Check: dmesg, /var/log/messages, LVM metadata with vgcfgrestore
Migrate data between storage systems	dd if=/dev/myvg/mylv of=/backup/lv_backup.img

15. Basic Kernal Management

Command	Arguments / Options	Use Case / Description	Important Notes
uname	-r	Shows the kernel release version.
uname	-a	Shows all system information.
lsmod	(none)	Lists all currently loaded kernel modules.
modinfo	<module_name>	Shows detailed information about a kernel module.
modprobe	<module_name>	Loads a kernel module and its dependencies.
modprobe	-r <module_name>	Unloads a kernel module and its dependencies.
insmod	/path/to/module.ko	Low-level command to insert a module (does not handle dependencies).
rmmod	<module_name>	Low-level command to remove a module (does not handle dependencies).
dmesg	(none)	Displays the kernel ring buffer (boot and kernel messages).
dmesg	-T	Shows dmesg output with human-readable timestamps.
lspci	-k	Lists PCI devices and shows the kernel driver in use for each.
udevadm	monitor	Monitors the udev daemon for device events in real-time.

Used Scenarios with Q/A:

Questions	Answers
List loaded kernel modules	lsmod
Load kernel module	modprobe module_name
Unload kernel module	modprobe -r module_name
Check kernel module parameters	modinfo module_name
Set kernel module parameters	Create /etc/modprobe.d/mymodule.conf: options module_name param=value
List available kernel modules	find /lib/modules/$(uname -r) -name “*.ko”
Check kernel version	uname -r
Check kernel boot parameters	cat /proc/cmdline
Modify kernel parameters at runtime	echo “value” > /proc/sys/kernel/parameter or sysctl -w kernel.parameter=value
Blacklist kernel module	echo “blacklist module_name” > /etc/modprobe.d/blacklist.conf
Update kernel	dnf install kernel-* then reboot
Boot from different kernel version	Select from GRUB menu or set default with grub2-set-default
Check hardware information	lspci, lsusb, lscpu
Monitor kernel messages	dmesg or journalctl –dmesg
Troubleshoot kernel issues	Check: dmesg | grep -i error, remove problematic modules, use previous kernel
Configure kernel module dependencies	modinfo module_name | grep depends
Create custom kernel module	dnf install kernel-devel, write module code, compile with make
Check kernel ring buffer	dmesg -w (follow)
Analyze system interrupts	cat /proc/interrupts
Monitor kernel performance	vmstat 1, mpstat 1, pidstat 1

16. Managing the Boot Process

Command	Arguments	Use Case / Description	Important Notes
systemctl –type=target	–all	List all targets (active/inactive)
systemctl isolate	rescue.target	Switch to rescue target immediately
systemctl get-default		Show current default target
systemctl set-default	multi-user.target	Set default boot target
grub2-mkconfig	-o /boot/grub2/grub.cfg	Regenerate GRUB config (BIOS)
grub2-mkconfig	-o /boot/efi/EFI/redhat/grub.cfg	Regenerate GRUB config (UEFI)
grub2-install	/dev/sda	Reinstall GRUB bootloader
systemctl cat	multi-user.target	View target unit file contents
grep Isolate	*.target	Find targets that allow isolation
dnf group list		List available package groups
dnf group install	“server with gui”	Install GUI packages

Used Scenarios with Q/A:

Question	Answer
Modify GRUB2 configuration	vim /etc/default/grub add to GRUB_CMDLINE_LINUX, then grub2-mkconfig -o /boot/grub2/grub.cfg
Set default kernel boot entry	grub2-set-default 0 (0 for first entry)
Add kernel boot parameters	vim /etc/default/grub add to GRUB_CMDLINE_LINUX=”rhgb quiet new_param”
Reinstall GRUB2 bootloader	grub2-install /dev/sda
Rebuild initramfs	dracut -f or mkinitrd -f
Boot into rescue mode	Reboot, in GRUB edit kernel line, add systemd.unit=rescue.target
Boot into emergency mode	Reboot, in GRUB edit kernel line, add systemd.unit=emergency.target
Set default boot target	systemctl set-default multi-user.target
Change systemd target at runtime	systemctl isolate multi-user.target
Troubleshoot boot failures	Boot from installation media, chroot /mnt/sysroot, reinstall GRUB, fix fstab
Recover from bootloader issues	Boot from live CD, mount root, chroot, grub2-install /dev/sda
Password protect GRUB	grub2-setpassword
Analyze boot performance	systemd-analyze, systemd-analyze blame, systemd-analyze critical-chain
Configure serial console for boot	Add console=ttyS0,115200 to kernel parameters
Backup boot configuration	cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.backup
Restore boot configuration	Restore from backup or recreate from /etc/default/grub
Work with UEFI boot manager	Use efibootmgr to manage UEFI entries
Set up dual boot configuration	Install both OS, configure GRUB to detect both
Monitor boot process messages	Remove rhgb quiet from kernel parameters, watch boot messages
Fix filesystem issues at boot	Boot from rescue media, run fsck on affected partitions

