Essential Linux Commands Every Sysadmin Should Know

1. Files and Directories

The cd command changes the current directory.

cd /home          # enter '/home' directory
cd ..             # go up one level
cd ../..          # go up two levels
cd                # go to the current user's home directory
cd ~user1         # switch to user1's home directory
cd -              # return to the previous directory

The pwd command prints the absolute path of the current working directory.

# pwd
/root

The ls command lists files and directories.

ls                # list names
ls -l             # long format with details
ls -a             # include hidden files
ls -R             # recursive listing
ls [0-9]          # show entries containing digits

Copying files uses cp with options:

-a : archive (preserve attributes, recursive)
-p : preserve mode, ownership and timestamps
-i : interactive (prompt before overwrite)
-r : recursive copy (for directories)
-u : copy only when source is newer than destination

Moving or renaming uses mv with options:

-f : force overwrite without prompting
-i : interactive (prompt before overwrite)
-u : overwrite only if source is newer

Deletion uses rm with options:

-f : ignore nonexistent files, never prompt
-i : interactive (prompt before each removal)
-r : recursive removal of directories (dangerous)

2. Viewing File Content

The cat command outputs file contents; it can be combined with more or less. Variants include:

cat file1                     # view from start
tac file1                     # view from end (reverse)
cat -n file1                  # show line numbers
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 filename | head -n 3000 | tail -n +1000   # lines 1000‑3000
cat filename | tail -n +3000 | head -n 1000   # lines 3000‑3999

3. File Search

The find command locates files and directories.

find / -name file1                     # search from root
find / -user user1                    # files owned by user1
find /usr/bin -type f -atime +100    # files not accessed in the last 100 days
find /usr/bin -type f -mtime -10     # files created/modified within the last 10 days

Additional utilities:

whereis halt          # locate binary, source, and man page
which halt            # show full path of the executable

Delete files larger than 50 MiB:

find /var/mail/ -size +50M -exec rm {} \;

4. Permissions

Change mode with chmod:

chmod ugo+rwx directory1          # grant read/write/execute to user, group, others
chmod go-rwx directory1           # revoke read/write/execute from group and others

Change ownership with chown:

chown user1 file1                 # set file owner
chown -R user1 directory1         # recursive ownership change
chown user1:group1 file1          # set owner and group

Change group with chgrp:

chgrp group1 file1               # set group ownership

5. Text Processing

Search text with grep (often piped with other commands):

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 in /var/log

Replace text with sed:

sed 's/string1/string2/g' example.txt   # replace all occurrences
sed '/^$/d' example.txt                # delete empty lines

Combine files side‑by‑side with paste:

paste file1 file2                         # merge columns
paste -d '+' file1 file2                  # use '+' as delimiter

Sort and deduplicate with sort and uniq:

sort file1 file2 | uniq                 # union (unique lines)
sort file1 file2 | uniq -u              # lines only in one file (difference)
sort file1 file2 | uniq -d              # common lines (intersection)

Compare files with comm:

comm -1 file1 file2   # suppress column 1 (lines only in file2)
comm -2 file1 file2   # suppress column 2 (lines only in file1)
comm -3 file1 file2   # suppress column 3 (common lines)

6. Archiving and Compression

The tar command creates and extracts archives. Common options:

-c : create archive
-t : list archive contents
-x : extract archive (optionally with -C to set destination)
-j : use bzip2 compression
-z : use gzip compression
-v : verbose output
-f filename : specify archive name
-C dir : change to directory before operation

Examples:

tar -jcv -f filename.tar.bz2 directory   # create bzip2 archive
tar -jtv -f filename.tar.bz2               # list contents
tar -jxv -f filename.tar.bz2 -C /dest      # extract to /dest

Other compression utilities:

bunzip2 file1.bz2
bzip2 file1
gunzip file1.gz
gzip file1
gzip -9 file1          # maximum compression
rar a file1.rar test_file
rar a file1.rar file1 file2 dir1
rar x file1.rar
zip file1.zip file1
unzip file1.zip
zip -r file1.zip file1 file2 dir1

7. System Shutdown and Reboot

shutdown -h now          # halt immediately
init 0                   # alternative halt
telinit 0                # alternative halt
shutdown -h 22:30        # schedule shutdown at 22:30
shutdown -c              # cancel scheduled shutdown
shutdown -r now          # reboot now
reboot                   # reboot
logout                   # log out of session
time command             # measure execution time of a command

8. Process Management

Java processes:

jps                     # list Java processes with PIDs

General process listing with ps:

ps aux                  # all processes with detailed info
ps ax                   # all processes not attached to a terminal
ps -lA                  # long format, all processes
ps axjf                 # show process tree

Terminate processes with kill (signal numbers optional):

kill -l                 # list all signal names
kill -9 3268            # force kill PID 3268
kill -l KILL            # get numeric value of SIGKILL

Kill by name with killall:

killall nginx           # kill all nginx processes
killall -9 bash        # force kill all bash processes
killall -TERM nginx    # send TERM signal to nginx

Monitor resources with top (Linux equivalent of Windows Task Manager).

Check listening ports:

netstat -tunlp | grep PORT