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Linux-Commands-Cheat-Sheet

A collection of common Linux commands for system navigation, file management, networking, and more.

Table of Contents

  1. Basic Commands
  2. Operators
  3. File and Directory Management
  4. File Permissions
  5. Screen
  6. Shutdown and Sleep
  7. User Management
  8. Package Management
  9. Cron Jobs and Scheduling
  10. Process Management
  11. System Monitoring
  12. Systemd Management
  13. Networking
  14. SSH Management
  15. Mail Management
  16. Kernel and Modules Management
  17. Boot, Bootloader (GRUB), and EFI Firmware

Basic Commands

Command Description
pwd Print the current working directory
ls List files in the directory
cd <directory> Change directory
whoami Display current user
echo <text> Print text to terminal
history Show command history

System Information

Command Description
uname -a Display detailed system information (kernel, hostname, etc.)
uname -r Show the kernel version
hostname Show or set the system hostname
hostnamectl Display or set the system's hostname and related settings
uptime Show how long the system has been running and system load
whoami Display the current user
id Show the current user’s ID and group information
who Show who is currently logged into the system
w Display who is logged in and what they are doing
date Show or set the system date and time
cal Display a calendar for the current month
df -h Show disk space usage in human-readable format
du -h <directory> Display the disk usage of files and directories in human-readable format
du -sh <directory> Display the total size of a directory
top Display real-time system resource usage and processes
free -h Show memory usage in human-readable format
lscpu Display CPU architecture information
lsblk List information about block devices (disks, partitions)
dmesg Display messages from the kernel ring buffer
lsusb List information about USB devices
lspci List information about PCI devices
uptime Display the system’s uptime and load averages
last Show the last logins of users
uname -m Show the machine hardware name (e.g., x86_64)

Operators

Operator Description
` ` (Pipe)
> Redirect output to a file, overwriting the file if it exists (echo "Hello" > file.txt)
>> Redirect output to a file, appending if the file exists (echo "Hello" >> file.txt)
< Redirect input from a file to a command (sort < file.txt)
2> Redirect error output to a file (command 2> error.log)
2>&1 Redirect error output to standard output (command > file.txt 2>&1)
& Run a command in the background (command &)
&& Run the next command only if the previous command succeeds (command1 && command2)
`
; Run multiple commands in sequence (command1; command2; command3)
$(command) Command substitution: use the output of a command as an argument (echo $(date))
&> Redirect both standard output and error output to a file (command &> output.log)

File and Directory Management

Command Description
ls List files and directories in the current directory
ls -l List files and directories with detailed information
ls -a List all files, including hidden files
cd <directory> Change to a specific directory
cd .. Move up one directory level
pwd Display the current working directory
mkdir <directory> Create a new directory
rmdir <directory> Remove an empty directory
rm <file> Delete a file
rm -r <directory> Remove a directory and its contents recursively
rm -rf <directory> Forcefully remove a directory and its contents
cp <source> <destination> Copy files or directories
cp -r <source-directory> <destination> Copy directories recursively
mv <source> <destination> Move or rename files and directories
touch <file> Create an empty file or update the timestamp of an existing file
ln -s <target> <link> Create a symbolic link (soft link)
ln <target> <link> Create a hard link
cat <file> Display the contents of a file
less <file> View the contents of a file page by page
more <file> View the contents of a file page by page (older than less)
head <file> Display the first 10 lines of a file
tail <file> Display the last 10 lines of a file
tail -f <file> Display the contents of a file in real-time (follow the file as it grows)
find <directory> -name <filename> Search for a file or directory by name
find <directory> -type d -name <dirname> Find directories matching a specific name
find <directory> -type f -name <filename> Find files matching a specific name
grep "<pattern>" <file> Search for a specific pattern in a file
grep -r "<pattern>" <directory> Search for a pattern recursively in a directory

File Permissions

Command Description
ls -l List files and show their permissions, owner, and group
chmod <permissions> <file> Change the permissions of a file or directory
chmod 644 <file> Set read/write for owner and read-only for group and others
chmod 755 <directory> Set read/write/execute for owner, read/execute for group and others
chown <owner>:<group> <file> Change the owner and group of a file or directory
chown <owner> <file> Change the owner of a file
chgrp <group> <file> Change the group of a file