17. Essential Troubleshooting Commands

Command	Arguments	Use Case / Description	Important Notes
systemctl	rescue	Boots into rescue mode
systemctl	emergency	Boots into emergency mode
journalctl	-b	Shows logs from the current boot
journalctl	-b -1	Shows logs from the previous boot
systemctl list-units		Show loaded units in rescue/emergency mode
systemctl show-environment		Show current shell environment variables
chroot	/mnt/sysimage	Change root to mounted system in rescue mode
dracut –force		Recreate initramfs
mount -o remount,rw	/	Remount root filesystem as read-write
passwd		Change password (e.g., root password reset)
touch /.autorelabel		Trigger SELinux relabel on next boot
exec /usr/lib/systemd/systemd		Replace current shell with Systemd (after password reset)
fsck	/dev/sdb1	Check and repair filesystem
journalctl -xb		Show boot logs with details
systemctl reboot		Reboot system

Used Scenarios with Q/A:

Questions	Answers
Reset forgotten root password	Reboot, edit kernel line adding rd.break, then: chroot /sysroot passwd root touch /.autorelabel exit twice
Repair corrupted filesystem	umount /dev/sda1 (if mounted), then fsck /dev/sda1
Recover from kernel panic	Check logs, identify faulty driver/hardware, boot previous kernel
Fix network connectivity issues	systemctl restart NetworkManager, check cables, ip link show, nmcli con show
Restore broken package database	rpm –rebuilddb
Recover deleted files	Use testdisk, photorec, or restore from backup
Fix bootloader issues	Boot from installation media, chroot /mnt/sysroot, grub2-install /dev/sda, grub2-mkconfig
Troubleshoot service failures	systemctl status servicename, journalctl -u servicename, check dependencies
Diagnose performance issues	top, vmstat 1, iostat 1, identify resource bottlenecks
Fix permission problems	ls -la, getfacl file, check user/group with id username
Recover from full filesystem	df -h, identify large files with du -sh /* 2>/dev/null | sort -hr, clean up
Troubleshoot hardware issues	dmesg | grep -i error, lspci, smartctl -a /dev/sda
Fix broken dependencies	dnf check, rpm -Va, reinstall broken packages
Recover from failed updates	Boot previous kernel, use dnf history undo
Diagnose memory issues	free -h, cat /proc/meminfo, identify memory leaks
Fix corrupted configuration files	Restore from backup or recreate from documentation
Troubleshoot login issues	Check PAM configuration, user shell, home directory permissions
Recover from accidental file deletion	Restore from backup or use file recovery tools
Fix broken symbolic links	find / -type l -! -exec test -e {} \; -print
Diagnose and fix security issues	grep -r “password” /etc/ 2>/dev/null, check file permissions, audit logs

18. Automation with Bash Shell Scripting

Command	Arguments	Use Case / Description	Important Notes
#!/bin/bash	(shebang)	Specify script interpreter
exit 0		Exit script with success status
$1, $2, …		Positional parameters (script arguments)
$#		Number of arguments
$@		All arguments
read	VARIABLE	Read user input into variable
test	-f file	Check if file exists
[ -f file ]		Alternative test syntax
$(command)		Command substitution
if … then … fi		Conditional execution
for i in $@; do … done		Loop through arguments
for (( i=0; i<10; i++ )); do … done		Arithmetic for loop
while condition; do … done		While loop
until condition; do … done		Until loop
case $var in … esac		Case statement
bash -x	script.sh	Debug script execution

Used Scenarios with Q/A:

Questions	Answers
Create script that displays system information	!/bin/bash echo “=== System Information ===” echo “Hostname: $(hostname)” echo “Kernel: $(uname -r)” echo “Uptime: $(uptime -p)” echo “Memory: $(free -h | grep Mem | awk ‘{print $3 “/” $2}’)” echo “Disk: $(df -h / | awk ‘NR==2 {print $3 “/” $2 ” (” $5 “)”}’)”
Write script that backs up specific directories	!/bin/bash BACKUP_DIR=”/backup” DATE=$(date +%Y%m%d) tar -czf “$BACKUP_DIR/backup_$DATE.tar.gz” /home /etc echo “Backup completed: $BACKUP_DIR/backup_$DATE.tar.gz”!/bin/bash BACKUP_DIR=”/backup” DATE=$(date +%Y%m%d) tar -czf “$BACKUP_DIR/backup_$DATE.tar.gz” /home /etc echo “Backup completed: $BACKUP_DIR/backup_$DATE.tar.gz”
Create script that monitors disk space and sends alerts	!/bin/bash THRESHOLD=80 USAGE=$(df / | awk ‘NR==2 {print $5}’ | sed ‘s/%//’) if [ $USAGE -gt $THRESHOLD ]; then echo “Warning: Root filesystem is ${USAGE}% full” | mail -s “Disk Alert” admin@example.com fi
Write script that processes log files	!/bin/bash LOG_FILE=”/var/log/application.log” ERRORS=$(grep -c “ERROR” “$LOG_FILE”) if [ $ERRORS -gt 0 ]; then echo “Found $ERRORS errors in $LOG_FILE” grep “ERROR” “$LOG_FILE” | tail -10 > /tmp/recent_errors.txt fi
Create script with command-line arguments	!/bin/bash echo “Script name: $0” echo “First argument: $1” echo “Second argument: $2” echo “All arguments: $@” echo “Number of arguments: $#”
Write script with conditional statements	!/bin/bash if [ -f “/etc/passwd” ]; then echo “File exists” else echo “File does not exist” fi if [ $UID -eq 0 ]; then echo “Running as root” else echo “Not running as root” fi
Create script with loops	!/bin/bash For loop for user in $(cut -d: -f1 /etc/passwd | head -5); do echo “User: $user” done While loop count=1 while [ $count -le 5 ]; do echo “Count: $count” ((count++)) done
Write script that creates user accounts from file	!/bin/bash USER_FILE=”users.txt” while IFS=: read -r username password; do useradd -m “$username” echo “$username:$password” | chpasswd echo “Created user: $username” done < “$USER_FILE”
Create script that monitors services	!/bin/bash SERVICES=(“httpd” “mariadb” “sshd”) for service in “${SERVICES[@]}”; do if systemctl is-active –quiet “$service”; then echo “$service is running” else echo “$service is NOT running” systemctl start “$service” fi done
Write script with functions	!/bin/bash log_message() { echo “$(date): $1” >> /var/log/myscript.log } backup_files() { tar -czf “$1” “$2” } log_message “Starting backup” backup_files “/backup/home.tar.gz” “/home” log_message “Backup completed”
Create script that handles errors	!/bin/bash set -e # Exit on any error cleanup() { echo “Cleaning up…” rm -f /tmp/tempfile } trap cleanup EXIT # Main script with error handling if [ ! -d “$1” ]; then echo “Error: Directory $1 does not exist” >&2 exit 1 fi
Write script that interacts with users	!/bin/bash read -p “Enter username: ” username read -s -p “Enter password: ” password echo read -p “Enter shell [bash]: ” shell shell=${shell:-bash} useradd -m -s “/bin/$shell” “$username” echo “$username:$password” | chpasswd echo “User $username created”
Create script for system maintenance	!/bin/bash Cleanup temporary files find /tmp -type f -mtime +7 -delete Rotate logs find /var/log -name “*.log” -mtime +30 -delete Update package cache dnf makecache
Write script that configures network	!/bin/bash INTERFACE=”eth0″ IP=”192.168.1.100″ NETMASK=”255.255.255.0″ GATEWAY=”192.168.1.1″ nmcli con add type ethernet con-name static-$INTERFACE ifname $INTERFACE \ ip4 $IP/$NETMASK gw4 $GATEWAY nmcli con up static-$INTERFACE
Create script that installs software	!/bin/bash PACKAGES=(“httpd” “mariadb-server” “php”) for pkg in “${PACKAGES[@]}”; do if ! rpm -q “$pkg” &>/dev/null; then dnf install -y “$pkg” systemctl enable “$pkg” fi done
Write script for log rotation	!/bin/bash LOG_DIR=”/var/log/myapp” find “$LOG_DIR” -name “.log” -mtime +7 -exec gzip {} \; find “$LOG_DIR” -name “.gz” -mtime +30 -delete echo “Log rotation completed: $(date)” >> “$LOG_DIR/rotation.log”
Create script that generates reports	!/bin/bash echo “=== System Report ===” > /tmp/system_report.txt echo “Date: $(date)” >> /tmp/system_report.txt echo “Uptime: $(uptime)” >> /tmp/system_report.txt echo “Memory: $(free -h)” >> /tmp/system_report.txt echo “Disk: $(df -h)” >> /tmp/system_report.txt echo “Top processes:” >> /tmp/system_report.txt ps aux –sort=-%cpu | head -10 >> /tmp/system_report.txt
Write script with array processing	!/bin/bash FILES=(“/etc/passwd” “/etc/group” “/etc/hosts”) for file in “${FILES[@]}”; do echo “=== $file ===” if [ -f “$file” ]; then cat “$file” else echo “File not found” fi echo done
Create script that uses regular expressions	!/bin/bash validate_ip() { if [[ $1 =~ ^[0-9]+.[0-9]+.[0-9]+.[0-9]+$ ]]; then return 0 else return 1 fi } read -p “Enter IP address: ” ip if validate_ip “$ip”; then echo “Valid IP: $ip” else echo “Invalid IP: $ip” fi
Write script for automated troubleshooting	!/bin/bash LOG_FILE=”/var/log/troubleshoot.log” check_service() { if ! systemctl is-active –quiet “$1”; then echo “Service $1 is not running. Starting…” >> “$LOG_FILE” systemctl start “$1” fi } check_disk() { USAGE=$(df / | awk ‘NR==2 {print $5}’ | sed ‘s/%//’) if [ $USAGE -gt 90 ]; then echo “Disk usage critical: ${USAGE}%” >> “$LOG_FILE” fi } echo “Troubleshooting started: $(date)” > “$LOG_FILE” check_service “sshd” check_service “httpd” check_disk echo “Troubleshooting completed: $(date)” >> “$LOG_FILE”

