Linux Basics Cheat Sheet for Sysadmins

Linux Basics Cheat Sheet for Sysadmins

A handy reference for day-to-day Linux system administration.

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🔹 File Management

• List Files:

  ls -l  # Long listing format
  

• List Hidden Files:

  ls -la  # Includes hidden files (those starting with .)
  

• Copy Files:

  cp source destination  # Copies a file or directory
  

• Move Files / Rename:

  mv source destination  # Moves or renames a file or directory
  

• Remove Files:

  rm file  # Deletes a file
  

• Search for Files:

  find /path -name "filename"  # Searches for a file by name
  

• Search Inside Files:

  grep "pattern" file  # Searches for a pattern inside a file
  

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🔹 User & Group Management

• Add a User:

  sudo useradd username  # Adds a new user
  

• Delete a User:

  sudo userdel username  # Deletes a user
  

• Add a User with Home Directory:

  sudo useradd -m username  # Creates a home directory for the user
  

• Change Password:

  passwd username  # Changes the password for a user
  

• Add User to Group:

  sudo usermod -aG groupname username  # Adds a user to a group
  

• List Groups:

  groups username  # Lists groups a user belongs to
  

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🔹 Process Management

• List Processes:

  ps aux  # Displays all running processes
  

• Tree View of Processes:

  pstree -p  # Shows processes in a tree format
  

• Kill a Process:

  kill <pid>  # Sends a termination signal to a process
  kill -9 <pid>  # Forcefully kills a process
  

• Monitor System Processes:

  top  # Displays real-time system processes
  htop  # Interactive process viewer (if installed)
  

• Check Resource Usage by Process:

  ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head  # Sorts processes by memory usage
  

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🔹 Service & Systemd Management

• Start a Service:

  sudo systemctl start service  # Starts a service
  

• Stop a Service:

  sudo systemctl stop service  # Stops a service
  

• Restart a Service:

  sudo systemctl restart service  # Restarts a service
  

• Enable Service at Boot:

  sudo systemctl enable service  # Enables a service to start at boot
  

• Check Service Status:

  sudo systemctl status service  # Displays the status of a service
  

• List Failed Services:

  systemctl --failed  # Lists all failed services
  

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🔹 Disk & Storage Management

• Check Disk Usage:

  df -h  # Displays disk usage in human-readable format
  

• Check Directory Size:

  du -sh /path/to/directory  # Displays the size of a directory
  

• List Block Devices:

  lsblk  # Lists information about block devices
  

• Check Disk Partitions:

  fdisk -l  # Lists disk partitions
  

• Mount a Filesystem:

  sudo mount /dev/sdX1 /mnt  # Mounts a filesystem to /mnt
  

• Unmount a Filesystem:

  sudo umount /mnt  # Unmounts a filesystem
  

• Check Inodes:

  df -i  # Displays inode usage
  

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🔹 Networking

• Check IP Address:

  ip addr show  # Displays network interfaces and IP addresses
  

• Check Default Route:

  ip route  # Displays the default gateway
  

• Ping a Host:

  ping -c 4 example.com  # Sends 4 ICMP echo requests to a host
  

• Check Open Ports:

  ss -tuln  # Displays listening ports
  

• DNS Lookup:

  dig example.com  # Performs a DNS lookup
  nslookup example.com  # Alternative DNS lookup tool
  

• Check Connectivity:

  curl -I http://example.com  # Fetches HTTP headers to check connectivity
  

• Test Port Connectivity:

  nc -zv host port  # Tests if a port is open on a host
  

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🔹 Permissions & Ownership

• Change Ownership:

  sudo chown user:group file  # Changes ownership of a file
  

• Change Permissions:

  chmod 755 file  # Sets read, write, execute permissions for user, and read/execute for others
  

• Symbolic Modes:

  chmod u+x file   # Adds execute permission for the user
  chmod g-w file   # Removes write permission for the group
  chmod o-r file   # Removes read permission for others
  

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🔹 Logs & Monitoring

• View System Logs:

  journalctl -xe  # Displays recent system logs with details
  

• Logs for a Service:

  journalctl -u servicename  # Displays logs for a specific service
  

• Follow Logs in Real Time:

  tail -f /var/log/syslog  # Follows the system log in real-time
  tail -f /var/log/messages  # Follows the messages log in real-time
  

• Check Last Boot Messages:

  dmesg | less  # Displays kernel messages from the last boot
  

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🔹 Package Management

Debian/Ubuntu

• Update Packages:

  sudo apt update && sudo apt upgrade  # Updates package lists and upgrades installed packages
  

• Install a Package:

  sudo apt install package  # Installs a package
  

• Remove a Package:

  sudo apt remove package  # Removes a package
  

RHEL/CentOS/Fedora

• Update Packages:

  sudo yum update  # Updates all packages
  sudo dnf upgrade  # Updates all packages (dnf for newer systems)
  

• Install a Package:

  sudo yum install package  # Installs a package
  sudo dnf install package  # Installs a package (dnf for newer systems)
  

• Remove a Package:

  sudo yum remove package  # Removes a package
  

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🔹 Security & Hardening

• Check for Active Users:

  who  # Displays logged-in users
  w  # Displays logged-in users with additional details
  

• Check Last Logins:

  last  # Displays login history
  

• Check Sudo Access:

  sudo -l  # Lists sudo privileges for the current user
  

• Lock a User Account:

  sudo usermod -L username  # Locks a user account
  

• Check Listening Ports:

  ss -tulnp  # Displays listening ports with process details
  

• Enable Firewall (UFW example):

  sudo ufw enable  # Enables the firewall
  sudo ufw allow ssh  # Allows SSH traffic
  sudo ufw status  # Displays firewall status
  

• Generate SSH Key Pair:

  ssh-keygen -t ed25519  # Generates an SSH key pair using the ed25519 algorithm
  

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🔹 Quick Recovery & Troubleshooting

• Check System Load:

  uptime  # Displays system uptime and load averages
  

• Check Memory Usage:

  free -h  # Displays memory usage in human-readable format
  

• Check Disk IO:

  iostat  # Displays disk I/O statistics
  

• Check Open Files:

  lsof | less  # Lists open files
  

• Check Failed SSH Logins:

  grep "Failed password" /var/log/auth.log  # Searches for failed SSH login attempts
  

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🔹 Legend (Linux Terms)

• PID – Process ID.
• UID – User ID.
• GID – Group ID.
• TTY – Terminal session.
• Daemon – Background process/service.
• Inode – Index node storing file metadata.
• Mount Point – Directory where a filesystem is attached.
• Systemd – Init system and service manager in modern Linux.
• SELinux/AppArmor – Linux security frameworks.

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🔹 Best Practices

• Keep your system updated regularly.
• Use man <command> or --help to learn command options.
• Use tmux or screen for persistent sessions over SSH.
• Always set up SSH key-based authentication instead of passwords.
• Monitor disk usage with alerts (df -h, du -sh).
• Never run daily tasks as root — use sudo.
• Automate repetitive tasks with shell scripts.
• Regularly review /var/log for errors and security issues.