Linux Documentation

cat /etc/issue          # Show Linux distribution info
cat /etc/passwd         # View user accounts
cat /etc/fstab          # View filesystem mount table
cat file1 file2 > combined.txt  # Concatenate files

cd /var/log             # Navigate to logs
cd ~                    # Go to home directory
cd -                    # Return to previous directory

chmod 755 script.sh           # rwxr-xr-x
chmod u+x file                # Add execute for owner
chmod -R 644 /path/to/dir     # Recursive permission change
chmod 4755 /usr/bin/program   # Set SUID bit
chmod 2755 /shared/dir        # Set SGID bit
chmod 1777 /tmp               # Set sticky bit

chown user:group file.txt
chown -R www-data:www-data /var/www/

cp source.txt destination.txt
cp -r /source/dir /destination/    # Recursive copy
cp -a /source/ /backup/            # Archive mode (preserves all attributes)

dd if=/dev/sda of=/dev/sdb bs=4M status=progress    # Clone disk
dd if=/dev/zero of=/swapfile bs=1M count=1024        # Create swap file
dd if=/dev/sda of=disk.img bs=4096                   # Create disk image

df -h                   # Human-readable format
df -i                   # Show inode usage
df -T                   # Show filesystem type

du -sh /path/to/folder          # Total size, human-readable
du -sh /path/to/folder/ | less  # Pipe to less for large output
du -h --max-depth=1 /var        # One level deep summary

file document.pdf
file /bin/ls

find / -name "*.conf"                       # Find by name
find /var/log -mtime -7                     # Modified in last 7 days
find / -type f -perm /4000                  # Find SUID files
find / -type f -size +100M                  # Files larger than 100MB
find /home -user username                   # Files owned by user
find / -nouser                              # Files with no owner

ln file hardlink              # Hard link
ln -s /path/to/file symlink   # Symbolic (soft) link

ls -la            # Long format, all files (including hidden)
ls -lh            # Human-readable sizes
ls -lt            # Sort by modification time
ls -li            # Show inode numbers
ls -lR            # Recursive listing

mkdir newdir
mkdir -p /path/to/nested/dir    # Create parent dirs as needed

mv oldname.txt newname.txt
mv file.txt /destination/path/

pwd

rm file.txt
rm -rf /path/to/dir     # Force recursive delete (use with caution!)

rmdir emptydir

rsync -av /source/folder/ /destination/folder/
rsync -avz --progress user@remote:/path/ /local/path/
rsync -av --delete /source/ /destination/   # Mirror (delete extra files at dest)

rsync -av /media/username/Drive1/SourceFolder/ /media/username/Drive2/

stat filename       # Inode, size, permissions, timestamps

touch newfile.txt
touch -t 202601011200 file.txt   # Set specific timestamp

tree /etc/systemd
tree -L 2 /var       # Limit depth to 2 levels

awk '{print $1, $3}' file.txt               # Print columns 1 and 3
awk -F: '{print $1}' /etc/passwd            # Use : as delimiter
awk '/error/ {print}' logfile.log           # Print lines containing "error"
df -h | awk '{print $1, $5}'                # Extract filesystem and usage

cut -d: -f1 /etc/passwd           # First field, colon delimiter
cut -c1-10 file.txt               # First 10 characters per line

diff file1.txt file2.txt
diff -u old.conf new.conf          # Unified format (for patches)

grep "pattern" file.txt
grep -r "error" /var/log/          # Recursive search
grep -i "warning" logfile          # Case-insensitive
grep -c "pattern" file             # Count matches
grep -v "exclude" file             # Invert match
grep -E "regex|pattern" file       # Extended regex (same as egrep)
grep -n "text" file                # Show line numbers

head -n 20 /var/log/syslog
head -c 100 file.bin               # First 100 bytes

less /var/log/syslog
journalctl | less

sed 's/old/new/g' file.txt                  # Replace all occurrences
sed -i 's/old/new/g' file.txt               # In-place edit
sed -n '10,20p' file.txt                    # Print lines 10-20
sed '/^#/d' config.conf                     # Delete comment lines