19. Configuring SSH

Command	Arguments	Use Case / Description	Important Notes
ssh	-p 2022 user@host	Connect via non-default port
ssh	-i ~/.ssh/key.pem user@host	Connect with specific private key
ssh	-L 8080:localhost:80 user@host	Local port forwarding
ssh	-R 8080:localhost:80 user@host	Remote port forwarding
ssh-keygen	-t rsa -b 4096	Generate RSA key pair
ssh-copy-id	-i ~/.ssh/key.pub user@host	Copy public key to host
ssh-agent	/bin/bash	Start SSH agent for current shell
ssh-add		Add private key passphrase to agent
semanage port	-a -t ssh_port_t -p tcp 2022	Add SELinux policy for custom SSH port
firewall-cmd	–add-port=2022/tcp	Open custom SSH port in firewall

Used Scenarios with Q/A:

Questions	Answers
Configure SSH to use different port	Edit /etc/ssh/sshd_config: change #Port 22 to Port 2222, then systemctl restart sshd, semanage port -a -t ssh_port_t -p tcp 2222, firewall-cmd –add-port=2222/tcp –permanent, firewall-cmd –reload
Disable root login over SSH	Edit /etc/ssh/sshd_config: change PermitRootLogin yes to PermitRootLogin no
Configure key-based authentication	ssh-keygen -t rsa -b 4096 -N “” -f ~/.ssh/id_rsa, ssh-copy-id -i ~/.ssh/id_rsa.pub user@host
Restrict SSH access to specific users	Edit /etc/ssh/sshd_config: add AllowUsers user1 user2 or AllowGroups sshusers
Configure SSH session timeout	Edit /etc/ssh/sshd_config: add ClientAliveInterval 300 and ClientAliveCountMax 2
Set up SSH tunneling	ssh -L 8080:localhost:80 user@remote (local forward) or ssh -R 8080:localhost:80 user@remote (remote forward)
Configure SSH X11 forwarding	Edit /etc/ssh/sshd_config: set X11Forwarding yes and on client use ssh -X user@host
Harden SSH configuration	Edit /etc/ssh/sshd_config: Protocol 2 PermitEmptyPasswords no PasswordAuthentication no PubkeyAuthentication yes ChallengeResponseAuthentication no UsePAM yes PrintMotd no
Set up SSH jump host	ssh -J jumpuser@jumpserver targetuser@target
Configure SSH bastion host	Configure bastion in ~/.ssh/config: Host bastion Hostname bastion.example.com User myuser IdentityFile ~/.ssh/bastion_key Host internal* ProxyJump bastion User internaluser IdentityFile ~/.ssh/internal_key Host internal1 Hostname 10.0.1.10 Host internal2 Hostname 10.0.1.11
Monitor SSH connections	ss -tlnp | grep :22, journalctl -u sshd -f
Troubleshoot SSH connection issues	Check: ssh -v user@host, verify keys, permissions (~/.ssh/ should be 700, keys 600), firewall, SELinux
Configure SSH client options	Create ~/.ssh/config: Host myserver Hostname server.example.com User myuser Port 2222 IdentityFile ~/.ssh/mykey ServerAliveInterval 60
Set up SSH for automated scripts	Use SSH keys in scripts: ssh -i /path/to/key user@host command
Configure SSH for port forwarding	ssh -L 3306:localhost:3306 user@dbserver (forward MySQL port)
Implement two-factor authentication for SSH	Use Google Authenticator: dnf install google-authenticator, google-authenticator, edit /etc/pam.d/sshd and /etc/ssh/sshd_config
Configure SSH to use specific ciphers	Edit /etc/ssh/sshd_config: Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
Set up SSH connection multiplexing	Edit ~/.ssh/config: Host * ControlMaster auto ControlPath ~/.ssh/%r@%h:%p ControlPersist 600
Configure SSH to use different authentication methods	Edit /etc/ssh/sshd_config: AuthenticationMethods publickey,password publickey
Backup and restore SSH keys	tar -czf ssh-backup.tar.gz ~/.ssh/ /etc/ssh/

