Unix Toolbox

uname -a                       # Kernel version (and BSD version)
lsb_release -a                 # Full release info of any LSB distribution
cat /etc/debian_version        # Debian version
cat /etc/redhat-release        # RedHat/Fedora version
uptime                         # How long the system has been running + load
hostname                       # System's host name
hostname -i                    # IP address of the host (Linux only)

dmesg                          # Detected hardware and boot messages
lspci -tv                      # Show PCI devices
lsusb -tv                      # Show USB devices
cat /proc/cpuinfo              # CPU model
cat /proc/meminfo              # Hardware memory
free -m                        # Used and free memory (-m for MB)
dmidecode                      # DMI/SMBIOS: hw info from the BIOS

top                            # Display and update the top cpu processes
mpstat 1                       # Processors related statistics
vmstat 2                       # Virtual memory statistics (2 s intervals)
iostat 2                       # I/O statistics (2 s intervals)
tail -n 500 /var/log/messages  # Last 500 kernel/syslog messages

id                             # Active user id with login and group
who                            # Show who is logged on
last                           # Show last logins on the system
useradd -c "Colin B" -g admin -m colin   # Add user colin (Linux/Solaris)
usermod -a -G <group> <user>    # Add existing user to group (Debian)
userdel colin                  # Delete user colin
groupadd admin                 # Add group "admin"

ulimit -a                      # Check shell limits
ulimit -n 10240                # Open files limit (valid within the shell only)
sysctl -a                      # View all system limits
sysctl fs.file-max             # View max open files limit
sysctl fs.file-max=102400      # Change max open files limit
# /etc/security/limits.conf: per-user/process limits
#   asterisk  hard  nofile  409600

ps -auxef                      # All processes with full listing
ps axjf                        # Process tree
top                            # Live view of processes
pgrep -l ssh                   # Find the PID of ssh processes
jobs                           # List background jobs of the shell

kill -9 1234                   # Force kill process with PID 1234
kill -1 1234                   # Reload config (SIGHUP)
killall -9 httpd               # Kill all httpd processes
pkill -9 ssh                   # Kill all ssh processes
pkill -TERM -u colin           # Terminate all processes of user colin

nice -n 5 command              # Run command with lower priority
renice +5 1234                 # Lower priority of PID 1234
renice -5 -u colin             # Higher priority for all colin's processes

df -h                          # Free disk space (human readable)
du -sh /path                   # Size of a directory
du -sh *                       # Size of each item in current dir
lsblk                          # List block devices
fdisk -l                       # List partitions
mount | column -t              # Mounted file systems

mount /dev/sda1 /mnt           # Mount a partition
umount /mnt                    # Unmount
mount -t nfs srv:/share /mnt   # Mount an NFS share
# /etc/fstab: persistent mounts
#   /dev/sda1  /mnt  ext3  defaults  0  1

find / -name "*.conf"          # Find files by name
find . -type f -size +10M      # Files larger than 10 MB
find . -mtime -1               # Files modified in last 24 h
find . -exec grep -l "foo" {} \;  # Files containing "foo"
locate pattern                 # Fast find via indexed db

chmod 755 file                 # rwxr-xr-x
chmod u+x file                 # Add execute for owner
chown colin:admin file         # Change owner and group
chgrp admin file               # Change group
chattr +i file                 # Make file immutable
umask 022                      # Default permissions for new files

ip addr                        # Show all interfaces and addresses
ip link show                   # Show link state
ifconfig                       # Legacy: show interfaces
hostname -I                    # All IP addresses of the host
ethtool eth0                   # Link speed and parameters of eth0

ip route                        # Show routing table
ip route add default via 192.168.1.1   # Add default gateway
ss -tulnp                      # Listening TCP/UDP ports with PID
netstat -an                    # All connections (legacy)
route -n                       # Routing table (legacy)

iptables -L -n -v              # List all rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT   # Allow SSH
iptables -A INPUT -j DROP      # Drop everything else
ufw status                     # Ubuntu/Debian firewall status
ufw allow 22/tcp               # Allow SSH (UFW)

ping -c 3 example.com           # Test reachability
traceroute example.com         # Trace the route
dig example.com                # DNS lookup
dig +short example.com MX      # MX records only
nslookup example.com           # DNS lookup (legacy)
tcpdump -i eth0 -nn port 80    # Capture HTTP traffic on eth0

ssh user@host                  # Connect to host as user
ssh -p 2222 user@host          # Connect on a custom port
ssh-keygen -t ed25519          # Generate an ed25519 key pair
ssh-copy-id user@host          # Install public key on remote host
ssh-agent bash                 # Start an agent
ssh-add ~/.ssh/id_ed25519      # Add key to the agent

scp file.txt user@host:/tmp/    # Copy local file to remote
scp user@host:/tmp/file.txt .  # Copy remote file to local
scp -r dir user@host:/tmp/     # Copy a whole directory
sftp user@host                 # Interactive secure FTP session

ssh -L 8080:localhost:80 user@host   # Local port forward
ssh -R 8080:localhost:80 user@host   # Remote port forward
ssh -D 1080 user@host                  # SOCKS proxy (dynamic forward)
ssh -N -f -L 5901:localhost:5901 host  # Background tunnel, no command

rsync -a /src/ /dest/          # Archive mode (recursive, preserve)
rsync -a /src/ user@host:/dest/  # Sync to a remote host
rsync -a user@host:/src/ /dest/  # Pull from a remote host
rsync -a --delete /src/ /dest/  # Delete files in dest not in src

rsync -a -v /src/ /dest/       # Verbose output
rsync -a -z /src/ user@host:/dest/  # Compress during transfer
rsync -a -n /src/ /dest/        # Dry run (show what would happen)
rsync -a --progress /src/ /dest/   # Show progress
rsync -a -e "ssh -p 2222" /src/ user@host:/dest/  # Custom SSH port

sudo command                   # Run command as root
sudo -u colin command         # Run as user colin
sudo -s                       # Start a root shell
sudo -k                       # Invalidate the cached credentials

visudo                         # Edit /etc/sudoers safely
# /etc/sudoers entries:
#   colin  ALL=(ALL) ALL              # colin may run anything
#   %admin ALL=(ALL) ALL             # members of admin group
#   colin  ALL=(ALL) NOPASSWD: ALL   # without password

# ~/.bashrc is sourced for interactive shells
export PATH=$PATH:/sbin:/usr/sbin
export EDITOR=vim
export PS1='\u@\h:\w\$ '      # Prompt: user@host:cwd$
alias ll='ls -la'
source ~/.bashrc                # Reload the config

Ctrl+A  / Ctrl+E   # Move to start / end of line
Ctrl+W            # Delete word before cursor
Ctrl+U  / Ctrl+K  # Delete to start / end of line
Ctrl+R            # Reverse search in history
Alt+.             # Insert last argument of previous command

#!/bin/bash
var="hello"                    # Assign a variable (no spaces around =)
echo $var                      # Use the variable
echo $0                        # Script name
echo $1 $2                     # First and second arguments
echo $#                        # Number of arguments
echo "$@"                      # All arguments as a string

if [ -f /etc/hostname ]; then
  echo "file exists"
elif [ -d /etc ]; then
  echo "is a directory"
else
  echo "not found"
fi

# Test operators: -f file, -d dir, -z empty string, -n non-empty
# Numbers: -eq, -ne, -lt, -le, -gt, -ge

for f in *.txt; do
  echo "processing $f"
done

for i in $(seq 1 5); do
  echo "number $i"
done

while read line; do
  echo "$line"
done < input.txt