Screen

Command Description
screen Start a new screen session
screen -S <session_name> Start a new screen session with a custom name
screen -ls List all active screen sessions
screen -r <session_id> Reattach to a detached screen session
Ctrl + a + d Detach from the current screen session
screen -X -S <session_id> quit Force quit a screen session
Ctrl + a + k Kill the current screen
screen -x <session_id> Attach to a running session shared by multiple users
Ctrl + a + n Switch to the next window in the screen session
Ctrl + a + p Switch to the previous window in the screen session
Ctrl + a + c Create a new window in the current screen session
Ctrl + a + " List all windows in the current session

Shutdown and Sleep

Command Description
shutdown now Shut down the system immediately
shutdown -h now Halt the system immediately
shutdown -r now Restart the system immediately
shutdown -h +<time> Schedule shutdown after a specified time (e.g., +10 for 10 minutes)
shutdown -c Cancel a scheduled shutdown
reboot Reboot the system
halt Halt the system without powering it off
systemctl poweroff Power off the system
systemctl reboot Reboot the system
systemctl suspend Suspend the system (sleep mode)
systemctl hibernate Hibernate the system
systemctl hybrid-sleep Hibernate and suspend the system
pm-suspend Suspend the system (if pm-utils is installed)
pm-hibernate Hibernate the system (if pm-utils is installed)
pm-suspend-hybrid Hibernate and suspend the system (if pm-utils is installed)

User Management

Command Description
adduser <username> Create a new user with home directory and default shell
useradd <username> Create a new user (minimal setup, no home directory by default)
userdel <username> Delete a user account
userdel -r <username> Delete a user account and their home directory
passwd <username> Change the password for a user
chage -l <username> Display password aging information for a user
usermod -aG <groupname> <username> Add a user to a group
groups <username> List groups a user is part of
id <username> Show user ID (UID), group ID (GID), and other group memberships
deluser <username> Remove a user from the system
deluser <username> <groupname> Remove a user from a specific group
sudo <command> Execute a command as the superuser
su <username> Switch to another user account (requires that user's password)
who Show who is logged in
whoami Display the current logged-in user
last Show last logins of users
w Display who is logged in and what they are doing
finger <username> Display detailed information about a user
getent passwd Display all users in the system
vipw Safely edit the /etc/passwd file (user accounts)
visudo Safely edit the /etc/sudoers file
chown <owner>:<group> <file> Change ownership of a file
chmod <permissions> <file> Change file permissions

Group Management

Command Description
groupadd <groupname> Create a new group
groupdel <groupname> Delete a group
gpasswd -a <username> <groupname> Add a user to a group (alternative to usermod -aG)
gpasswd -d <username> <groupname> Remove a user from a group
newgrp <groupname> Switch to a new group for the current session
groups <username> Display the groups a user belongs to
getent group <groupname> Show group entry in /etc/group

Account and Session Management

Command Description
chage -E <date> <username> Set an account expiration date for a user
chage -M <days> <username> Set maximum number of days a password remains valid
chage -m <days> <username> Set minimum number of days before a password can be changed
chage -I <days> <username> Set the number of days after a password expires before an account is disabled
faillog -u <username> Show login failure statistics for a user
faillog -r <username> Reset failed login count for a user
lastlog Show the last login of all users
pkill -u <username> Terminate all processes owned by a user
passwd -l <username> Lock a user account (disallow login)
passwd -u <username> Unlock a user account
nologin Prevent a user from logging in by setting their shell to /sbin/nologin

Package Management

Debian/Ubuntu

Command Description
apt-get update Update package list
apt-get upgrade Upgrade all installed packages
apt-get install <package> Install a package
apt-get remove <package> Remove a package

Red Hat/CentOS

Command Description
yum update Update package list
yum install <package> Install a package
yum remove <package> Remove a package

Cron Jobs and Scheduling

Managing Cron Jobs

Command Description
crontab -e Edit the current user's crontab file
crontab -l List the current user's cron jobs
crontab -r Remove the current user's crontab file
sudo crontab -e -u <username> Edit the crontab for a specified user
sudo crontab -l -u <username> List the crontab for a specified user
sudo crontab -r -u <username> Remove the crontab for a specified user

Cron Job Syntax

Cron jobs are defined using the following syntax:

* * * * * command_to_run

Example Cron Job Entries

Entry Description
0 5 * * * /path/to/script.sh Run a script every day at 5:00 AM
*/15 * * * * /path/to/backup.sh Run a backup script every 15 minutes
0 0 1 * * /path/to/report.sh Run a report script at midnight on the first day of every month
30 2 * * 1 /path/to/cleanup.sh Run a cleanup script every Monday at 2:30 AM

System-Wide Cron Jobs

System-wide cron jobs are configured in /etc/crontab and can also be found in the /etc/cron.d/ directory.