20. Managing Apache HTTP Services

Command	Arguments	Use Case / Description	Important Notes
dnf install	httpd	Install Apache web server
systemctl enable –now	httpd	Start and enable Apache
systemctl status	httpd	Check Apache status
curl	http://localhost	Test web server from command line
httpd	-t	Test Apache configuration
apachectl	start	Start Apache (legacy)
apachectl	stop	Stop Apache (legacy)
apachectl	restart	Restart Apache (legacy)
apachectl	graceful	Graceful restart (legacy)
setenforce	0	Temporarily disable SELinux (for testing)

Used Scenarios with Q/A:

Questions	Answers
Install Apache web server	dnf install httpd
Start and enable Apache service	systemctl start httpd && systemctl enable httpd
Configure virtual hosts	Create /etc/httpd/conf.d/vhost.conf: <VirtualHost *:80> ServerName www.example.com DocumentRoot /var/www/html/example ErrorLog /var/log/httpd/example_error.log CustomLog /var/log/httpd/example_access.log combined </VirtualHost *:80>
Set up SSL/TLS for website	dnf install mod_ssl, create SSL cert: openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout /etc/pki/tls/private/apache.key -out /etc/pki/tls/certs/apache.crt, configure in VirtualHost: <VirtualHost *:443> SSLEngine on SSLCertificateFile /etc/pki/tls/certs/apache.crt SSLCertificateKeyFile /etc/pki/tls/private/apache.key </VirtualHost>
Configure directory access controls	In VirtualHost or directory block: <Directory “/var/www/html/secure”> Require all denied Require ip 192.168.1.0/24 </Directory>
Set up password protection for directory	htpasswd -c /etc/httpd/.htpasswd username, then in VirtualHost: <Directory “/var/www/html/secure”> AuthType Basic AuthName “Restricted Area” AuthUserFile /etc/httpd/.htpasswd Require valid-user </Directory>
Configure custom error pages	ErrorDocument 404 /errors/404.html and create the error page
Set up logging for virtual hosts	In VirtualHost: CustomLog /var/log/httpd/vhost_access.log combined
Configure PHP support	dnf install php php-mysqlnd
Set up CGI scripts	In main config: ScriptAlias /cgi-bin/ “/var/www/cgi-bin/”
Configure virtual host aliases	ServerAlias example.com *.example.com
Set up reverse proxy	ProxyPass “/app” “http://backend:8080/”, ProxyPassReverse “/app” “http://backend:8080/”
Configure load balancing	BalancerMember http://backend1:8080 BalancerMember http://backend2:8080 ProxyPass “/app” “balancer://mycluster/”
Implement URL rewriting	RewriteEngine On, RewriteRule ^oldpage$ newpage [R=301,L]
Set up web application firewall	dnf install mod_security, configure in /etc/httpd/conf.d/mod_security.conf
Monitor web server performance	apachectl status, server-status page, ps aux | grep httpd
Troubleshoot web server issues	Check: systemctl status httpd, journalctl -u httpd, apachectl configtest
Configure Apache modules	dnf install mod_php (for PHP), enable modules with a2enmod module_name
Set up virtual host for different ports	Listen 8080 in VirtualHost
Backup web server configuration	tar -czf httpd-backup.tar.gz /etc/httpd/ /var/www/html/

21. Managing SELinux

Command	Arguments	Use Case / Description	Important Notes
sestatus	-v	Shows SELinux status	Use -v for detailed information
setenforce	0	Sets SELinux to permissive mode	Temporary change only
setenforce	1	Sets SELinux to enforcing mode	Temporary change only
getenforce	(none)	Gets the current SELinux mode	Shows enforcing/permissive/disabled
chcon	-t httpd_sys_content_t /path	Changes SELinux context of file/directory	Avoid using – changes lost on relabel
semanage	fcontext -a -t httpd_sys_content_t “/dir(/.*)?”	Adds file context to policy	Recommended method for context changes
restorecon	-R -v /path	Restores default SELinux context recursively	Use after semanage fcontext
semanage	boolean -l	Lists SELinux booleans	Shows current and permanent settings
setsebool	-P boolean_name on/off	Sets SELinux boolean persistently	-P makes changes permanent
getsebool	-a	Lists all SELinux booleans	Use grep to filter results
sealert	-l UUID	Analyzes SELinux log messages	Install setroubleshoot-server package
sepolicy	generate	Generates SELinux policies for applications	Advanced troubleshooting
ls	-Z file	Shows SELinux context of files
ps	-Z	Shows SELinux context of processes

