Using GNU Screen for Long-Running Processes

Ever started a long-running process over SSH, only to have your connection drop and kill everything? GNU screen solves this problem by creating persistent terminal sessions that keep running even after you disconnect.

What is Screen?

Screen is a terminal multiplexer that allows you to: - Run processes that survive SSH disconnects - Detach and reattach to sessions anytime - Run multiple terminal windows in one SSH session - Share terminal sessions between users

Think of it as "tabs for your terminal" that persist even when you close your SSH connection.

Installation

Fedora/RHEL/CentOS

sudo dnf install screen

Debian/Ubuntu

sudo apt install screen

Verify Installation

screen --version
# Output: Screen version 4.09.01 (GNU) 20-Aug-23

Basic Usage

Starting a New Session

Start with a name (recommended):

screen -S build

Start without a name:

screen

You're now inside a screen session. It looks exactly like a regular terminal, but it's persistent.

Detaching from a Session

To leave a session running and return to your normal terminal:

Press: Ctrl+A then D

You'll see:

[detached from 12345.build]

Your process keeps running in the background!

Listing Active Sessions

screen -ls

Output example:

There are screens on:
    12345.build     (Detached)
    67890.download  (Detached)
2 Sockets in /run/screen/S-martijn.

Reattaching to a Session

If you have one session:

screen -r

If you have multiple sessions:

screen -r build

Or use the PID:

screen -r 12345

Killing a Session

From inside the session:

exit

From outside:

screen -X -S build quit

Real-World Example: Building a RAG System

Here's how I use screen for long-running AI model builds:

# Start a named session
screen -S rag-build

# Navigate to project
cd ~/lennyhub-rag
source venv/bin/activate

# Start long-running process
python build_lightrag_direct.py 2>&1 | tee build.log

# Detach safely: Ctrl+A, then D

Later, even after closing SSH:

# Reconnect to SSH
ssh user@server

# Reattach to see progress
screen -r rag-build

# Process is still running!

Essential Screen Commands

All screen commands start with Ctrl+A, then another key:

Advanced Tips

Name Your Sessions Descriptively

screen -S model-training
screen -S data-processing
screen -S web-scraper

Better than generic names or PIDs!

Monitor a Session Without Attaching

screen -x build

This creates a "shared" view - useful for watching progress without interrupting.

Start Screen with a Command

screen -S download wget https://example.com/large-file.zip

Starts screen, runs the command, and you can detach immediately.

Log Session Output

# Inside screen, press Ctrl+A then H
# Creates screenlog.0 in current directory

Or start with logging enabled:

screen -L -S build

Reattach Even if "Attached" Elsewhere

screen -dr build

The -d flag detaches it from the other location first, then reattaches to you.

Common Use Cases

1. Long-Running Builds

screen -S compile
make -j$(nproc)
# Ctrl+A, D to detach

2. Server Monitoring

screen -S monitor
htop
# Ctrl+A, D to detach

3. Multiple Services

screen -S services

# Create windows for each service
Ctrl+A, C  # New window
./start-database.sh

Ctrl+A, C  # Another window  
./start-api.sh

Ctrl+A, C  # Another window
tail -f logs/app.log

# Switch between with Ctrl+A, N (next)

4. Remote Pair Programming

# Person A creates shared session
screen -S pairing

# Person B joins the same session
screen -x pairing

# Both see the same terminal in real-time!

Troubleshooting

"Cannot open your terminal '/dev/pts/X'"

script /dev/null
screen

Orphaned Screen Sessions After Reboot

# Clean up dead sessions
screen -wipe

Accidentally Nested Screen Sessions

Never run screen inside a screen session! Check if you're already in one:

echo $STY
# Empty = not in screen
# Has value = already in screen

Screen vs tmux

Screen: - Simpler, easier to learn - Available everywhere - Perfect for basic use cases

tmux: - More modern, more features - Better window management - Scriptable configuration

For most users, screen is perfect. I've used it for 15+ years without needing tmux.

Quick Reference Card

# Session Management
screen -S name          # Start named session
screen -ls              # List sessions  
screen -r name          # Reattach to session
screen -dr name         # Detach & reattach
Ctrl+A, D               # Detach from session
exit                    # End session

# Window Management (inside screen)
Ctrl+A, C               # Create window
Ctrl+A, N               # Next window
Ctrl+A, P               # Previous window  
Ctrl+A, "               # List windows
Ctrl+A, K               # Kill window

# Other
Ctrl+A, [               # Scrollback mode (q to exit)
Ctrl+A, ?               # Help

Conclusion

Screen is an essential tool for anyone working with remote servers or long-running processes. Once you start using it, you'll wonder how you ever managed without it.

Key takeaways: - Always use named sessions (-S name) - Detach with Ctrl+A, D - Reattach with screen -r name - Use screen -ls to see what's running

Now you'll never lose work to a dropped SSH connection again!

Resources

• Official documentation: man screen
• Quick start: screen -h
• Help inside screen: Ctrl+A, ?

Pro tip: Set up aliases in your .bashrc:

alias sls='screen -ls'
alias sr='screen -r'
alias ss='screen -S'

Then starting a session is just: ss build 🚀