Scheduling with at

The at command is used to schedule one-time tasks.

Command Description
at 10:00 Schedule a command to run at 10:00 AM (prompt for the command)
`echo "command_to_run" at 10:00`
atq List scheduled jobs for the current user
atrm <job_number> Remove a scheduled job by its job number

Process Management:

Viewing Processes

Command Description
ps aux List all running processes in detail
ps -ef List processes with full-format output
top Show real-time system resource usage (CPU, memory, processes)
htop Enhanced interactive process viewer (htop needs to be installed separately)
pgrep <name> Find the PID(s) of a process by name
pidof <process> Find the PID of a running process
pstree Display processes in a tree-like format

Managing Processes

Command Description
kill <PID> Terminate a process by PID
kill -9 <PID> Forcefully kill a process
killall <name> Kill all processes by name
pkill <name> Kill processes by name (supports regular expressions)
bg Resume a suspended job in the background
fg Bring a background job to the foreground
jobs List all background jobs in the current shell
nice -n <priority> <command> Start a process with a specified priority (lower value = higher priority)
renice <priority> <PID> Change the priority of a running process

Monitoring Processes

Command Description
top Display real-time information about running processes and resource usage
htop Interactive version of top (install separately with sudo apt-get install htop)
uptime Show how long the system has been running
free -m Display system memory usage in megabytes
vmstat Report virtual memory statistics
lsof List open files by processes
strace -p <PID> Trace system calls made by a process
watch <command> Execute a command repeatedly and monitor its output in real-time

Signals

Linux processes can be sent various signals to control their behavior.

Signal Number Description
SIGHUP 1 Hangup, reload configuration
SIGINT 2 Interrupt from keyboard (Ctrl + C)
SIGKILL 9 Kill signal, cannot be ignored
SIGTERM 15 Termination signal
SIGSTOP 19 Stop process (cannot be ignored)
SIGCONT 18 Continue a stopped process

Background & Foreground Process Management

Command Description
command & Run a command in the background
bg Resume a suspended job in the background
fg Bring a background job to the foreground
jobs List current jobs and their statuses
nohup <command> & Run a command immune to hangups, in the background
disown <job> Remove a job from the job table, leaving it running

Process Ownership and Security

Command Description
ps -u <username> List all processes belonging to a user
sudo -u <username> <command> Run a command as a different user
chown <owner>:<group> <file> Change ownership of files or directories
chmod <permissions> <file> Modify permissions for a file or directory
setfacl -m u:<user>:rwx <file> Set file access control lists (ACLs) for a user on a file
getfacl <file> View ACL permissions of a file

Monitoring CPU and Memory Usage by Process

Command Description
top Display processes sorted by CPU usage
htop Interactive real-time process viewer
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu List processes sorted by CPU usage
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem List processes sorted by memory usage

Systemd Management

systemd is the system and service manager for Linux operating systems. It is responsible for managing services, processes, and various units.

Command Description
systemctl Main command for managing systemd services and units
systemctl status <service> Check the status of a service
systemctl start <service> Start a service
systemctl stop <service> Stop a running service
systemctl restart <service> Restart a service
systemctl reload <service> Reload the configuration of a service without restarting it
systemctl enable <service> Enable a service to start on boot
systemctl disable <service> Disable a service from starting on boot
systemctl is-enabled <service> Check if a service is enabled to start at boot
systemctl daemon-reload Reload systemd manager configuration files after changes
systemctl list-units --type=service List all systemd services
systemctl list-units --failed List failed services
journalctl -u <service> View logs for a specific service
journalctl -xe View systemd logs with more details on errors
systemctl mask <service> Completely disable a service, even if another service tries to start it
systemctl unmask <service> Re-enable a masked service
systemctl show <service> Show detailed properties of a service
systemctl cat <service> View the unit file for a service
systemctl isolate <target> Switch to a specific system target (e.g., multi-user.target, rescue.target)

Systemd Unit Types

Systemd manages various types of units, not just services.