Used Scenarios with Q/A:

Questions	Answers
Check SELinux status	sestatus
Set SELinux to enforcing mode	setenforce 1
Set SELinux to permissive mode	setenforce 0
View SELinux context of files	ls -Z /etc/passwd
Change SELinux context of files	chcon -t httpd_sys_content_t /var/www/html/index.html
Restore default SELinux context	restorecon -R /var/www/html/
Configure SELinux booleans	setsebool -P httpd_can_network_connect on
Troubleshoot SELinux denials	sealert -a /var/log/audit/audit.log or ausearch -m avc -ts recent
Analyze SELinux audit logs	ausearch -m avc -ts today
Create custom SELinux policy	audit2allow -a to see suggested rules, audit2allow -a -M mypolicy to create module
Configure SELinux for web server	setsebool -P httpd_can_network_connect on, chcon -t httpd_sys_content_t /web/content
Configure SELinux for database	setsebool -P mysql_connect_http on, chcon -t mysqld_db_t /var/lib/mysql/
Configure SELinux for network services	semanage port -a -t http_port_t -p tcp 8080
Set SELinux context for custom applications	chcon -t bin_t /opt/myapp/bin/, semanage fcontext -a -t bin_t “/opt/myapp/bin(/.*)?”
Monitor SELinux events	tail -f /var/log/audit/audit.log | grep avc
Backup SELinux configuration	tar -czf selinux-backup.tar.gz /etc/selinux/ /var/lib/selinux/
Troubleshoot SELinux issues	Check: sealert -a /var/log/audit/audit.log, verify contexts, check booleans
Configure SELinux users	semanage user -l, semanage login -a -s user_u username
Set up SELinux policy modules	audit2allow -M mypolicy, semodule -i mypolicy.pp
Implement SELinux in mixed environment	Set permissive mode during migration: setenforce 0, monitor logs, create policies

22. Configuring Firewall

Command	Arguments	Use Case / Description	Important Notes
firewall-cmd	–list-all	Lists all firewall rules for default zone
firewall-cmd	–list-all –zone=zone_name	Lists rules for specific zone
firewall-cmd	–get-services	Lists all available services
firewall-cmd	–add-service=service_name	Adds a service to firewall	Runtime only
firewall-cmd	–add-service=service_name –permanent	Adds service permanently	Requires –reload
firewall-cmd	–remove-service=service_name	Removes a service from firewall
firewall-cmd	–add-port=port/protocol	Adds a port to firewall
firewall-cmd	–remove-port=port/protocol	Removes a port from firewall
firewall-cmd	–runtime-to-permanent	Makes runtime rules permanent	Alternative to –permanent
firewall-cmd	–reload	Reloads firewall rules	Activates permanent changes
firewall-cmd	–get-default-zone	Shows current default zone
firewall-cmd	–set-default-zone=zone_name	Changes default zone
firewall-cmd	–get-zones	Lists all available zones
firewall-cmd	–list-all-zones	Shows configuration for all zones

Used Scenarios with Q/A:

Questions	Answers
Check firewall status	firewall-cmd –state
List all firewall rules	firewall-cmd –list-all or firewall-cmd –list-all-zones
Open port for specific service	firewall-cmd –add-port=80/tcp or firewall-cmd –add-service=http
Block specific IP address	firewall-cmd –add-rich-rule=’rule family=”ipv4″ source address=”192.168.1.100″ reject’
Configure firewall zones	firewall-cmd –get-zones, firewall-cmd –get-default-zone
Set default firewall zone	firewall-cmd –set-default-zone=public
Create custom firewall service	firewall-cmd –new-service=myservice, firewall-cmd –service=myservice –add-port=8080/tcp
Configure rich rules	firewall-cmd –add-rich-rule=’rule family=”ipv4″ source address=”192.168.1.0/24″ service name=”ssh” accept’
Set up port forwarding	firewall-cmd –add-forward-port=port=80:proto=tcp:toport=8080
Configure masquerading	firewall-cmd –add-masquerade
Implement DMZ zone	firewall-cmd –new-zone=dmz, firewall-cmd –zone=dmz –add-interface=eth1
Monitor firewall activity	firewall-cmd –list-rich-rules, journalctl -u firewalld -f
Backup firewall configuration	firewall-cmd –runtime-to-permanent
Restore firewall configuration	firewall-cmd –reload
Troubleshoot firewall issues	Check: systemctl status firewalld, firewall-cmd –state, verify services/ports
Configure time-based rules	firewall-cmd –add-rich-rule=’rule family=”ipv4″ source address=”192.168.1.0/24″ port port=”22″ protocol=”tcp” accept’ with time constraints
Set up logging for firewall rules	firewall-cmd –set-log-denied=all
Implement source NAT	firewall-cmd –add-rich-rule=’rule family=”ipv4″ source address=”192.168.1.0/24″ masquerade’
Configure destination NAT	firewall-cmd –add-forward-port=port=80:proto=tcp:toport=8080:toaddr=192.168.1.100
Set up firewall for specific applications	Create service definition for custom application ports

