Cheatsheet

23 entries

commands and tricks for day to day

Reset a repository's commit history by creating an orphan branch

git checkout --orphan latest_branch
git add -A
git commit -am "reset configs"
git branch -D main
git branch -m main
git push -f origin main

Extract URLs from a specific Chrome bookmark folder using jq

cat ~/.config/google-chrome/Default/Bookmarks \
  | jq -r '.roots.bookmark_bar.children[]
      | select(.name == "tree")
      | .children[]
      | .url'

List all bookmarks from Chrome's JSON file showing name and URL

cat ~/.config/google-chrome/Default/Bookmarks \
  | jq '.roots.bookmark_bar.children[] | {name: .name, url: .url}'

List all Chrome bookmark folders using jq

cat ~/.config/google-chrome/Default/Bookmarks \
  | jq -r '[.. | objects | select(.type == "folder") | .name] | unique | sort | .[]'

Chrome Internals Pages
Useful chrome:// internal pages for debugging DNS, events, and proxy settings
```
chrome://net-internals/#dns
chrome://net-internals/#events
chrome://net-internals/#proxy
```

List all entries from the root of Chrome's bookmarks bar

jq -r '.roots.bookmark_bar.children[]' \
  "$HOME/Library/Application Support/Google/Chrome/Default/Bookmarks"

Post-install Docker
Add current user to docker group after installation
```
sudo usermod -aG docker $USER
exec su -l $USER
groups $USER
docker ps
```

Basic k6 script with ramp-up, sustained load, and ramp-down stages

import http from 'k6/http';
import { sleep } from 'k6';

export const options = {
    stages: [
        { duration: '10s', target: 100 },
        { duration: '30s', target: 100 },
        { duration: '10s', target: 0 }
    ]
}

export default function () {
    http.get('http://192.168.68.108:3000');
    sleep(1);
}

ntfy Send Notification
Send a push notification via ntfy.sh using curl
```
curl -d "Backup successful" ntfy.sh/mytopic
```

Force curl to resolve a hostname to a specific IP address, bypassing DNS

curl --resolve <hostname>:<porta>:<ip> <protocolo>://<hostname>:<porta>
curl --resolve meuservidor.com:8080:192.168.0.100 http://meuservidor.com:8080
curl --resolve google.com:8888:127.0.0.1 https://google.com:8888

Windows Activation (massgrave)
Activate Windows using the massgrave open-source script
```
irm https://massgrave.dev/get | iex
```

Retrieve saved WiFi passwords on Linux using NetworkManager

ls /etc/NetworkManager/system-connections/
sudo cat /etc/NetworkManager/system-connections/[network-name]
nmcli device wifi list
sudo nmcli connection show "<wifi_name>" --show-secrets

Use OpenSSL AES-256-CBC with PBKDF2 to encrypt and decrypt files

# Encrypt
echo "Este é um arquivo de teste" > original.txt
openssl enc -aes-256-cbc -salt -pbkdf2 -in original.txt -out secreto.enc

# Decrypt
openssl enc -aes-256-cbc -d -salt -pbkdf2 -in secreto.enc

# Decrypt from remote URL
curl -s https://raw.githubusercontent.com/apolzek/apolzek.github.io/main/secreto.enc \
  | openssl enc -aes-256-cbc -d -salt -pbkdf2

Linux Network Connections
View all saved NetworkManager connection configs
```
sudo cat /etc/NetworkManager/system-connections/*
```
Set Network Interface MTU
Lower the MTU on a network interface to avoid packet drops (e.g. on iPhone tethering)
```
sudo ip link set wlp3s0 mtu 1280
```

Commands for viewing, configuring, and troubleshooting DNS on macOS

# Show current DNS resolver configuration (system-wide)
scutil --dns

# Display DNS servers currently in use (per interface)
networksetup -getdnsservers Wi-Fi

# Set custom DNS servers for Wi-Fi (replace with your desired servers)
sudo networksetup -setdnsservers Wi-Fi 1.1.1.1 8.8.8.8