Unit Type Description
.service A system service (e.g., web servers, databases)
.socket A socket for inter-process communication
.device A device unit exposed by the Linux kernel
.mount A file system mount point
.automount A mount point that is automatically mounted on access
.swap A swap device or file
.target A group of systemd units (e.g., multi-user.target)
.timer A scheduling unit similar to cron jobs

Common Targets

Command Description
systemctl get-default Show the current default target (e.g., multi-user, graphical)
systemctl set-default <target> Set the default target (e.g., multi-user.target, graphical.target)
systemctl isolate <target> Switch the system to the specified target (e.g., rescue.target)

Networking

Network Interface Management

Command Description
ip addr Show all network interfaces and their IP addresses
ip addr show <interface> Display information about a specific network interface
ip link set <interface> up Bring a network interface up
ip link set <interface> down Bring a network interface down
ifconfig Display or configure network interface (older command, use ip for new systems)
ifconfig <interface> Show details about a specific network interface
ifdown <interface> Take a network interface down (Debian-based systems)
ifup <interface> Bring a network interface up (Debian-based systems)
ethtool <interface> Display or change Ethernet device settings
ip route Display or manipulate the routing table
ip route add <destination> via <gateway> Add a new route to the routing table
ip route del <destination> Remove a route from the routing table
hostname Show or set the system’s hostname
hostname -I Display all IP addresses of the host
nmcli dev show <interface> Display the connection status of a specific network interface (NetworkManager)
nmcli device status Show status of all network interfaces (NetworkManager)
nmcli connection show Display saved network connections (NetworkManager)
nmcli connection up <connection-name> Bring up a specific network connection (NetworkManager)
nmcli connection down <connection-name> Bring down a specific network connection (NetworkManager)
nmtui Text-based user interface for managing network connections

DNS Management

Command Description
dig <domain> Query DNS information for a domain
dig +short <domain> Get the short output (e.g., only the IP address)
dig @<nameserver> <domain> Query a specific DNS server for domain resolution
nslookup <domain> Query DNS information for a domain
host <domain> Perform DNS lookup and reverse lookup
systemd-resolve --status Show DNS resolution status (systemd-resolved)
resolvectl query <domain> Query DNS using systemd-resolved
cat /etc/resolv.conf Display DNS server configuration

Network Troubleshooting

Command Description
ping <host> Check connectivity to a host
ping -c <count> <host> Send a specific number of ICMP echo requests to a host
ping6 <host> Send an ICMP echo request to a host using IPv6
traceroute <host> Trace the path packets take to reach a host
tracepath <host> Similar to traceroute, but does not require superuser privileges
mtr <host> Combines ping and traceroute to provide continuous network analysis
netstat -tuln Show listening ports and services
ss -tuln Show listening sockets (more modern than netstat)
nc -zv <host> <port> Check if a specific port is open on a host (TCP/UDP connection test)
nc -l <port> Open a port and listen for incoming connections
telnet <host> <port> Check TCP connectivity to a host and port
curl <url> Perform a basic HTTP request and display the response
curl -I <url> Display HTTP headers for a URL
curl -O <url> Download a file from a URL
wget <url> Download files from the web using HTTP, HTTPS, or FTP
arp -a Display the system's ARP table (shows MAC to IP mappings)
tcpdump Capture and analyze network traffic
tcpdump -i <interface> Capture packets on a specific network interface
tcpdump -nn -X port <port> Capture packets on a port and display the packet contents in hexadecimal

Firewall Management (iptables and firewalld)