23. Accessing Network Storage

Command	Arguments	Use Case / Description	Important Notes
mount	-t nfs server:/share /mnt	Mounts an NFS share
mount	server.example.com:/ /mnt	NFSv4 pseudo root mount	Mounts all shares
umount	/mnt	Unmounts an NFS share
showmount	-e server	Shows exports on NFS server	May not work through firewalls
systemctl	start nfs-server	Starts NFS server service
systemctl	enable nfs-server	Enables NFS server service
systemctl	start autofs	Starts automount service
systemctl	enable autofs	Enables automount service
firewall-cmd	–add-service nfs –permanent	Opens firewall for NFS	Also add rpc-bind and mountd

Used Scenarios with Q/A:

Questions	Answers
Mount NFS share manually	mount -t nfs server:/export /mnt/nfs
Configure persistent NFS mounts	echo “server:/export /mnt/nfs nfs defaults 0 0” >> /etc/fstab
Set up automount for NFS shares	Install autofs: dnf install autofs, configure /etc/auto.master and /etc/auto.nfs
Configure NFS exports	On server: vim /etc/exports add: /export 192.168.1.0/24(rw,sync,no_root_squash), then exportfs -a
Secure NFS exports	/export 192.168.1.0/24(rw,sync,no_root_squash,all_squash)
Mount SMB/CIFS share	mount -t cifs //server/share /mnt/smb -o username=user,password=pass
Configure automount for SMB shares	In /etc/auto.master: /mnt/smb /etc/auto.smb –timeout=60
Set up iSCSI initiator	dnf install iscsi-initiator-utils, iscsiadm -m discovery -t st -p target_ip, iscsiadm -m node -T iqn -p target_ip -l
Configure iSCSI persistent connections	iscsiadm -m node -T iqn -p target_ip -o update -n node.startup -v automatic
Mount FTP share	curlftpfs ftp://user:pass@server /mnt/ftp
Troubleshoot network storage issues	Check: showmount -e server, rpcinfo -p server, network connectivity, firewall
Monitor network storage performance	iostat -x 1, nfsiostat
Configure storage authentication	For iSCSI: configure CHAP in /etc/iscsi/iscsid.conf
Set up multipath storage	dnf install device-mapper-multipath, configure /etc/multipath.conf
Configure storage encryption	cryptsetup luksFormat /dev/sdb1 for encrypted storage
Backup network storage configuration	tar -czf storage-config-backup.tar.gz /etc/fstab /etc/exports /etc/auto.*
Restore network storage access	Restore from backup or recreate configurations
Set up high availability storage	Configure DRBD or cluster-aware filesystems
Configure storage quotas on network shares	edquota -u username for user quotas on NFS
Monitor network storage usage	df -h, du -sh /mnt/nfs/*

24. Configuring Time Services

Command	Arguments	Use Case / Description	Important Notes
timedatectl	(none)	Shows current time settings	Recommended command
timedatectl	set-time “YYYY-MM-DD HH:MM:SS”	Sets system time and date
timedatectl	set-timezone Timezone	Sets system timezone
timedatectl	list-timezones	Lists available timezones
timedatectl	set-ntp true/false	Enables/disables NTP synchronization
date	(none)	Shows current date and time
date	–date ‘@epoch_time’	Converts epoch time to human readable
hwclock	–systohc	Syncs system time to hardware clock
hwclock	–hctosys	Syncs hardware time to system clock
chronyc	sources	Shows NTP sources	For chrony service
chronyc	tracking	Shows NTP tracking information
tzselect	(none)	Interactive timezone selection

Used Scenarios with Q/A:

