Essential Linux Commands Every Sysadmin Should Know

System Information

# uname -a

– display kernel, OS, and CPU details # head -n 1 /etc/issue – show OS version # cat /proc/cpuinfo – view CPU information # hostname – display the host name # env – list environment variables # dmidecode – retrieve detailed hardware data

Resources

# free -m

– show memory and swap usage # df -h – display disk space usage for all partitions # du -sh – report size of a specific directory # uptime – show system uptime, user count, and load average

Network

# ifconfig

– list network interface properties # iptables -L – view firewall rules # route -n – display routing table # netstat -lntp – show all listening ports # netstat -antp – list established connections # netstat -s – network statistics # lsof – list open files and network connections

Processes

# ps -ef

– list all processes # top – real‑time process monitoring

Users

# w

– display currently logged‑in users # id – show user identity information # last – view login history # finger root – display user details # cut -d: -f1 /etc/passwd – list all system users # cut -d: -f1 /etc/group – list all groups # crontab -l – show current user's scheduled jobs

Services

# chkconfig --list

– list all services # chkconfig --list | grep on – list services enabled at boot

Packages

# rpm -qa

– list all installed packages

Security / History

To disable Bash command history:

export HISTSIZE=0

export HISTFILE=/dev/null

FTP Commands

Common FTP client commands (options in brackets are optional): bin – use binary transfer mode bye – exit the FTP session cdup – change to the parent directory on the remote host chmod mode filename – change remote file permissions (e.g., chmod 777 a.out) delete remote-file – remove a file on the remote host get remote-file [local-file] – download a file ls [remote-dir] [local-file] – list remote directory contents and optionally save to a local file mkdir dir-name – create a directory on the remote host put local-file – upload one or more files to the remote host quit – exit the FTP session (same as bye) system – display the remote host's operating system type