Command Description
iptables -L List all firewall rules
iptables -A <chain> -p <protocol> --dport <port> -j <target> Add a rule to a chain (e.g., ACCEPT, DROP)
iptables -D <chain> <rule-number> Delete a specific rule from a chain
iptables -F Flush (delete) all firewall rules
firewall-cmd --list-all List all active firewalld rules (CentOS, Fedora)
firewall-cmd --add-service=<service> Temporarily allow a service through the firewall
firewall-cmd --permanent --add-service=<service> Permanently allow a service through the firewall
firewall-cmd --remove-service=<service> Remove a service from the firewall
ufw status Display the status of UFW (Uncomplicated Firewall) (Ubuntu)
ufw enable Enable UFW
ufw disable Disable UFW
ufw allow <port>/<protocol> Allow traffic on a port (e.g., ufw allow 22/tcp)
ufw deny <port>/<protocol> Deny traffic on a port

Network File Transfer

Command Description
scp <source> <user>@<host>:<destination> Securely copy files to a remote host
scp <user>@<host>:<source> <destination> Securely copy files from a remote host
rsync -avz <source> <user>@<host>:<destination> Synchronize files between local and remote hosts
rsync -avz <user>@<host>:<source> <destination> Synchronize files from remote to local
sftp <user>@<host> Start a secure file transfer session
ftp <host> Start an FTP session (unencrypted)

SSH and Remote Connections

Command Description
ssh <user>@<host> Connect to a remote host via SSH
ssh -i <keyfile> <user>@<host> Connect to a remote host using a specific private key
ssh -L <local-port>:<remote-host>:<remote-port> <user>@<host> Create an SSH tunnel with port forwarding
ssh-copy-id <user>@<host> Copy local SSH key to a remote host to enable passwordless login
scp <user>@<host>:<source> <destination> Securely copy files between local and remote hosts
sftp <user>@<host> Securely transfer files to/from a remote host using SFTP
tmux Start a terminal multiplexer session (maintain SSH sessions)
screen Start a screen session (maintain SSH sessions)

System Monitoring

This section provides commands for monitoring system performance, processes, CPU, memory, disk I/O, and network activity.

General System Monitoring

Command Description
top Real-time view of CPU and memory usage by processes
htop Enhanced, interactive version of top with additional features (may require installation)
vmstat Report virtual memory statistics, including processes, memory, paging, block I/O, and CPU activity
glances Comprehensive system monitoring tool (requires installation)
uptime Show how long the system has been running and system load
watch <command> Run a command repeatedly at regular intervals and display the output live

CPU Monitoring

Command Description
mpstat Display CPU usage per core (from sysstat package)
sar Collect, report, and save system activity information (including CPU, memory, I/O, etc.)
lscpu Display CPU architecture information (model, cores, speed)
cat /proc/cpuinfo Display detailed CPU information
iostat CPU and I/O statistics, part of the sysstat package

Memory Monitoring

Command Description
free -h Display memory usage in human-readable format (total, used, free, swap)
vmstat -s Memory and swap statistics in a detailed format
cat /proc/meminfo View detailed memory statistics
watch free -h Monitor memory usage in real-time
dmidecode --type memory Display detailed information about installed memory (requires root)

Disk Usage and I/O Monitoring

Command Description
df -h Display disk space usage in human-readable format
du -sh <directory> Display the total size of a directory in human-readable format
iostat Show disk I/O and CPU usage statistics (from sysstat package)
iotop Display real-time disk I/O usage by processes (requires installation)
dstat Comprehensive resource statistics (including disk I/O)
lsblk List block devices, partitions, and file systems
blkid Display block device attributes such as UUIDs and file system types
df -i Show inode usage for file systems
du -ah <directory> Show disk usage for all files and directories
hdparm -t /dev/sda Test read speed of a disk (may require installation)

Process Monitoring

Command Description
ps aux List all running processes with detailed information
pgrep <process-name> Search for a running process by name
pidof <process-name> Find the process ID (PID) of a running process
pstree Show processes in a tree-like format
kill <PID> Send a signal to terminate a process using its PID
killall <process-name> Terminate all processes matching a specific name
pkill <process-name> Send a signal to processes based on name and other attributes
renice <priority> <PID> Change the priority of a running process
strace -p <PID> Trace system calls and signals for a specific process
lsof List open files by processes
lsof -i :<port> List processes using a specific network port
nice -n <priority> <command> Start a command with a specified priority

