Linux Cheatsheet

Dummy Values

Unless stated otherwise, we use the following strings for the examples below:

User: simshadows
Group: mygroup
Arbitrary File: myfile.txt
Arbitrary Directory: mydir
Other values such as IP addresses will be implicitly assumed to be example values.

General

bash and Common CLI Utilities

See bash Cheatsheet.

SSH and Remote Access

Client commands:

# Open remote terminal
ssh 192.168.0.2

# Generate `ed25519` key
ssh-keygen -t ed25519 -C "simshadows@myuri 1970-01-01"
# Check your public key
cat ~/.ssh/id_ed25519.pub

# Copy your default key to SSH server
ssh-copy-id 192.168.0.2
# Copy a key to SSH server with more specificity (`-p` is the server port)
ssh-copy-id -i ~/.ssh/id_ed25519.pub -p 8765 [email protected]

Server commands:

# Check accepted public keys
cat ~/.ssh/authorized_keys

Also see Other Common Tasks.

Admin

Package/Distro/Hypervisor

Debian

# Full system update
## Step 1
apt update
## Step 2
apt upgrade
## Step 3
apt full-upgrade

# Install package `nmap`
apt install nmap

# Search for packages with keyword `nmap`
apt search nmap

Arch Linux

# Full system update
## Step 1: Update keyring
pacman -Sy --needed archlinux-keyring
## Step 2: Update all packages
pacman -Syu

# Install package `gvim`
pacman -Syu gvim

Windows WSL

See this article for more commands.

# Start WSL
wsl
# Start WSL as user `root`
wsl -u root

User Management

# List users (each line is a user)
cat /etc/passwd
# List groups (each line is a group, GID, and its members)
cat /etc/group

# Create new user
useradd -m -s /bin/bash simshadows

# Change password for a user
passwd simshadows

# Append group `sudo` to user
usermod -a -G sudo simshadows

# Update sudoers file
visudo

# Read user uid
id -u simshadows

Disk Management

# Listing things out
fdisk -l
lsblk -o +MODEL,SERIAL
mount

# Manage partition table for disk `/dev/sda`
fdisk /dev/sda

# Format partition `/dev/sda1` as ext4
mkfs.ext4 /dev/sda1
# Format partition `/dev/sda1` as FAT32
mkfs.fat -F32 /dev/sda1

# Mount partition `/dev/sda1` to `/mnt/foobar`
mount /dev/sda1 /mnt/foobar
# Umount `/mnt/foobar`
umount /mnt/foobar

# Print all SMART info for disk `/dev/sda`
smartctl /dev/sda -a

Common fdisk commands:

Filesystem Permissions

# View permissions
ls -la

# Change file owner
chown simshadows myfile.txt
# Change file group
chgrp mygroup myfile.txt

# Set file permissions to `rx-r-xr--` (octal mode)
chmod 654 myfile.txt
#     rwx 7  |  -wx 3
#     rw- 6  |  -w- 2
#     r-x 5  |  --x 1
#     r-- 4  |  --- 0

# Set `x` for user, group, and other (symbolic mode)
chmod a+x myfile.txt
# Same effect as `a+x` except the bits set by the umask are not affected
chmod +x myfile.txt
# Set `x` for user, set `r` for group and other, and remove `w` for other
chmod u+x,go+r,o-w myfile.txt
#     u User   |  - Remove             |  r Read
#     g Group  |  + Add                |  w Write
#     o Other  |  = (see `man chmod`)  |  x Execute
#     a All    |                       |

# View ACL
getfacl myfile.txt
getfacl mydir

# TODO: What about setting ACL values?
# TODO: What about setuid, setguid, and sticky bit?

Network Management

ip a

# List all open ports (numeric addresses)
ss -ln
# List all open TCP/UDP ports (numeric addresses)
ss -tuln

# Check route table
ip r
# It's short for `ip route show`. Manpage `man ip route`.
# Default gateway starts with `default via`.

Network Auditing

ping -c 5 8.8.8.8
traceroute 8.8.8.8

nmap (we use 192.168.0.2 and 192.168.0.0/24 for these examples):

# Host discovery
nmap -sn 192.168.0.0/24

# TCP scan (SYN --> SYN ACK --> ACK)
nmap -sT 192.168.0.0/24
# TCP scan (SYN --> SYN ACK) (Use this if you're having firewall issues.)
nmap -sS 192.168.0.0/24
# UDP scan
nmap -sU 192.168.0.0/24

# What OS is a host running? (Also does other things.)
nmap -A 192.168.0.2

# Full TCP scan and `-A` on a host. Assume the host exists.
nmap -sT -p0- -A -Pn -T4 192.168.0.2

# Run all vulnerability scan scripts on a host
nmap --script vuln 192.168.0.2

Common subnet masks:

IPv4 Subnet Mask	Prefix	Bits
255.255.255.255	/32	0
255.255.255.240	/28	4
255.255.255.0	/24	8

systemd

The examples below use a placeholder unit name foobar.service.

# Start unit now, and always auto-start on boot
systemctl enable --now foobar.service
# Restart unit
systemctl restart foobar.service
# Check the status of a unit
systemctl status foobar.service

# User unit files are located at:
#     ~/.config/systemd/user/
# Add `--user` to operate on them like:
systemctl --user enable --now foobar.service

More References

My Arch Linux setup guide and setup script