50 Essential Linux Commands: Functions, Examples, and Practical Tips

1. netstat command (view network connection status)

Function: Displays all active network connections on the system, including TCP, UDP, and UNIX domain sockets.

Usage: netstat [options] Common options: -a show all connections and listening ports; -t show TCP connections; -u show UDP connections; -n display addresses and ports numerically; -p show PID and program name; -r show routing table; -s show network interface statistics; -l show only listening sockets; -c continuously refresh.

Example: netstat – show all connections; netstat -t – show only TCP connections.

Tip: Use netstat -an to display all connections with numeric addresses; netstat -p shows process IDs and names (may require root).

2. pgrep command (find process IDs by name)

Function: Finds the PID(s) of processes matching a given name.

Usage: pgrep [options] pattern Options: -l also display process name; -u limit to a specific user; -n show newest matching process; -o show oldest; -v invert match; -a show full command line; -f match against the full command line; -c output only the count.

Example: pgrep -l nginx – find processes named "nginx" and show name and PID; pgrep -u www-data – find processes run by user www-data.

Tip: pgrep -f searches the full command line; pgrep -l shows name and PID.

3. tar command (archive and compress files)

Function: Packages files and directories into an archive and optionally compresses it.

Usage: tar [options] filename Example: tar -cvf archive.tar file1 file2 – create archive.tar containing file1 and file2; tar -czvf archive.tar.gz dir – archive dir and compress with gzip.

Tip: tar -xvf extracts; tar -tf lists archive contents.

4. scp command (secure copy files)

Function: Copies files and directories between local and remote hosts.

Usage: scp [options] source destination Example: scp file.txt user@remote:/path/to/dir – copy file.txt to remote host; scp -r dir user@remote:/path/to/dir – copy directory recursively.

Tip: Use scp -P port to specify a non‑default SSH port; scp -q suppresses progress output.

5. netstat command (view network connections) – duplicate entry

Function: Shows all active network connections, including TCP, UDP, and UNIX sockets.

Usage: netstat [options] Example: netstat – display all connections; netstat -t – show only TCP.

Tip: netstat -a shows listening and non‑listening sockets; netstat -n displays numeric IPs and ports.

6. pgrep command (find process IDs by name) – duplicate entry

Function: Finds the PID(s) of processes matching a given name.

Usage: pgrep process_name Example: pgrep apache2 – find PIDs of "apache2"; pgrep -u user sshd – find sshd processes started by user.

Tip: pgrep -f searches the full command line; pgrep -l shows name and PID.

7. top command (monitor system processes)

Function: Continuously displays running processes and resource usage.

Usage: top Example: Run top directly.

Tip: Default sort is by CPU usage; Shift+M sorts by memory; top -u username shows only that user’s processes; -c shows full command line; -p PID monitors a specific PID; -H shows thread tree; -i ignores zombie processes.

8. free command (show memory usage)

Function: Displays total, used, and free memory.

Usage: free [options] Example: free – show memory in kilobytes; free -m – show in megabytes.

Tip: Remember to count cached and buffered memory as available; for more detail use cat /proc/meminfo.

9. df command (disk space usage)

Function: Shows filesystem disk space usage.

Usage: df [options] filesystem Example: df – all mounted filesystems; df -h – human‑readable format.

Tip: df -i shows inode usage; df -T shows filesystem type.

10. du command (disk usage of files/directories)

Function: Shows size of files and directories.

Usage: du [options] file/dir Example: du file.txt – size of a file; du -h /path/to/dir – human‑readable size of a directory.

Tip: Append a trailing slash to avoid counting symlinks or mount points; du -s shows total size only.

11. ps command (view process information)

Function: Displays information about running processes.

Usage: ps [options] Example: ps – processes of current user; ps -ef – full‑format list of all processes.

Tip: ps -u username shows processes of a specific user; ps -ef | grep process_name filters by name; ps aux shows all processes including system ones.

12. grep command (search file contents)

Function: Searches files for a given pattern.

Usage: grep [options] pattern file Example: grep "keyword" file.txt – lines containing "keyword"; grep -r "pattern" dir – recursive search.

Tip: grep -i ignores case; grep -v shows non‑matching lines.

13. awk command (text processing and reporting)

Function: Processes and analyzes text files line by line, can generate reports.

Usage: awk [options] 'script' file Example: awk '{print $1}' file.txt – print first field of each line; awk '3 >= 90 {print $1,$2}' file.txt – print first and second fields where third field ≥ 90.

Tip: awk -F sets field separator;

awk 'BEGIN{print "header"} {print $0} END{print "footer"}' file.txt

adds header/footer.

14. sed command (stream editor)

Function: Edits and transforms text streams.

Usage: sed [options] 'script' file Example: sed 's/old/new/g' file.txt – replace all "old" with "new"; sed '/pattern/d' file.txt – delete lines matching pattern.

Tip: sed -i edits files in place; sed -n prints only matched lines.