sort file.txt
sort -n file.txt            # Numeric sort
sort -r file.txt            # Reverse sort
sort -k2 -t: /etc/passwd    # Sort by second field with : delimiter
du -h | sort -h             # Sort human-readable sizes

tail -n 50 /var/log/syslog
tail -f /var/log/syslog         # Follow (real-time output)
tail -f /var/log/*.log          # Follow multiple files

command | tee output.log              # Display and save
command | tee -a output.log           # Append to file

echo "HELLO" | tr 'A-Z' 'a-z'        # Convert to lowercase
cat file | tr -d '\r'                 # Remove carriage returns

sort file.txt | uniq
sort file.txt | uniq -c      # Count occurrences
sort file.txt | uniq -d      # Show only duplicates

wc -l file.txt      # Line count
wc -w file.txt      # Word count
wc -c file.txt      # Byte count

find / -name "*.tmp" | xargs rm
cat urls.txt | xargs -n1 curl -O

chage -l username                # List aging info
chage -M 90 username            # Max password age 90 days
chage -E 2026-12-31 username    # Set account expiry date

getent passwd username
getent group groupname

groupadd developers
groupdel oldgroup
groupmod -n newname oldname

id
id username

last                    # Recent logins
lastb                   # Failed login attempts
who                     # Currently logged in users
w                       # Who is logged in and what they are doing

passwd                  # Change own password
passwd username         # Change another user's password (root)
passwd -l username      # Lock account
passwd -u username      # Unlock account

su - username               # Switch user (login shell)
sudo command                # Run command as root
sudo -u user command        # Run as specific user
visudo                      # Edit sudoers safely

useradd -m -s /bin/bash newuser       # Create user with home dir and shell
userdel -r username                    # Delete user and home directory
usermod -aG sudo username             # Add user to supplementary group
usermod -L username                    # Lock user account
usermod -s /sbin/nologin username     # Disable shell access

free -h                 # Human-readable
free -h -s 5            # Refresh every 5 seconds

top                     # Basic process viewer
htop                    # Enhanced interactive viewer (if installed)
top -u username         # Filter by user

ionice -c 3 command             # Idle priority (only when no other I/O)
ionice -c 2 -n 0 command       # Best-effort, highest priority

iostat
iostat -xz 1 5          # Extended stats, every 1 sec, 5 times

kill PID                    # Send SIGTERM (graceful)
kill -9 PID                 # Send SIGKILL (force)
kill -HUP PID              # Send SIGHUP (reload config)
killall processname         # Kill by name
pkill -f "pattern"          # Kill by pattern match

lsof /var/log/syslog        # Who has this file open?
lsof -i :80                 # What process uses port 80?
lsof -u username            # Files open by user
lsof +D /directory          # All open files in directory

mpstat -P ALL 1 5           # All CPUs, every 1 sec, 5 times

nice -n 10 command          # Start with lower priority
renice -n 5 -p PID         # Change running process priority

ps aux                      # All processes, detailed
ps -ef                      # Full-format listing
ps aux --sort=-%mem         # Sort by memory usage
ps -eo pid,user,%cpu,cmd    # Custom columns

strace command              # Trace a command
strace -p PID              # Attach to running process
strace -e open command     # Filter specific syscalls

uptime

vmstat 1 5                  # Every 1 sec, 5 times

blkid                       # Show all block device UUIDs and types
blkid /dev/sda1

fdisk -l                    # List all partitions (MBR)
fdisk /dev/sdb              # Interactive partition editor
gdisk /dev/sdb              # GPT partition editor
parted /dev/sdb print       # Show partition table