Network Monitoring

Command Description
netstat -tuln Show listening ports and their associated services
ss -tuln More modern version of netstat, shows listening ports
iftop Display real-time network bandwidth usage (requires installation)
nload Show incoming and outgoing network traffic (requires installation)
tcpdump -i <interface> Capture and display network packets on a specific interface
iptraf-ng Real-time network traffic monitoring (requires installation)
ping <hostname/IP> Test network connectivity by sending ICMP echo requests
traceroute <hostname/IP> Trace the path packets take to reach a network host
ss -s Show detailed socket statistics
arp -a Display the system's ARP table (address resolution protocol)
dig <hostname> Query DNS servers for information about a hostname
host <hostname> Perform DNS lookup for a domain or IP address
nslookup <hostname> Query DNS to resolve a hostname to an IP address
mtr <hostname> Network diagnostic tool combining ping and traceroute (requires installation)

Miscellaneous System Monitoring

Command Description
uptime Show how long the system has been running and the current load average
last Show the last logins of users on the system
sar Collect and display system activity statistics (CPU, memory, I/O, network)
dstat Tool for generating detailed performance and resource statistics
vmstat Report information about processes, memory, paging, and I/O
iostat -x Show extended I/O statistics, including disk utilization
tload Show a graphical representation of system load in the terminal
uptime Display system uptime and load averages
w Show who is logged in and what they are doing
who Show who is logged in to the system

SSH Management

SSH (Secure Shell) is a protocol used for securely accessing remote machines over a network. This section covers commands for managing SSH connections, keys, and configuration.

Basic SSH Commands

Command Description
ssh <user>@<host> Connect to a remote host as a specified user
ssh -p <port> <user>@<host> Connect to a remote host on a specific port
ssh -i <keyfile> <user>@<host> Connect using a specific private key file
ssh -L <local_port>:<remote_host>:<remote_port> <user>@<host> Create an SSH tunnel for local port forwarding
ssh -R <remote_port>:<local_host>:<local_port> <user>@<host> Create an SSH tunnel for remote port forwarding
ssh -D <local_port> <user>@<host> Set up a SOCKS proxy on the specified local port
scp <file> <user>@<host>:<remote_path> Copy a file to a remote host using SSH
scp <user>@<host>:<remote_path> <local_path> Copy a file from a remote host to the local machine
rsync -avz -e "ssh -p <port>" <source> <user>@<host>:<destination> Sync files with a remote host over SSH

SSH Key Management

SSH keys provide a secure way to authenticate without using passwords.

Command Description
ssh-keygen -t rsa -b 4096 -C "[email protected]" Generate a new SSH key pair (RSA, 4096 bits)
ssh-add <keyfile> Add a private key to the SSH authentication agent
ssh-copy-id <user>@<host> Install your public key on a remote host for passwordless authentication
cat ~/.ssh/id_rsa.pub Display your public key for sharing
ssh-agent bash Start a new shell with the SSH agent running
ssh-keygen -R <host> Remove the specified host from the known_hosts file
ssh -Q key List supported key types

Common SSH Configuration Options

Option Description
Host <name> Alias for a host configuration
HostName <hostname> Actual hostname or IP address to connect to
User <username> Default username to log in as
Port <port_number> Port to connect to (default is 22)
IdentityFile <path> Path to the private key file for authentication
ForwardAgent yes Enable SSH agent forwarding
StrictHostKeyChecking no Disable host key verification (not recommended for security)

SSH Key Management for Users

Command Description
ssh-keygen Generate a new SSH key pair
ssh-copy-id <username>@<host> Copy SSH key to a remote host for passwordless login
cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys Manually add SSH public key for user authentication
chmod 600 ~/.ssh/authorized_keys Set correct permissions for SSH authorized keys

SSH Security Best Practices

Best Practice Description
Use SSH keys instead of passwords SSH keys provide better security than passwords.
Disable root login Prevent direct SSH login as root by editing /etc/ssh/sshd_config:
```plaintext
PermitRootLogin no
```
Use strong passphrases for SSH keys Protect private keys with strong passphrases.
Change the default SSH port Edit /etc/ssh/sshd_config and change Port 22 to another port.
Enable two-factor authentication Add an extra layer of security with 2FA using tools like Google Authenticator.

SSH Configuration

The SSH client can be configured using the ~/.ssh/config file for convenience and ease of use.

