Linux Commands
File System Commands
Disk Usage
df -h # disk free — show filesystem usage
df -h /home # show usage for a specific path
du -sh directory/ # disk usage of a directory (human readable)
du -sh * # usage of all items in current dir
du -ah | sort -rh | head -20 # top 20 largest files/dirs
File Linking
ln -s /path/to/original link-name # create symbolic (soft) link
ln original hard-link # create hard link
readlink -f symlink # resolve symlink to real path
Archiving and Compression
# tar — tape archive
tar -czf archive.tar.gz directory/ # create compressed archive
tar -xzf archive.tar.gz # extract compressed archive
tar -tzf archive.tar.gz # list contents
tar -czf - directory/ | ssh user@host "tar -xzf - -C /dest" # stream over SSH
# zip / unzip
zip -r archive.zip directory/
unzip archive.zip
unzip archive.zip -d /destination/
# gzip / bzip2
gzip file.txt # compress (replaces original)
gzip -d file.txt.gz # decompress
bzip2 file.txt
bunzip2 file.txt.bz2
User Management
whoami # current username
id # uid, gid, and group memberships
id alice # info for specific user
groups # groups current user belongs to
w # who is logged in and what they're doing
who # who is logged in
last # login history
Managing Users (as root/sudo)
# Create user
useradd -m -s /bin/bash alice # -m creates home dir, -s sets shell
useradd -m -G sudo,docker alice # add to groups on creation
# Set/change password
passwd alice
# Modify user
usermod -aG docker alice # add to group (append)
usermod -s /bin/zsh alice # change shell
usermod -l newname oldname # rename user
# Delete user
userdel alice # keep home directory
userdel -r alice # remove home directory too
# Lock/unlock account
usermod -L alice # lock
usermod -U alice # unlock
Managing Groups
groupadd developers # create group
groupdel developers # delete group
gpasswd -a alice developers # add user to group
gpasswd -d alice developers # remove user from group
cat /etc/group # list all groups
Switch Users and Sudo
su - alice # switch to alice (login shell)
sudo command # run as root
sudo -u alice command # run as alice
sudo -l # list sudo privileges
Text Processing
grep — Search Text
grep "pattern" file.txt # basic search
grep -i "pattern" file.txt # case-insensitive
grep -r "pattern" /var/log/ # recursive search
grep -n "pattern" file.txt # show line numbers
grep -v "pattern" file.txt # invert (lines NOT matching)
grep -c "pattern" file.txt # count matching lines
grep -l "pattern" *.txt # list files with matches
grep -E "error|warning" file.txt # extended regex (OR)
grep -A 3 -B 3 "pattern" file.txt # show 3 lines before and after
awk — Column Processor
awk '{print $1}' file.txt # print first column
awk '{print $1, $3}' file.txt # print columns 1 and 3
awk -F: '{print $1}' /etc/passwd # use : as delimiter
awk '/pattern/ {print $0}' file # print lines matching pattern
awk '{sum += $2} END {print sum}' f # sum column 2
awk 'NR > 1 {print}' file.txt # skip header line
sed — Stream Editor
sed 's/old/new/' file.txt # replace first occurrence per line
sed 's/old/new/g' file.txt # replace all occurrences
sed -i 's/old/new/g' file.txt # in-place replacement
sed -n '5,10p' file.txt # print lines 5-10
sed '/pattern/d' file.txt # delete lines matching pattern
sed -n '/start/,/end/p' file.txt # print between patterns
sort, uniq, wc
sort file.txt # alphabetical sort
sort -n file.txt # numeric sort
sort -r file.txt # reverse sort
sort -k2 file.txt # sort by column 2
sort -u file.txt # sort and remove duplicates
uniq file.txt # remove consecutive duplicates
uniq -c file.txt # count occurrences
uniq -d file.txt # show only duplicates
wc file.txt # lines, words, bytes
wc -l file.txt # count lines only
wc -w file.txt # count words only
cut, tr, paste
cut -d: -f1 /etc/passwd # extract field 1 with : delimiter
cut -c1-10 file.txt # extract characters 1-10
tr 'a-z' 'A-Z' < file.txt # lowercase to uppercase
tr -d '\n' < file.txt # delete newlines
tr -s ' ' < file.txt # squeeze repeated spaces
paste file1.txt file2.txt # merge lines side by side
System Monitoring
Process Management
ps aux # all running processes
ps aux | grep nginx # find specific process
pgrep nginx # get PID of nginx processes
top # interactive process viewer
htop # enhanced top (if installed)
kill PID # send SIGTERM
kill -9 PID # send SIGKILL (force)
killall nginx # kill by name
pkill -f "python script.py" # kill by pattern
# Background jobs
command & # run in background
jobs # list background jobs
fg %1 # bring job 1 to foreground
bg %1 # resume job 1 in background
nohup command & # run immune to hangups
uname -a # kernel info
hostname # system hostname
uptime # how long system has been running
free -h # memory usage
vmstat 1 5 # virtual memory stats (5 samples, 1s interval)
iostat -x 1 # I/O statistics
lscpu # CPU info
lsmem # memory info
lsblk # block devices
lspci # PCI devices
lsusb # USB devices
Network Commands
ip addr # show network interfaces
ip route # show routing table
ss -tulnp # listening ports (sockets)
netstat -tulnp # older equivalent
ping -c 4 google.com # test connectivity
traceroute google.com # trace route to host
curl -I https://example.com # HTTP headers only
wget -q https://example.com # download file
nslookup example.com # DNS lookup
dig example.com # detailed DNS lookup
Log Files
journalctl -f # follow systemd journal
journalctl -u nginx # logs for nginx service
journalctl --since "1 hour ago"
tail -f /var/log/syslog # follow syslog
tail -f /var/log/auth.log # authentication events
grep "error" /var/log/syslog | tail -50