fsck /dev/sda1
fsck -y /dev/sda1           # Auto-fix

lsblk
lsblk -f                   # Show filesystem type and mount points

# Physical Volume
pvcreate /dev/sdb1

# Volume Group
vgcreate myvg /dev/sdb1
vgdisplay

# Logical Volume
lvcreate -L 10G -n mylv myvg
lvextend -L +5G /dev/myvg/mylv
resize2fs /dev/myvg/mylv            # Resize ext4 after extending
xfs_growfs /mountpoint              # Resize XFS after extending
lvdisplay

mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
mdadm --detail /dev/md0
mdadm --manage /dev/md0 --add /dev/sdd1    # Add spare
cat /proc/mdstat                            # Check RAID status

mkfs.ext4 /dev/sda1
mkfs.xfs /dev/sdb1
mkfs.btrfs /dev/sdc1

mount /dev/sda1 /mnt/data
mount -t nfs server:/share /mnt/nfs
mount -o remount,rw /                       # Emergency remount read-write
umount /mnt/data

smartctl -a /dev/sda                # All SMART info
smartctl -t short /dev/sda          # Run short self-test
smartctl -H /dev/sda                # Health status

mkswap /dev/sda2            # Initialize swap partition
swapon /dev/sda2            # Enable swap
swapoff /dev/sda2           # Disable swap
swapon --show               # Show active swap

curl -vo /dev/null https://google.com

curl -vo /dev/null -L https://google.com

curl -H 'Host: www.example.com' http://server-ip-address/

curl -I https://example.com                         # HEAD request only
curl -X POST -d '{"key":"value"}' -H "Content-Type: application/json" https://api.example.com
curl -u user:pass https://secure.example.com        # Basic auth
curl -k https://self-signed.example.com             # Skip SSL verification
curl -o output.html https://example.com             # Save to file

dig example.com                     # Query A record
dig example.com MX                  # Query MX records
dig @8.8.8.8 example.com           # Query specific DNS server
dig +short example.com              # Short output
dig -x 8.8.8.8                     # Reverse DNS lookup

host example.com
host 8.8.8.8                        # Reverse lookup

ip addr show                        # Show all interfaces and IPs
ip addr add 192.168.1.10/24 dev eth0   # Assign IP
ip link set eth0 up                 # Bring interface up
ip link set eth0 down               # Bring interface down
ip route show                       # Show routing table
ip route add default via 192.168.1.1   # Add default gateway
ip neigh show                       # Show ARP table (neighbors)

mtr google.com
mtr -r -c 10 google.com            # Report mode, 10 cycles

ss -tulnp                           # TCP/UDP listening ports with PIDs
ss -s                               # Summary statistics
ss -tnp state established          # Active connections
netstat -tulnp                      # Legacy equivalent

nmap 192.168.1.0/24                 # Scan subnet
nmap -sV 192.168.1.1                # Service version detection
nmap -p 80,443 target               # Scan specific ports

ping -c 4 google.com               # Send 4 packets
ping -i 0.5 host                   # Interval 0.5 seconds
ping6 ::1                          # IPv6 ping

resolvectl status
resolvectl query example.com

scp file.txt user@remote:/path/
scp -r localdir/ user@remote:/path/
scp user@remote:/path/file.txt ./

ssh user@hostname
ssh -p 2222 user@host              # Custom port
ssh -i ~/.ssh/key.pem user@host    # Specific key
ssh -L 8080:localhost:80 user@host # Local port forwarding
ssh -D 1080 user@host             # SOCKS proxy (dynamic forwarding)

tcpdump -i eth0                         # Capture on interface
tcpdump -i any port 80                  # Filter by port
tcpdump -i eth0 host 192.168.1.1       # Filter by host
tcpdump -w capture.pcap                 # Write to file
tcpdump -r capture.pcap                 # Read from file

traceroute google.com
tracepath google.com                    # Does not require root

wget https://example.com/file.tar.gz
wget -r -l 2 https://example.com       # Recursive, depth 2
wget -c https://example.com/large.iso  # Resume interrupted download