Example SSH Configurations

# Default settings
Host *
    User your_username
    Port 22
    IdentityFile ~/.ssh/id_rsa

# Specific host configuration
Host myserver
    HostName myserver.example.com
    User myuser
    Port 2222
    IdentityFile ~/.ssh/myserver_id_rsa

Mail Management

Sending Mail

Command Description
mail <recipient> Open the mail client to send an email
`echo "Email body" mail -s "Subject" `
sendmail <recipient> Send an email using the sendmail command
mailx -s "Subject" <recipient> < <file> Send the contents of a file as the body of the email
mutt -s "Subject" <recipient> < <file> Send an email with a subject using mutt
ssmtp <recipient> Send an email using ssmtp (simple SMTP client)
mail -c <cc_recipient> Send a carbon copy (CC) of the email to another recipient
mail -b <bcc_recipient> Send a blind carbon copy (BCC) of the email to another recipient

Common Mail Services

Service Description
postfix A widely used mail transfer agent (MTA)
sendmail A traditional mail transfer agent, often used for sending mail
exim Another popular mail transfer agent
ssmtp A simple utility to send emails via SMTP
mutt A text-based email client for Unix
mailx An enhanced version of the mail command

Mail Troubleshooting

Command Description
tail -f /var/log/mail.log Monitor mail logs for sending issues (Debian/Ubuntu)
grep "error" /var/log/mail.log Search for errors in the mail log
mailq View the mail queue
sendmail -bv <recipient> Verify if an email can be sent to a recipient

Mail Configuration

Configuration files for mail services can typically be found in /etc/mail/ or /etc/.

Example /etc/ssmtp/ssmtp.conf Configuration

root=postmaster
mailhub=smtp.example.com:587
AuthUser=username
AuthPass=password
UseSTARTTLS=YES

Kernel and Modules Management

This section provides commands for managing the Linux kernel and its modules, including loading, unloading, and querying kernel modules.

Kernel Information

Command Description
uname -r Display the current kernel version
uname -a Display all system information, including the kernel version
hostnamectl Show information about the host, including kernel version
cat /proc/version Display kernel version information from the /proc filesystem
dmesg Display boot and kernel log messages
`dmesg grep `
ls /boot/ List the files related to the kernel, including kernel images

Kernel Module Management

Command Description
lsmod List currently loaded kernel modules
modinfo <module_name> Display detailed information about a specific kernel module
modprobe <module_name> Load a kernel module into the running kernel
modprobe -r <module_name> Remove a kernel module from the running kernel
insmod <path_to_module> Insert a module into the kernel (requires the full path to the module file)
rmmod <module_name> Remove a kernel module
depmod -a Generate a list of module dependencies
modprobe --show-depends <module_name> Show module dependencies before loading it
ls /lib/modules/$(uname -r)/kernel/ List the kernel modules available for the current kernel

Kernel Parameters Management

Command Description
sysctl -a Display all kernel parameters (sysctl settings)
sysctl <parameter> View the current value of a specific kernel parameter
sysctl -w <parameter>=<value> Set the value of a kernel parameter at runtime
cat /proc/sys/<parameter> View a kernel parameter's current value directly from the /proc filesystem
echo <value> > /proc/sys/<parameter> Temporarily change a kernel parameter (until reboot)
nano /etc/sysctl.conf Edit the sysctl configuration file to apply kernel parameter changes permanently
sysctl -p Reload sysctl settings from the configuration file

Kernel Boot Parameters

Command Description
cat /proc/cmdline Show the kernel parameters passed at boot time
nano /etc/default/grub Edit GRUB configuration to permanently modify kernel boot parameters
grub-mkconfig -o /boot/grub/grub.cfg Rebuild the GRUB configuration file after making changes
update-grub Update GRUB to apply boot configuration changes

Rebuilding the Initramfs

Command Description
mkinitcpio -p linux Rebuild the initramfs for Arch-based systems
update-initramfs -u Update the initramfs for the current kernel (Debian/Ubuntu)
dracut -f Regenerate initramfs on systems using Dracut

Kernel Upgrade and Management

Command Description
apt-cache search linux-image Search for available kernel versions (Debian/Ubuntu)
apt install linux-image-<version> Install a specific kernel version (Debian/Ubuntu)
dnf list kernel List available kernel versions (Fedora)
dnf install kernel-<version> Install a specific kernel version (Fedora)
yum update kernel Update the kernel to the latest version (CentOS/Red Hat)
grubby --default-kernel Show the default kernel used at boot time
grubby --set-default /boot/vmlinuz-<version> Set a specific kernel as the default for booting (RHEL/CentOS)
reboot Reboot the system to load the newly installed kernel