Questions	Answers
Check current system time and date	date and timedatectl
Set system timezone	timedatectl set-timezone America/New_York
Configure NTP time synchronization	dnf install chrony, systemctl enable –now chronyd
Set up chrony as NTP client	Edit /etc/chrony.conf: server 0.rhel.pool.ntp.org iburst server 1.rhel.pool.ntp.org iburst server 2.rhel.pool.ntp.org iburst server 3.rhel.pool.ntp.org iburst
Configure chrony with multiple time servers	Add multiple servers in /etc/chrony.conf as shown above
Set up local time server	Edit /etc/chrony.conf: add allow 192.168.1.0/24 and use local stratum 10
Troubleshoot time synchronization	chronyc sources -v, chronyc tracking, chronyc makestep
Monitor time service status	systemctl status chronyd, chronyc activity
Configure time service logging	Edit /etc/chrony.conf: set logdir /var/log/chrony
Set up time service for virtual machines	Add clock=pit to kernel parameters for better time in VMs
Configure hardware clock synchronization	hwclock –systohc to sync hardware clock
Backup time service configuration	tar -czf chrony-backup.tar.gz /etc/chrony.conf /var/lib/chrony/
Restore time service configuration	Restore from backup or reconfigure
Troubleshoot time drift issues	chronyc tracking check offset, chronyc makestep to force sync
Configure time service security	Use NTP authentication with keys in /etc/chrony.conf
Set up time service monitoring	chronyc sourcestats, set up monitoring with Nagios/Zabbix
Configure time service for different timezones	timedatectl list-timezones, set per-user if needed
Implement redundant time servers	Configure multiple independent time sources
Monitor time service performance	chronyc tracking check stability and offset
Troubleshoot certificate time validation	Check certificate validity dates with openssl x509 -in cert.pem -text | grep -A 2 -B 2 Validity

25. Managing Containers

Command	Arguments	Use Case / Description	Important Notes
podman	run -d –name name image	Runs container in background
podman	run -it image /bin/sh	Runs container with interactive shell
podman	ps	Lists running containers
podman	ps -a	Lists all containers
podman	stop container	Stops a container
podman	start container	Starts a container
podman	restart container	Restarts a container
podman	rm container	Removes a container
podman	images	Lists container images
podman	rmi image	Removes container image
podman	pull image	Pulls image from registry
podman	exec -it container /bin/sh	Executes command in running container
podman	inspect image/container	Shows detailed container/image info
podman	search term	Searches for container images
podman	login registry	Logs into container registry
podman	info	Shows container environment info
podman	build -t name .	Builds image from Containerfile
podman	generate systemd –name container –files	Generates systemd service file
skopeo	inspect docker://image	Inspects remote images without pulling
buildah	bud -t image .	Builds image from Containerfile	Alternative to podman build

Used Scenarios with Q/A:

Questions	Answers
Install container runtime (Podman)	dnf install podman
Pull container image from registry	podman pull docker.io/nginx:latest
List available container images	podman images
Run container in foreground	podman run nginx:latest
Run container in background	podman run -d –name mynginx nginx:latest
Execute commands in running container	podman exec -it mynginx /bin/bash
Stop and remove containers	podman stop mynginx && podman rm mynginx
Create container from Containerfile	Create Containerfile: FROM docker.io/centos:8 RUN dnf install -y httpd COPY index.html /var/www/html/ EXPOSE 80 CMD [“httpd”, “-DFOREGROUND”] Then: podman build -t myapp .
Configure container networking	podman run –network=host nginx or podman run -p 8080:80 nginx
Set up container storage	podman run -v /host/data:/container/data nginx
Manage container logs	podman logs mynginx
Configure container resource limits	podman run –memory=512m –cpus=1.5 nginx
Set up container health checks	Create healthcheck in Containerfile: HEALTHCHECK –interval=30s –timeout=3s \ CMD curl -f http://localhost/ || exit 1
Create container pod	podman pod create –name mypod, podman run –pod mypod nginx
Configure container registries	podman login docker.io, configure registries in /etc/containers/registries.conf
Backup container data	podman commit mynginx mynginx-backup or podman export mynginx > backup.tar
Monitor container performance	podman stats mynginx, podman top mynginx
Troubleshoot container issues	Check: podman logs mynginx, podman inspect mynginx, verify resources
Set up container as systemd service	Create /etc/systemd/system/container-myservice.service: [Unit] Description=My Container Service [Service] Restart=always ExecStart=/usr/bin/podman run –name myservice myimage:latest ExecStop=/usr/bin/podman stop myservice [Install] WantedBy=multi-user.target
Implement container security best practices	Use SELinux labels: podman run –security-opt label=type:container_t myapp, run as non-root, use trusted images