apt update                          # Update package index
apt upgrade                         # Upgrade all packages
apt install package-name            # Install package
apt remove package-name             # Remove package
apt search keyword                  # Search packages
apt list --installed                # List installed packages
apt show package-name               # Package details

dnf update                          # Update all packages
dnf install package-name
dnf remove package-name
dnf search keyword
dnf list installed
dnf info package-name
dnf provides /usr/bin/command       # Find which package provides a file

dpkg -i package.deb                 # Install .deb file
dpkg -l                             # List installed packages
dpkg -L package-name                # List files in package
dpkg -S /path/to/file               # Find which package owns a file

rpm -ivh package.rpm                # Install
rpm -qa                             # List all installed
rpm -ql package-name                # List files in package
rpm -qf /path/to/file              # Find owning package

snap install package
snap list
snap refresh                        # Update all snaps
snap remove package

systemctl start service-name
systemctl stop service-name
systemctl restart service-name
systemctl reload service-name           # Reload config without restart
systemctl status service-name
systemctl enable service-name           # Start at boot
systemctl disable service-name
systemctl is-active service-name
systemctl is-enabled service-name
systemctl list-units --type=service     # List running services
systemctl list-unit-files               # List all unit files
systemctl daemon-reload                 # Reload unit files after changes
systemctl mask service-name             # Completely prevent starting
systemctl unmask service-name

systemctl get-default                   # Current default target
systemctl set-default multi-user.target # Set default to CLI (no GUI)
systemctl set-default graphical.target  # Set default to GUI
systemctl isolate rescue.target         # Switch to rescue mode

journalctl                              # All logs
journalctl -u service-name             # Logs for specific service
journalctl -f                           # Follow (like tail -f)
journalctl --since "1 hour ago"
journalctl --since "2026-01-01" --until "2026-01-02"
journalctl -p err                       # Only error priority and above
journalctl -b                           # Current boot only
journalctl -b -1                        # Previous boot
journalctl --disk-usage                 # How much disk logs use

update-grub                             # Debian/Ubuntu: regenerate grub.cfg
grub2-mkconfig -o /boot/grub2/grub.cfg  # RHEL/CentOS

dmesg
dmesg | grep -i error
dmesg --level=err,warn
dmesg -T                                # Human-readable timestamps

getfacl file.txt                        # View ACLs
setfacl -m u:username:rw file.txt       # Set ACL for user
setfacl -m g:groupname:r file.txt       # Set ACL for group
setfacl -x u:username file.txt          # Remove ACL entry
setfacl -b file.txt                     # Remove all ACLs

aa-status                               # Show AppArmor status
aa-complain /path/to/profile           # Set profile to complain mode
aa-enforce /path/to/profile            # Set profile to enforce mode

auditctl -l                             # List audit rules
auditctl -w /etc/passwd -p wa           # Watch file for writes/attributes
ausearch -m avc                         # Search for SELinux denials
ausearch -k mykey                       # Search by key
aureport --summary                      # Summary report

firewall-cmd --state
firewall-cmd --list-all
firewall-cmd --add-service=http --permanent
firewall-cmd --add-port=8080/tcp --permanent
firewall-cmd --reload
firewall-cmd --zone=public --list-services

gpg --gen-key                           # Generate key pair
gpg --encrypt -r recipient file.txt     # Encrypt
gpg --decrypt file.txt.gpg             # Decrypt
gpg --sign file.txt                    # Sign file
gpg --verify file.txt.sig             # Verify signature
gpg --list-keys                        # List public keys

# iptables (legacy)
iptables -L -n -v                       # List all rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -j DROP
iptables-save > /etc/iptables/rules.v4