Kernel Compilation (Advanced)

Command Description
make menuconfig Configure kernel options before compiling
make -j$(nproc) Compile the kernel using all available CPU cores
make modules_install Install compiled kernel modules
make install Install the compiled kernel
make mrproper Clean up all kernel build files and configurations
cp /boot/config-$(uname -r) .config Copy the current kernel config file as a base for compiling

Boot, Bootloader (GRUB), and EFI Firmware

This section covers commands related to system boot, managing the GRUB bootloader, and working with EFI/UEFI firmware.


GRUB Bootloader Management

GRUB (GRand Unified Bootloader) is the default bootloader for many Linux distributions, and it manages which kernel or operating system to boot.

Common GRUB Commands

Command Description
grub-install /dev/sda Install GRUB bootloader on the specified disk (e.g., /dev/sda)
grub-mkconfig -o /boot/grub/grub.cfg Generate the GRUB configuration file (run after installing or updating GRUB)
update-grub Update GRUB configuration (Debian/Ubuntu)
grub-set-default <entry> Set the default boot entry by specifying the menu entry index or title
grub-reboot <entry> Reboot the system into a specific GRUB entry once
grub-editenv list List the saved GRUB environment variables
grub2-mkconfig -o /boot/grub2/grub.cfg Generate GRUB configuration (Red Hat/CentOS/Fedora)
grub2-install /dev/sda Install GRUB on Red Hat-based systems (RHEL/CentOS/Fedora)
nano /etc/default/grub Edit GRUB configuration file for setting timeout, default OS, etc.

GRUB Configuration Example

You can manually edit /etc/default/grub to modify boot options, such as default kernel, timeout, or adding kernel boot parameters.

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_CMDLINE_LINUX="quiet splash"
GRUB_GFXMODE=1920x1080

After making changes, update the GRUB configuration:

sudo update-grub

EFI/UEFI Firmware Management

EFI (Extensible Firmware Interface) or UEFI (Unified Extensible Firmware Interface) is the modern replacement for the traditional BIOS. It manages the system boot process and offers advanced features like secure boot, boot entries management, and boot order control.

EFI/UEFI Commands

Command Description
efibootmgr Manage UEFI boot entries and settings
efibootmgr -v List all UEFI boot entries in verbose mode
efibootmgr -n <boot-number> Set the next boot entry for the system to boot from once
efibootmgr -o 0001,0002 Change the boot order to prioritize boot entry 0001 followed by 0002
efibootmgr -b 0003 -B Remove the UEFI boot entry number 0003
efibootmgr -c -d /dev/sda -p 1 -L "Linux" -l /vmlinuz Create a new boot entry for Linux on the EFI partition on /dev/sda1
efibootmgr -t 5 Set a 5-second timeout for the UEFI boot menu
efibootmgr -a <boot-number> Activate a specific boot entry

Viewing and Modifying EFI Variables

EFI variables contain information about system settings and configuration.

Command Description
ls /sys/firmware/efi/efivars/ List all EFI variables on the system
cat /sys/firmware/efi/fw_platform_size Display whether the platform is 32-bit or 64-bit UEFI
efivar --list List all UEFI variables in a readable format
efivar --print <variable> Display detailed information about a specific EFI variable
modprobe efivars Load the efivars module to access UEFI variables

Secure Boot Management

Secure Boot is a feature of UEFI that only allows signed software (such as bootloaders and operating systems) to boot.

Command Description
mokutil --sb-state Check the current state of Secure Boot
mokutil --enable-validation Enable Secure Boot validation
mokutil --disable-validation Disable Secure Boot validation
mokutil --list-enrolled List the keys enrolled in Secure Boot
mokutil --import <keyfile> Import a new key to be used for Secure Boot
mokutil --reset Reset all Secure Boot keys to their default state

Booting into UEFI Firmware

To enter the UEFI firmware settings from Linux:

  1. Systemd-based distributions:
    systemctl reboot --firmware-setup