# Reset DNS back to automatic (DHCP-provided DNS)
sudo networksetup -setdnsservers Wi-Fi empty

# Flush DNS cache (modern macOS)
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

# Query DNS resolution for a domain (low-level system resolver test)
dig example.com

# Alternative DNS lookup tool (simpler output)
nslookup example.com

# Test which DNS server is resolving a domain
dig example.com +short

# Show detailed resolver behavior and search domains
scutil --dns | grep 'nameserver'

# Check current network services (to identify correct interface name)
networksetup -listallnetworkservices

# View DNS configuration for a specific interface (example: Ethernet)
networksetup -getdnsservers Ethernet

# Set DNS for Ethernet interface
sudo networksetup -setdnsservers Ethernet 9.9.9.9 1.1.1.1

Commands for viewing, configuring, and troubleshooting DNS on Ubuntu (systemd-resolved and NetworkManager based systems)

# Show current DNS configuration (systemd-resolved)
resolvectl status

# Alternative command (older systems)
systemd-resolve --status

# Show DNS servers currently in use per interface
nmcli dev show | grep DNS

# Show NetworkManager connection profiles
nmcli connection show

# Set DNS servers for a specific connection (replace 'Wired connection 1')
sudo nmcli connection modify "Wired connection 1" ipv4.dns "1.1.1.1 8.8.8.8"

# Ignore DHCP-provided DNS and use manual DNS
sudo nmcli connection modify "Wired connection 1" ipv4.ignore-auto-dns yes

# Restart connection to apply DNS changes
sudo nmcli connection down "Wired connection 1" && sudo nmcli connection up "Wired connection 1"

# Flush DNS cache (systemd-resolved)
sudo resolvectl flush-caches

# Alternative cache restart (forces full reload of resolver)
sudo systemctl restart systemd-resolved

# Query DNS resolution using system resolver
getent hosts example.com

# Query DNS records with dig (recommended tool)
dig example.com

# Query specific DNS record type (A record)
dig example.com A

# Show trace of DNS resolution path
dig example.com +trace

# Simple DNS lookup tool (basic output)
nslookup example.com

# Check /etc/resolv.conf (often symlinked to systemd stub)
cat /etc/resolv.conf

# Verify if systemd stub resolver is active
ls -l /etc/resolv.conf

# Temporarily set DNS by editing resolv.conf (not persistent on most modern systems)
sudo bash -c 'echo -e "nameserver 1.1.1.1\nnameserver 8.8.8.8" > /etc/resolv.conf'

macOS Clipboard Basic Commands
Minimal commands to copy file content to clipboard and paste it back
```
# Copy file content to clipboard using pipe
cat arquivo.txt | pbcopy

# Paste clipboard content in terminal
pbpaste
```

Setup and commands to copy file content to clipboard and paste it back on Ubuntu using xclip or xsel

# Install clipboard utility (xclip)
sudo apt update && sudo apt install -y xclip

# Copy file content to clipboard (equivalent to: cat arquivo.txt | pbcopy)
cat arquivo.txt | xclip -selection clipboard

# Paste clipboard content to terminal (equivalent to pbpaste)
xclip -selection clipboard -o

# Alternative install (xsel instead of xclip)
sudo apt install -y xsel

# Copy file content using xsel
cat arquivo.txt | xsel --clipboard --input

# Paste clipboard content using xsel
xsel --clipboard --output

# Optional aliases for convenience (add to ~/.bashrc or ~/.zshrc)
alias pbcopy='xclip -selection clipboard'
alias pbpaste='xclip -selection clipboard -o'

Finds all files larger than 10 MB in the current directory tree
Searches recursively for files larger than 10 MB, filters out any files inside .git directories, removes the leading ./ from the paths, and appends the resulting file paths to .gitignore.
```
find . -type f -size +10M | grep -v ".git" | sed 's|^\./||' >> .gitignore 
```

Quick reference for checking, increasing, and troubleshooting Linux Inotify limits, including watches, instances, event queues, and common filesystem monitoring errors.