# nftables (modern replacement)
nft list ruleset
nft add table inet filter
nft add chain inet filter input { type filter hook input priority 0 \; }
nft add rule inet filter input tcp dport 22 accept

openssl version
openssl s_client -connect example.com:443    # Test TLS connection
openssl x509 -in cert.pem -text -noout       # View certificate details
openssl req -new -x509 -nodes -out cert.pem -keyout key.pem -days 365  # Self-signed cert
openssl dgst -sha256 file.txt                # Hash a file

getenforce                                   # Show current mode
setenforce 0                                 # Set permissive (temporary)
setenforce 1                                 # Set enforcing (temporary)
sestatus                                     # Detailed status
ls -Z file.txt                               # Show SELinux context
restorecon -Rv /path                         # Restore default contexts
audit2allow -a                               # Generate policy from denials
semanage fcontext -l                         # List file contexts

ufw status verbose
ufw enable
ufw allow 22/tcp
ufw allow from 192.168.1.0/24
ufw deny 23
ufw delete allow 22/tcp

ansible all -m ping                          # Test connectivity
ansible-playbook playbook.yml                # Run playbook
ansible all -m shell -a "uptime"            # Ad-hoc command
ansible-inventory --list                     # List inventory

at 10:00 AM tomorrow
at> /path/to/script.sh
at> Ctrl+D

atq                     # List pending jobs
atrm 1                  # Remove job #1

crontab -e              # Edit current user's crontab
crontab -l              # List current user's crontab
crontab -r              # Remove crontab

# Example: Run backup daily at 2:30 AM
30 2 * * * /usr/local/bin/backup.sh

# Example: Every 5 minutes
*/5 * * * * /path/to/script.sh

systemctl list-timers                       # List active timers
systemctl status mytimer.timer
systemctl enable --now mytimer.timer

#!/bin/bash
# Variables
NAME="world"
echo "Hello, $NAME"

# Conditionals
if [ -f /etc/passwd ]; then
    echo "File exists"
fi

# Loops
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# While loop
while read -r line; do
    echo "$line"
done < file.txt

# Functions
my_function() {
    echo "Argument: $1"
    return 0
}

# Exit codes
command
if [ $? -eq 0 ]; then
    echo "Success"
fi

git init
git clone https://repo.example.com/project.git
git status
git add .
git commit -m "message"
git pull
git push
git log --oneline -10
git branch feature-branch
git checkout feature-branch
git merge feature-branch
git diff

# Container lifecycle
docker ps                               # List running containers
docker ps -a                            # All containers (running + stopped)
docker run -d -p 8080:80 --name myapp nginx
docker exec -it container_name /bin/bash
docker stop container_name
docker rm container_name
docker rm -f container_name             # Force remove

# Images
docker images
docker pull image:tag
docker build -t myimage:latest .
docker image prune -f                   # Clean old images

# Volumes and networks
docker volume ls
docker volume create mydata
docker network ls

# Docker Compose
docker compose up -d
docker compose down
docker compose logs -f

docker run -d -p 3000:8080 \
  --add-host=host.docker.internal:host-gateway \
  -v open-webui:/app/backend/data \
  --name open-webui --restart always \
  ghcr.io/open-webui/open-webui:main

docker stop open-webui
docker rm open-webui
docker pull ghcr.io/open-webui/open-webui:main
docker run -d -p 3000:8080 \
  -v open-webui:/app/backend/data \
  --name open-webui --restart always \
  ghcr.io/open-webui/open-webui:main

podman run -d -p 8080:80 nginx
podman ps
podman exec -it container_name bash
podman stop container_name
podman rm container_name
podman images
podman build -t myimage .

virsh list --all                        # List all VMs
virsh start vm-name
virsh shutdown vm-name
virsh destroy vm-name                   # Force power off
virsh console vm-name                   # Attach to console
virsh dominfo vm-name                   # VM info
virt-install --name=myvm --ram=2048 --vcpus=2 --disk size=20 --os-variant=ubuntu22.04

lscpu

lshw -short
lshw -class network

lsmod                                   # List loaded modules
modprobe module_name                    # Load module
modprobe -r module_name                 # Remove module
modinfo module_name                     # Module details

lspci
lspci -v                                # Verbose
lsusb
lsusb -t                                # Tree format

