Computer Use Skill

Full desktop GUI control for headless Linux servers. Creates a virtual display (Xvfb + XFCE) so you can run and control desktop applications on VPS/cloud instances without a physical monitor.

Environment

Display: :99
Resolution: 1024x768 (XGA, Anthropic recommended)
Desktop: XFCE4 (minimal — xfwm4 + panel only)

Quick Setup

Run the setup script to install everything (systemd services, flicker-free VNC):

./scripts/setup-vnc.sh

This installs:

Xvfb virtual display on :99
Minimal XFCE desktop (xfwm4 + panel, no xfdesktop)
x11vnc with stability flags
noVNC for browser access

All services auto-start on boot and auto-restart on crash.

Actions Reference

Action	Script	Arguments	Description
screenshot	screenshot.sh	—	Capture screen → base64 PNG
cursor_position	cursor_position.sh	—	Get current mouse X,Y
mouse_move	mouse_move.sh	x y	Move mouse to coordinates
left_click	click.sh	x y left	Left click at coordinates
right_click	click.sh	x y right	Right click
middle_click	click.sh	x y middle	Middle click
double_click	click.sh	x y double	Double click
triple_click	click.sh	x y triple	Triple click (select line)
left_click_drag	drag.sh	x1 y1 x2 y2	Drag from start to end
left_mouse_down	mouse_down.sh	—	Press mouse button
left_mouse_up	mouse_up.sh	—	Release mouse button
type	type_text.sh	"text"	Type text (50 char chunks, 12ms delay)
key	key.sh	"combo"	Press key (Return, ctrl+c, alt+F4)
hold_key	hold_key.sh	"key" secs	Hold key for duration
scroll	scroll.sh	dir amt [x y]	Scroll up/down/left/right
wait	wait.sh	seconds	Wait then screenshot
zoom	zoom.sh	x1 y1 x2 y2	Cropped region screenshot

Usage Examples

export DISPLAY=:99

# Take screenshot
./scripts/screenshot.sh

# Click at coordinates
./scripts/click.sh 512 384 left

# Type text
./scripts/type_text.sh "Hello world"

# Press key combo
./scripts/key.sh "ctrl+s"

# Scroll down
./scripts/scroll.sh down 5

Workflow Pattern

Screenshot — Always start by seeing the screen
Analyze — Identify UI elements and coordinates
Act — Click, type, scroll
Screenshot — Verify result
Repeat

Tips

Screen is 1024x768, origin (0,0) at top-left
Click to focus before typing in text fields
Use ctrl+End to jump to page bottom in browsers
Most actions auto-screenshot after 2 sec delay
Long text is chunked (50 chars) with 12ms keystroke delay

Live Desktop Viewing (VNC)

Watch the desktop in real-time via browser or VNC client.

Connect via Browser

# SSH tunnel (run on your local machine)
ssh -L 6080:localhost:6080 your-server

# Open in browser
http://localhost:6080/vnc.html

Connect via VNC Client

# SSH tunnel
ssh -L 5900:localhost:5900 your-server

# Connect VNC client to localhost:5900

SSH Config (recommended)

Add to ~/.ssh/config for automatic tunneling:

Host your-server
  HostName your.server.ip
  User your-user
  LocalForward 6080 127.0.0.1:6080
  LocalForward 5900 127.0.0.1:5900

Then just ssh your-server and VNC is available.

System Services

# Check status
systemctl status xvfb xfce-minimal x11vnc novnc

# Restart if needed
sudo systemctl restart xvfb xfce-minimal x11vnc novnc

Service Chain

xvfb → xfce-minimal → x11vnc → novnc

xvfb: Virtual display :99 (1024x768x24)
xfce-minimal: Watchdog that runs xfwm4+panel, kills xfdesktop
x11vnc: VNC server with -noxdamage for stability
novnc: WebSocket proxy with heartbeat for connection stability

Opening Applications

export DISPLAY=:99

# Chrome — only use --no-sandbox if the kernel lacks user namespace support.
# Check: cat /proc/sys/kernel/unprivileged_userns_clone
#   1 = sandbox works, do NOT use --no-sandbox
#   0 = sandbox fails, --no-sandbox required as fallback
# Using --no-sandbox when unnecessary causes instability and crashes.
if [ "$(cat /proc/sys/kernel/unprivileged_userns_clone 2>/dev/null)" = "0" ]; then
    google-chrome --no-sandbox &
else
    google-chrome &
fi

xfce4-terminal &                # Terminal
thunar &                        # File manager

Note: Snap browsers (Firefox, Chromium) have sandbox issues on headless servers. Use Chrome .deb instead:

wget https://dl.google.com/linux/direct/google-chrome-stable_current_amd64.deb
sudo dpkg -i google-chrome-stable_current_amd64.deb
sudo apt-get install -f

Manual Setup

If you prefer manual setup instead of setup-vnc.sh:

# Install packages
sudo apt install -y xvfb xfce4 xfce4-terminal xdotool scrot imagemagick dbus-x11 x11vnc novnc websockify

# Run the setup script (generates systemd services, masks xfdesktop, starts everything)
./scripts/setup-vnc.sh

If you prefer fully manual setup, the setup-vnc.sh script generates all systemd service files inline -- read it for the exact service definitions.

Troubleshooting

VNC shows black screen

Check if xfwm4 is running: pgrep xfwm4
Restart desktop: sudo systemctl restart xfce-minimal

VNC flickering/flashing

Ensure xfdesktop is masked (check /usr/bin/xfdesktop)
xfdesktop causes flicker due to clear→draw cycles on Xvfb

VNC disconnects frequently

Check noVNC has --heartbeat 30 flag
Check x11vnc has -noxdamage flag

x11vnc crashes (SIGSEGV)

Add -noxdamage -noxfixes flags
The DAMAGE extension causes crashes on Xvfb

Requirements

Installed by setup-vnc.sh:

xvfb xfce4 xfce4-terminal xdotool scrot imagemagick dbus-x11 x11vnc novnc websockify

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