15. find command (search files and directories)

Function: Searches for files or directories matching criteria.

Usage: find [path] [options] [expression] Example: find /path/to/dir -name "*.txt" – find all .txt files; find /path/to/dir -type d -mtime +7 – directories modified more than 7 days ago.

Tip: find -exec runs a command on each match; find -print0 | xargs -0 safely handles spaces in names.

16. curl command (transfer data)

Function: Transfers data from servers, supports HTTP, HTTPS, FTP, etc.

Usage: curl [options] URL Example: curl https://www.example.com – fetch page; curl -o file.txt https://www.example.com/data.txt – download to file.

Tip: curl -H "header" adds custom header; curl -X POST -d "data" sends POST request.

17. crontab command (schedule recurring tasks)

Function: Sets or removes scheduled system tasks.

Usage: crontab [options] Example: crontab -e – edit current user’s crontab; crontab -l – list tasks.

Tip: crontab -r removes all tasks; crontab -u username views another user’s tasks.

18. sort command (sort lines of text)

Function: Sorts lines in a text file.

Usage: sort [options] file Example: sort file.txt – alphabetical sort; sort -rn file.txt – numeric reverse sort.

Tip: sort -u removes duplicate lines; sort -t sets field delimiter.

19. wc command (word, line, byte count)

Function: Counts lines, words, and characters in a file.

Usage: wc [options] file Example: wc file.txt – shows line, word, byte counts; cat file.txt | wc -l – count lines only.

Tip: wc -c counts bytes; wc -m counts characters (including multibyte).

20. kill command (terminate processes)

Function: Sends a signal to a process to terminate it.

Usage: kill [options] PID Example: kill 1234 – send default SIGTERM; kill -9 1234 – force kill.

Tip: killall terminates by process name.

21. ifconfig command (network interface configuration)

Function: Views and configures network interface information.

Usage: ifconfig [options] interface Example: ifconfig – show all interfaces; ifconfig eth0 up – enable eth0.

Tip: ifconfig -a shows all interfaces, including inactive; ifconfig eth0 down disables the interface.

22. ping command (test network connectivity)

Function: Sends ICMP echo requests to a target IP to test reachability.

Usage: ping [options] host Example: ping www.example.com; ping -c 5 192.168.0.1 – send 5 packets.

Tip: ping -i sets interval; ping -s sets packet size.

23. ssh command (secure remote login)

Function: Executes commands or logs into a remote host over an encrypted channel.

Usage: ssh [options] user@host Example: ssh [email protected]; ssh -p 2222 [email protected] – specify port.

Tip: ssh -X enables X11 forwarding; ssh-copy-id copies local public key for password‑less login.

24. wget command (non‑interactive download)

Function: Downloads files or webpages from a URL.

Usage: wget [options] URL Example: wget https://www.example.com/file.txt; wget -P /path/to/save https://www.example.com/file.txt.

Tip: wget -c continues interrupted downloads; wget -r recursively downloads a site.

25. vim command (text editor)

Function: Edits text files in the terminal.

Usage: vim [options] filename Example: vim file.txt – open file; vim -O2 file1.txt file2.txt – split window horizontally.

Tip: :w save; :q quit; :wq save and quit; i enters insert mode.

26. tail command (view file end)

Function: Shows the last few lines of a file.

Usage: tail [options] filename Example: tail file.txt – last 10 lines; tail -f file.txt – follow new additions.

Tip: tail -n sets number of lines; tail -F follows even if file is renamed.

27. cat command (concatenate and view files)

Function: Displays file contents.

Usage: cat [options] filename Example: cat file.txt; cat file1.txt file2.txt > file3.txt – concatenate.

Tip: cat -n numbers lines; cat -E shows $ at line ends.

28. more command (page through file)

Function: Paginates file output.

Usage: more [options] filename Example: more file.txt; ls -l | more.

Tip: Space advances a page, q quits, /keyword searches.

29. iostat command (disk I/O statistics)

Function: Shows I/O statistics for disks.

Usage: iostat [options] [interval] [count] Example: iostat – all disks; iostat -x sda – detailed stats for sda.

Tip: iostat -p shows per‑partition stats; iostat -d shows per‑device stats.

30. vmstat command (virtual memory statistics)

Function: Displays virtual memory and system activity.

Usage: vmstat [options] [interval] [count] Example: vmstat; vmstat 1 5 – five samples at 1‑second intervals.

Tip: vmstat -s shows detailed memory stats; vmstat -d shows disk I/O.

31. mpstat command (CPU performance monitoring)

Function: Shows performance statistics for multiple CPUs.

Usage: mpstat [options] [interval] [count] Example: mpstat; mpstat -P ALL – all CPUs.

Tip: mpstat -u shows CPU utilization; mpstat -I ALL shows interrupt stats.

32. pidstat command (process performance monitoring)

Function: Shows performance stats for a specific process or all processes.