uname -a                # All information
uname -r                # Kernel release
uname -m                # Machine hardware (x86_64, aarch64)

sysctl -a                                   # Show all parameters
sysctl net.ipv4.ip_forward                 # Check specific value
sysctl -w net.ipv4.ip_forward=1            # Set temporarily

logger "Custom log message"
logger -p auth.warning "Security warning from script"

lsb_release -a

Distributor ID: Ubuntu
Description:    Ubuntu 22.04.5 LTS
Release:        22.04
Codename:       jammy

timedatectl                             # Show current settings
timedatectl set-timezone America/New_York
timedatectl set-ntp true                # Enable NTP sync
timedatectl list-timezones

logrotate -f /etc/logrotate.conf        # Force rotation
logrotate -d /etc/logrotate.conf        # Debug/dry run

gzip file.txt                   # Compress → file.txt.gz
gunzip file.txt.gz              # Decompress
bzip2 file.txt                  # Better compression → file.txt.bz2
bunzip2 file.txt.bz2
xz file.txt                    # Best compression → file.txt.xz
unxz file.txt.xz

# Create archives
tar -cvf archive.tar /path/to/dir               # Create tar
tar -czvf archive.tar.gz /path/to/dir           # Create gzipped tar
tar -cjvf archive.tar.bz2 /path/to/dir         # Create bzip2 tar
tar -cJvf archive.tar.xz /path/to/dir          # Create xz tar

# Extract archives
tar -xvf archive.tar
tar -xzvf archive.tar.gz
tar -xjvf archive.tar.bz2
tar -xJvf archive.tar.xz
tar -xvf archive.tar -C /destination/          # Extract to specific dir

# List contents
tar -tvf archive.tar

zip -r archive.zip /path/to/dir
unzip archive.zip
unzip -l archive.zip            # List contents without extracting

vim filename

nano filename

tmux                            # Start new session
tmux new -s mysession           # Named session
tmux ls                         # List sessions
tmux attach -t mysession        # Reattach

iotop
iotop -o                        # Only show active I/O

smem -tk                        # Totals in KB
smem -u                         # Per-user summary

sar -u 1 5                      # CPU usage, 1 sec interval, 5 times
sar -r 1 5                      # Memory usage
sar -n DEV 1 5                  # Network statistics
sar -d 1 5                      # Disk I/O

tuned-adm active                        # Show current profile
tuned-adm list                          # List available profiles
tuned-adm profile throughput-performance   # Set profile
tuned-adm recommend                     # Get recommended profile

# 1. Check interface status
ip addr show
ip link show

# 2. Check routing
ip route show
ping gateway-ip

# 3. Check DNS
dig example.com
cat /etc/resolv.conf
resolvectl status

# 4. Check application / port
ss -tulnp
curl -v http://service:port

alias ll='ls -la'
alias update='sudo apt update && sudo apt upgrade'
unalias ll                      # Remove alias

env                             # Show all environment variables
export MY_VAR="value"           # Set and export variable
echo $PATH                      # Show PATH
export PATH="$PATH:/new/path"   # Add to PATH

history
history | grep "pattern"
!123                            # Re-run command #123
!!                              # Re-run last command
!$                              # Last argument of previous command

man command                     # Manual page
man 5 passwd                    # Section 5 (file formats)
info command                    # GNU info page
command --help                  # Quick help
apropos keyword                 # Search man pages by keyword

mpv --ytdl-format=best "https://youtu.be/Cwm6krslrOc?si=vODFbOAlI6inrhWR"

ollama list                             # List downloaded models
ollama run llama3                       # Run a model
ollama pull mistral                     # Download a model

screen                          # Start new session
screen -S name                  # Named session
screen -ls                      # List sessions
screen -r name                  # Reattach

shutdown -h now                 # Halt immediately
shutdown -r +5 "Rebooting in 5 minutes"
reboot
poweroff

type ls                         # How shell interprets command
which python3                   # Path to executable
whereis nginx                   # Binary, source, and man page locations