System Information
uname -a: Displays all system information, including kernel version and hardware architecture.
top: Provides a dynamic real-time view of running system processes.
htop (needs installation on some distros): An interactive process viewer, similar to top but more user-friendly.
df -h: Shows disk space usage on all mounted filesystems, in human-readable form.
free -h: Displays the amount of free and used memory in the system, in human-readable form.
User Management
useradd <username>: Creates a new user account.
usermod -aG <groupname> <username>: Adds a user to a specified group.
passwd <username>: Sets or changes the password for a user account.
chage -l <username>: Lists password aging information for a user.
File and Directory Management
ls -lah: Lists all files and directories in the current directory, including hidden files, with detailed information in a human-readable format.
find / -name <filename>: Searches the entire filesystem for a file or directory named <filename>.
tar czvf archive.tar.gz /path/to/directory: Creates a compressed tar archive of a directory.
chmod +x script.sh: Makes a script executable.
Package Management
apt update && apt upgrade (Debian, Ubuntu, and derivatives): Updates the package index and upgrades all packages to their latest versions.
yum update (CentOS, RHEL): Updates all installed packages to their latest version.
dnf update (Fedora, newer RHEL): Similar to yum, but uses the DNF package manager.
Network Configuration and Troubleshooting
ip addr show: Displays IP addresses and all network interfaces.
netstat -tulnp: Lists all active listening ports along with the corresponding service.
ping google.com: Sends ICMP ECHO_REQUEST packets to google.com to test connectivity.
traceroute google.com: Traces the route packets take from your machine to google.com
Process Management
ps aux: Shows running processes with detailed information.
kill <PID>: Sends a SIGTERM to a process with the specified PID, requesting its termination.
killall <processname>: Terminates all processes with the given name.
systemctl restart <servicename>: Restarts a systemd service.
System Monitoring and Performance
vmstat: Reports virtual memory statistics.
iostat: Provides CPU and input/output statistics for devices and partitions.
lsof: Lists open files and the processes that opened them.
Disk Usage and Files
du -sh /path/to/directory: Shows the total size of a directory and its subdirectories in a human-readable format.
ncdu (needs installation on some distros): An interactive disk usage viewer, providing a detailed breakdown of which files and directories are using disk space.