Essential Linux Commands Every Sysadmin Should Master

File and Directory Operations

Basic navigation and manipulation of the filesystem are performed with the following commands:

cd – change the current directory.

cd /home            # absolute path
cd ..               # parent directory
cd ../..            # two levels up
cd                  # $HOME
cd ~user1           # another user's home
cd -                # previous directory

pwd – print the absolute pathname of the current working directory. pwd ls – list directory contents.

ls                     # simple list
ls -l                  # long format with permissions, size, etc.
ls -a                  # include hidden entries
ls -R                  # recursive listing
ls [0-9]               # entries containing digits

cp – copy files or directories.

# Common options
-a   Preserve attributes and copy recursively
-p   Preserve mode, ownership, timestamps
-i   Prompt before overwriting
-r   Copy directories recursively
-u   Copy only when the source is newer

mv – move or rename files/directories.

# Common options
-f   Force overwrite without prompting
-i   Prompt before overwriting
-u   Overwrite only if the source is newer

rm – remove files or directories.

# Common options
-f   Ignore nonexistent files, never prompt
-i   Interactive mode, ask before each removal
-r   Recursive removal (dangerous for directories)

Viewing File Content

These utilities display or paginate the contents of text files.

cat – concatenate and print files.

cat file1                     # whole file
cat -n file1                 # with line numbers
tac file1                     # reverse order
more file1                   # paginated view
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 1000‑3000
cat file1 | tail -n +3000 | head -n 1000   # lines 3000‑3999

File Search

The find utility searches the filesystem based on criteria such as name, owner, timestamps, etc.

find / -name file1                     # start at root
find / -user user1                     # owned by user1
find /usr/bin -type f -atime +100     # not accessed in 100 days
find /usr/bin -type f -mtime -10      # modified within last 10 days
whereis halt                           # binary, source, man page
which halt                             # full path of executable

Delete files larger than 50 MiB:

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

Permissions Management

chmod – change file or directory mode.

# Show permissions
ls -lh
# Grant read/write/execute to everyone
chmod ugo+rwx directory1
# Revoke all permissions from group and others
chmod go-rwx directory1

chown – change ownership.

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

chgrp – change group ownership.

chgrp group1 file1

Text Processing

grep – filter lines matching a pattern.

grep Aug /var/log/messages               # simple search
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

sed – stream editor for in‑place substitutions.

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

paste – merge columns from multiple files.

paste file1 file2               # default tab delimiter
paste -d '+' file1 file2       # use '+' as delimiter

sort and uniq – ordering and deduplication.

sort file1 file2               # sort combined input
sort file1 file2 | uniq        # unique lines (union)
sort file1 file2 | uniq -u     # lines appearing only once (difference)
sort file1 file2 | uniq -d     # duplicate lines (intersection)

comm – compare two sorted files.

comm -1 file1 file2   # suppress lines unique to file1
comm -2 file1 file2   # suppress lines unique to file2
comm -3 file1 file2   # suppress common lines

Archiving and Compression

The tar command creates archives; optional flags invoke compression utilities.

# Core tar flags
-c   create archive
-t   list archive contents
-x   extract archive (use -C to specify destination)
-j   bzip2 compression
-z   gzip compression
-v   verbose output
-f   specify archive file name
-C   change to directory before extracting

Examples:

tar -jcvf archive.tar.bz2 file1 file2          # bzip2 archive
tar -jtvf archive.tar.bz2                       # list contents
tar -jxvf archive.tar.bz2 -C /dest               # extract to /dest

Other compression utilities:

bzip2 file1               # compress with bzip2
bunzip2 file1.bz2         # decompress bzip2
gzip file1                # compress with gzip (use -9 for maximum)
gunzip file1.gz           # decompress gzip
rar a archive.rar file1 file2 dir1   # create RAR archive
rar x archive.rar        # extract RAR
zip -r archive.zip file1 file2 dir1 # recursive zip
unzip archive.zip        # extract ZIP

System Shutdown and Reboot

shutdown -h now            # immediate power‑off
init 0                     # alternative shutdown
telinit 0                  # another shutdown method
shutdown -h HH:MM          # schedule shutdown
shutdown -c                # cancel scheduled shutdown
shutdown -r now            # immediate reboot
reboot                     # reboot
logout                     # end current session
time command               # measure execution time of a command

Process Management

jps – list Java processes with their PID and class name.

ps – snapshot of current processes.

# Common options
-A   all processes
-a   processes not attached to a terminal
-u   processes owned by a specific user
-x   processes without a controlling terminal
-l   long format with detailed info

ps aux          # all processes, detailed
ps ax           # all processes, no terminal
ps -lA          # long format for all
ps axjf         # process tree

kill – send signals to a process.

kill -l                 # list all signal names
kill -9 3268           # force kill PID 3268
kill -TERM 1234        # graceful termination
kill -l KILL           # numeric value of SIGKILL
kill -l SIGTERM        # numeric value of SIGTERM

killall – kill processes by name.

killall nginx               # terminate all nginx processes
killall -9 bash            # force kill all bash instances
killall -TERM nginx        # send SIGTERM to nginx

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

Identify the port a process is listening on:

netstat -tunlp | grep PORT