Essential Linux Commands Every Ops Engineer Should Master

1. Files and Directories

cd – change the current directory. Examples: <code>cd /home # go to /home cd .. # up one level cd ../../ # up two levels cd # go to the user’s home directory cd ~user1 # go to user1’s home directory cd - # return to the previous directory</code>

pwd – print the current working directory. <code># pwd /root</code>

ls – list files and directories. <code>ls # simple list ls -l # long format with details ls -a # include hidden files ls -R # recursive listing ls [0-9] # show entries containing digits</code>

cp – copy files or directories. <code>-a preserve attributes -p preserve mode, ownership, timestamps (useful for backups) -i interactive, ask before overwriting -r copy directories recursively -u copy only when source is newer</code>

mv – move or rename files/directories. <code>-f force overwrite without prompting -i ask before overwriting -u overwrite only if the destination is older</code>

rm – remove files or directories. <code>-f force, ignore nonexistent files -i interactive, ask before each removal -r recursive removal of directories (dangerous)</code>

2. Viewing File Content

cat – display file contents; can be combined with more or less . <code>cat file1 # show entire file cat -n file1 # number lines more file1 # paginate long file head -n 2 file1 # first two lines tail -n 2 file1 # last two lines tail -n +1000 file1 # from line 1000 onward cat file1 | head -n 3000 | tail -n +1000 # lines 1000‑2999 cat file1 | tail -n +3000 | head -n 1000 # lines 3000‑3999</code>

tac – display a file in reverse order (last line first).

3. File Search

find – locate files and directories. <code>find / -name file1 # search from root find / -user user1 # files owned by user1 find /usr/bin -type f -atime +100 # executable not accessed in 100 days find /usr/bin -type f -mtime -10 # modified within last 10 days</code>

whereis and which – locate binaries, sources, or manuals. <code>whereis halt # show binary, source, man page which halt # show full path of executable</code>

Delete files larger than 50 MiB: <code>find /var/mail/ -size +50M -exec rm {} \;</code>

4. Permissions Management

chmod – change file/directory permissions. <code>ls -lh # view permissions chmod ugo+rwx directory1 # grant read/write/execute to user, group, others chmod go-rwx directory1 # remove all permissions for group and others</code>

chown – change ownership. <code>chown user1 file1 # set owner chown -R user1 directory1 # recursive ownership change chown user1:group1 file1 # set owner and group</code>

chgrp – change group ownership. <code>chgrp group1 file1</code>

5. Text Processing

grep – filter lines matching patterns. <code>grep Aug /var/log/messages # find "Aug" in log grep ^Aug /var/log/messages # lines starting with "Aug" grep [0-9] /var/log/messages # lines containing digits grep Aug -R /var/log/* # recursive search</code>

sed – stream editor for substitutions and deletions. <code>sed 's/string1/string2/g' example.txt # replace all occurrences sed '/^$/d' example.txt # delete empty lines</code>

paste – merge files column‑wise. <code>paste file1 file2 # default tab delimiter paste -d '+' file1 file2 # use '+' as delimiter</code>

sort – sort file contents; can be combined with uniq . <code>sort file1 file2 | uniq # unique lines (union) sort file1 file2 | uniq -u # lines only in one file (difference) sort file1 file2 | uniq -d # common lines (intersection)</code>

comm – compare two sorted files. <code>comm -1 file1 file2 # suppress column 1 (unique to file1) comm -2 file1 file2 # suppress column 2 (unique to file2) comm -3 file1 file2 # suppress column 3 (common lines)</code>

6. Archiving and Compression

tar – create and manipulate archive files. <code>-c create archive -t list archive contents -x extract archive (optionally with -C to specify directory) -j use bzip2 compression -z use gzip compression -v verbose output -f filename # archive name -C dir # change to directory before extracting</code>

Common usage examples: <code>tar -jcvf filename.tar.bz2 /path/to/dir # create bzip2 archive tar -jtvf filename.tar.bz2 # list contents tar -jxvf filename.tar.bz2 -C /dest # extract</code>

Other tools: <code>bunzip2 file1.bz2 # decompress bzip2 bzip2 file1 # compress with bzip2 gunzip file1.gz # decompress gzip gzip file1 # compress with gzip ("-9" for maximum compression) rar a file1.rar file1 # create RAR archive zip file1.zip file1 # create ZIP archive unzip file1.zip # extract ZIP archive zip -r file1.zip file1 file2 dir1 # recursive ZIP</code>

7. System Control and Shutdown

shutdown – power off or reboot. <code>shutdown -h now # immediate halt shutdown -h 22:30 # schedule halt at 22:30 shutdown -c # cancel scheduled shutdown shutdown -r now # immediate reboot</code>

reboot – immediate reboot. <code>reboot</code>

logout – end the current session. <code>logout</code>

time – measure execution time of a command. <code>time ls -R /</code>

8. Process Management

jps – list Java processes. <code># Example output shows PID and class name of each Java process</code>

ps – snapshot of current processes. <code>-A show all processes -a show processes not attached to a terminal -u show processes for a specific user -x include processes without a controlling terminal -l long format with detailed info ps aux # all processes with user/CPU/memory info ps axjf # process tree</code>

kill – send signals to processes. <code>kill -l # list all signal names kill -9 3268 # force kill PID 3268 kill -l KILL # show numeric value of SIGKILL</code>

killall – kill processes by name. <code>killall nginx # terminate all nginx processes killall -9 bash # force kill all bash instances killall -TERM nginx # send SIGTERM to nginx</code>

top – interactive real‑time process monitor (similar to Windows Task Manager).

Check which port a process is listening on: <code>netstat -tunlp | grep 8080</code>