screen

GNU Screen Complete Cheat Sheet for Sysadmins

Overview

GNU Screen is a terminal multiplexer that allows you to manage multiple terminal sessions within a single window. It’s an essential tool for sysadmins to:

• Run long-running processes that persist after disconnection
• Manage multiple remote sessions simultaneously
• Split terminal views for monitoring multiple tasks
• Share terminal sessions with team members
• Maintain scrollback history and copy/paste between windows

Default escape key: Ctrl+a (all screen commands start with this)

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Quick Start

Starting Screen

# Start a new named session (recommended)
screen -S mysession

# Start a detached session (runs in background)
screen -dmS mysession command-to-run

# List all screen sessions
screen -ls

# Attach to a session
screen -r mysession

# Attach to a detached session (if multiple, specify PID)
screen -r <PID>

# Reattach or create if doesn't exist
screen -xRR mysession

# Force detach and attach (when session is attached elsewhere)
screen -d -r mysession

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Session Management

Basic Session Operations

Command	Description
Ctrl+a d	Detach current session (keeps processes running)
Ctrl+a D D	Detach and logout (quick exit)
Ctrl+a :sessionname name	Rename current session
Ctrl+a :sessionname	Show current session name
Ctrl+a ?	Display help with all key bindings

Session Commands (Command Mode)

Press Ctrl+a : to enter command mode, then type:

:quit                    # Close all windows and exit screen
:sessionname name        # Rename session
:source ~/.screenrc      # Reload configuration file
:hardcopy -h             # Copy scrollback buffer to file
:log                     # Toggle logging for current window
:logfile filename        # Set log file name
:loglevel [0-8]          # Set log verbosity (0=off, 8=debug)
:time                    # Show system time in caption
:caption always          # Always show caption bar
:caption splitonly       # Show caption only in split mode
:bell_msg "message"      # Custom bell message
:vbell off               # Disable visual bell

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Window Management

Copy and Paste Operations

Command	Description
Enter (in copy mode)	Start selection at cursor
Space (in copy mode)	Toggle selection, mark start/end
Enter (after selection)	Copy selected text to buffer
Ctrl+a ]	Paste from buffer
Ctrl+a >	Write paste buffer to file
Ctrl+a <	Read paste buffer from file
Ctrl+a :buffer	List paste buffers
Ctrl+a :clear	Clear paste buffer

Copy/Paste Workflow:

1. Ctrl+a [ - Enter copy mode
2. Navigate to start position
3. Space - Start selection
4. Navigate to end position
5. Space or Enter - Copy selection to buffer
6. Ctrl+a ] - Paste at cursor position

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Logging and Monitoring

Session Logging

Command	Description
Ctrl+a H	Start/stop logging current window to screenlog.0
Ctrl+a :logfile filename	Set custom log filename
Ctrl+a :log on/off	Toggle logging
Ctrl+a :loglevel N	Set log level (0-8)

Window Monitoring

Command	Description
Ctrl+a M	Monitor current window for activity (bell on output)
Ctrl+a _	Monitor current window for silence (bell if no output)
Ctrl+a :silence N	Set silence timeout in seconds
Ctrl+a :activity	Toggle activity monitoring

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Advanced Features

Nested Screen Sessions

When running screen inside another screen session:

Command	Description
Ctrl+a a	Send escape key to inner screen
Ctrl+a a c	Create window in inner screen
Ctrl+a a d	Detach inner screen session
Ctrl+a a k	Kill inner screen session

Tip: Change escape key in outer or inner screen to avoid confusion:

# In ~/.screenrc for outer screen
escape ^Jj  # Use Ctrl+j instead of Ctrl+a

# In ~/.screenrc for inner screen  
escape ^Tt  # Use Ctrl+t instead of Ctrl+a

Screen with SSH

# Create screen session before SSH disconnect
ssh user@host
screen -S work
# Run commands...
Ctrl+a d  # Detach, disconnect SSH, processes keep running
# Later...
ssh user@host
screen -r work  # Reattach to session

Locking Screen

Command	Description
Ctrl+a x	Lock current screen session (password protected)
Ctrl+a :password	Set screen password (encrypted in ~/.screenrc)

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Configuration (.screenrc)

Location and Loading

• User config: ~/.screenrc
• System config: /etc/screenrc or /usr/local/etc/screenrc
• Load config on the fly: Ctrl+a :source ~/.screenrc

Essential Configuration Options

# Change escape key (recommended: Ctrl+j or Ctrl+`)
escape ^Jj          # Use Ctrl+j as escape, Ctrl+j j sends literal Ctrl+j
# escape ^^^       # Use Ctrl+^ as escape
# escape ```       # Use backtick as escape

# Start windows at 1 instead of 0
screen 1
bind 0 select 10

# Always show window titles in status line
hardstatus alwayslastline

# Custom hardstatus line
hardstatus string "%{= kw}%-Lw%{= BW}%n*%f %t%{-}%+Lw %<"

# Enable 256 colors
term screen-256color

# Auto-detach on hangup
autodetach on

# Don't display copyright
startup_message off

# Scrollback buffer size (default 100 lines)
defscrollback 5000

# Enable alternate screen support
altscreen on

# Set default shell
shell /bin/bash

# Set terminal title
caption always "%{= kw}%n %t%{-}"

# Bind function keys for window switching
bindkey -k k1 select 1
bindkey -k k2 select 2
bindkey -k k3 select 3
bindkey -k k4 select 4
bindkey -k k5 select 5
bindkey -k k6 select 6
bindkey -k k7 select 7
bindkey -k k8 select 8
bindkey -k k9 select 9
bindkey -k k0 select 10