# Show current inotify limits
sysctl fs.inotify.max_user_instances
sysctl fs.inotify.max_user_watches
sysctl fs.inotify.max_queued_events

# Show all inotify-related kernel parameters
sysctl -a | grep inotify

# Temporarily increase the maximum number of inotify instances per user
sudo sysctl -w fs.inotify.max_user_instances=1024

# Temporarily increase the maximum number of watched files/directories per user
sudo sysctl -w fs.inotify.max_user_watches=1048576

# Temporarily increase the inotify event queue size
sudo sysctl -w fs.inotify.max_queued_events=65536

# Persist custom inotify limits across reboots
sudo tee /etc/sysctl.d/99-inotify.conf > /dev/null <<EOF
fs.inotify.max_user_instances=1024
fs.inotify.max_user_watches=1048576
fs.inotify.max_queued_events=65536
EOF

# Reload all sysctl configuration files
sudo sysctl --system

# Reload a specific sysctl configuration file
sudo sysctl -p /etc/sysctl.d/99-inotify.conf

# Check how many watches are currently allocated system-wide
sudo find /proc/*/fdinfo -type f 2>/dev/null | xargs grep -h '^inotify' 2>/dev/null | wc -l

# Show processes using inotify
sudo lsof | grep inotify

# Show per-process inotify usage (PID and count)
for pid in /proc/[0-9]*; do
  count=$(grep -c '^inotify' "$pid"/fdinfo/* 2>/dev/null)
  [ "$count" -gt 0 ] && echo "$(basename "$pid"): $count"
done

# Monitor filesystem events in real time (requires inotify-tools)
inotifywait -m /path/to/watch

# Monitor recursively including subdirectories
inotifywait -mr /path/to/watch

# Watch for specific events only
inotifywait -m -e create,modify,delete /path/to/watch

# Continuously print filesystem events
inotifywatch -v /path/to/watch

# Install inotify-tools (Debian/Ubuntu)
sudo apt install inotify-tools

# Install inotify-tools (RHEL/CentOS/Fedora)
sudo dnf install inotify-tools

# Check kernel messages for inotify-related errors
dmesg | grep -i inotify

# Find common watch-limit errors in logs
journalctl -xe | grep -Ei 'inotify|watch'

Linux Performance Analysis Commands

uptime
dmesg | tail
vmstat 1
mpstat -P ALL 1
pidstat 1
iostat -xz 1
free -m
sar -n DEV 1
sar -n TCP,ETCP 1
top

Install Guest Additions on an Ubuntu guest, plus the ISO/Extension Pack setup on the host

### HOST (your machine running VirtualBox) ###

# Ships the ISO at /usr/share/virtualbox/VBoxGuestAdditions.iso
# Skip it if VirtualBox came from Oracle's .deb (ISO already bundled)
sudo apt install virtualbox-guest-additions-iso

# Needed for USB passthrough
sudo usermod -aG vboxusers $USER

# Optional Extension Pack (USB 2.0/3.0, RDP) - version MUST match
VBoxManage --version
VBoxManage extpack install ~/Downloads/Oracle_VM_VirtualBox_Extension_Pack-*.vbox-extpack
VBoxManage list extpacks

### GUEST (inside the Ubuntu VM) ###

# Build deps - dkms rebuilds the module after kernel upgrades
sudo apt update
sudo apt install build-essential dkms linux-headers-$(uname -r)

# Menu: Devices > Insert Guest Additions CD image, then:
sudo mkdir -p /mnt/cdrom
sudo mount /dev/cdrom /mnt/cdrom
sudo /mnt/cdrom/VBoxLinuxAdditions.run
sudo reboot

# Alternative without the ISO (may lag the host version slightly)
sudo apt install virtualbox-guest-x11   # virtualbox-guest-utils if headless

# Shared folders access - logout/login afterwards
sudo usermod -aG vboxsf $USER

# Verify (in the guest, after reboot)
lsmod | grep vboxguest
modinfo vboxguest | grep version