Usage: pidstat [options] [interval] [count] Example: pidstat; pidstat -p 1234 – stats for PID 1234.

Tip: pidstat -d shows disk I/O; pidstat -r shows memory; pidstat -p ALL shows all processes.

33. ls command (list files and directories)

Function: Lists files and sub‑directories in the current directory.

Usage: ls [options] [file/dir] Example: ls; ls -l – long format; ls -a – include hidden files.

Tip: ls -lh – human‑readable sizes; ls -t – sort by modification time.

34. cd command (change directory)

Function: Changes the current working directory.

Usage: cd [directory] Example: cd /home; cd ..; cd ~.

Tip: cd - returns to previous directory; cd .. moves up one level.

35. mkdir command (create directories)

Function: Creates new directories.

Usage: mkdir [options] dirname Example: mkdir new_dir; mkdir -p /path/to/dir – create parent directories as needed.

Tip: mkdir -m mode dir sets permissions; mkdir -v dir1 dir2 shows creation details.

36. rm command (remove files and directories)

Function: Deletes files and directories.

Usage: rm [options] file/dir Example: rm file.txt; rm -r dir – recursive delete.

Tip: rm -i prompts before each removal; rm -rf forces deletion (use with caution).

37. cp command (copy files and directories)

Function: Copies files and directories.

Usage: cp [options] source target Example: cp file.txt /path/to/destination; cp -r dir /path/to/destination.

Tip: cp -i prompts before overwrite; cp -u copies only when source is newer.

38. mv command (move or rename files and directories)

Function: Moves or renames files and directories.

Usage: mv [options] source target Example: mv file.txt /path/to/destination; mv old_name new_name.

Tip: mv -i prompts before overwrite; mv -u moves only when source is newer.

39. chmod command (change file permissions)

Function: Modifies file or directory permissions.

Usage: chmod [options] mode file/dir Example: chmod +x script.sh; chmod 644 file.txt.

Tip: Numeric mode chmod 755 file.txt is quick; chmod -R applies recursively.

40. chown command (change file owner)

Function: Changes the owner (and optionally group) of a file or directory.

Usage: chown [options] owner[:group] file/dir Example: chown user1 file.txt; chown user1:group1 file.txt.

Tip: chown -R changes ownership recursively; chown --reference=ref_file target_file copies ownership from reference.

41. nohup (run program in background)

Function: Runs a command that continues after the terminal is closed, redirecting output to a file.

Usage: nohup command [args...] Example: nohup python my_script.py & – runs script in background, output goes to nohup.out.

Tip:

nohup java -Dserver.port=10086 -jar order.jar > output.log 2>&1 &

runs a JAR in background, redirecting stdout and stderr to output.log.

Note: By default output goes to nohup.out; you can redirect with > and 2>. Use jobs to list background jobs, fg to bring one to foreground, and Ctrl+C to stop.

42. java command (run Java applications)

Function: Starts the Java Virtual Machine and runs a Java program.

Usage: java [options] class [args...] Example: java -version; java -jar app.jar.

Tip: java -Xmx sets maximum heap size; java -Xms sets initial heap size.

43. javac command (compile Java source)

Function: Compiles Java source files into bytecode.

Usage: javac [options] sourcefile Example: javac HelloWorld.java.

Tip: javac -d sets output directory; javac -cp sets classpath.

44. jps command (list Java processes)

Function: Shows Java processes running on the system.

Usage: jps [options] Example: jps – list Java processes.

Tip: jps -l shows full class name; jps -v shows JVM arguments.

45. jstat command (JVM statistics monitor)

Function: Displays JVM statistical information.

Usage: jstat [options] pid [interval] [count] Example: jstat -gcutil 12345 – garbage‑collection stats for PID 12345.

Tip: jstat -gc shows GC stats; jstat -class shows class loading info.

46. jstack command (Java stack trace)

Function: Generates stack traces of Java threads.

Usage: jstack [options] pid Example: jstack 12345.

Tip: jstack -l includes lock information.

47. jmap command (Java memory map)

Function: Generates a memory snapshot of a Java process.

Usage: jmap [options] pid Example: jmap -heap 12345 – heap info for PID 12345.

Tip: jmap -histo produces an object histogram.

48. jcmd command (JVM diagnostic tool)

Function: Executes various JVM diagnostic commands.

Usage: jcmd pid command [options] Example: jcmd 12345 GC.run – forces garbage collection.

Tip: jcmd 12345 help lists all supported commands.

49. jinfo command (JVM configuration info)

Function: Shows JVM configuration details.

Usage: jinfo [options] pid Example: jinfo 12345.

Tip: jinfo -flags lists all JVM flags; jinfo -sysprops shows system properties.

50. jhat command (JVM heap dump analysis)

Function: Loads a Java heap dump file into the JVM for analysis.

Usage: jhat [options] heapdump_file Example: jhat heapdump.bin.

Tip: Use jmap to generate the heap dump; jhat -port sets the web server port.