# Bind F11/F12 for previous/next window
bindkey -k F11 prev
bindkey -k F12 next

# Disable visual bell
vbell off

# Set UTF-8 encoding
defencoding utf8
encoding utf8

# Auto-window title from running command
shelltitle "$ |"

# Bind Ctrl+a a to send literal Ctrl+a
bind a

# Set border color for split regions
caption splitonly "%{= kw}%n %t%{-}"

Complete Example .screenrc

# ~/.screenrc - Comprehensive configuration

# Use Ctrl+j as escape key (easier than Ctrl+a)
escape ^Jj

# Start at window 1
screen 1
bind 0 select 10

# 5000 lines scrollback
defscrollback 5000

# Enable 256 colors
term screen-256color

# Always show status bar with window list
hardstatus alwayslastline
hardstatus string "%{= kw}%-Lw%{= BW}%n*%f %t%{-}%+Lw %<"

# Auto-detach on connection loss
autodetach on

# No startup message
startup_message off

# UTF-8 support
defencoding utf8
encoding utf8

# Function keys for window switching
bindkey -k k1 select 1
bindkey -k k2 select 2
bindkey -k k3 select 3
bindkey -k k4 select 4
bindkey -k k5 select 5
bindkey -k k6 select 6
bindkey -k k7 select 7
bindkey -k k8 select 8
bindkey -k k9 select 9
bindkey -k k0 select 10

# F11/F12 for prev/next window
bindkey -k F11 prev
bindkey -k F12 next

# Disable bell
vbell off

# Bind Ctrl+a a to send literal Ctrl+a
bind a

# Set shell
shell /bin/bash

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Command-Line Options

Starting Screen

# Basic session creation
screen                          # Start new session
screen -S name                  # Start named session
screen -d -m command            # Start detached, run command
screen -dmS name command        # Start named detached session

# Session attachment
screen -r                       # Attach to single detached session
screen -r name                  # Attach to named session
screen -r PID                   # Attach by PID
screen -x                       # Attach without detaching others
screen -x name                  # Attach to named session (multi-attach)
screen -d -r name               # Force detach then attach
screen -D -RR                   # Smart attach (create if needed)

# Session listing
screen -ls                      # List all sessions
screen -list                    # Same as -ls

# Session control
screen -S name -X command       # Send command to session
screen -S name -X quit          # Kill named session
screen -S name -X detach        # Detach named session

# Advanced options
screen -c file                  # Use alternate config file
screen -e x y                   # Change escape keys (x=escape, y=literal)
screen -h lines                 # Set scrollback buffer size
screen -i                       # Enable flow control
screen -m                       # Ignore $STY, force new session
screen -p window                # Go to specific window on start
screen -t title                 # Set window title
screen -U                       # Enable UTF-8
screen -x                       # Multi-attach mode

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Common Sysadmin Use Cases

1. Long-Running Processes

# Start a backup that survives SSH disconnect
screen -S backup
# Run your backup command
rsync -avz /data /backup/
# Detach with Ctrl+a d
# Reattach later: screen -r backup

2. Monitoring Multiple Servers

# Create session with multiple windows
screen -S monitoring
# Window 1: tail logs on server1
ssh user@server1 "tail -f /var/log/syslog"
# Ctrl+a c (new window)
# Window 2: monitor disk space
ssh user@server2 "watch df -h"
# Ctrl+a c (new window)
# Window 3: check processes
ssh user@server3 "top"
# Ctrl+a d to detach

3. Split Screen Monitoring

# Start screen
screen -S monitor
# Create first window with server1
ssh user@server1 "top"
# Ctrl+a S (horizontal split)
# Ctrl+a tab (switch to new region)
# Ctrl+a " (select window 2 or create new)
# Window 2: server2 monitoring
ssh user@server2 "tail -f /var/log/auth.log"
# Ctrl+a tab to switch between views

4. Shared Session for Collaboration

# User A creates shared session
screen -S shared -x  # -x allows multi-attach
# User B attaches to same session
ssh user@host
screen -x shared
# Both users see same screen, can collaborate

5. Running Commands in Background

# Start detached session running command
screen -dmS jobname long-running-command
# Check if running
screen -ls
# View output later
screen -r jobname
# Or just view logs if logging enabled
tail -f screenlog.0

6. Reattach After SSH Disconnect

Add to ~/.bashrc:

# Auto-attach to existing screen or create new
if [ -z "$STY" ]; then
    screen -xRR main 2>/dev/null || screen -S main
fi

Now every SSH login automatically attaches to “main” session or creates it.

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Troubleshooting

Common Issues

Problem: There is a screen on: error when attaching

# Force detach and attach
screen -d -r sessionname

# Or kill the stale session
screen -S sessionname -X quit

Problem: Can’t type Ctrl+a in applications (like bash, vim)

# Send literal Ctrl+a to application
Ctrl+a a  # Then type your command (e.g., Ctrl+a a a goes to bash beginning-of-line)

Problem: Nested screen confusion

# Change escape key in one of the sessions
# In outer screen: Ctrl+a :escape ^Jj
# Now use Ctrl+j for outer, Ctrl+a for inner

Problem: No colors or 256-color support

# In ~/.screenrc:
term screen-256color
# Or set TERM before starting screen:
export TERM=screen-256color
screen

Problem: Scrollback not working

# Increase scrollback buffer
# In ~/.screenrc:
defscrollback 5000
# Or on command line:
screen -h 5000

Problem: Window titles not showing

# Enable hardstatus in ~/.screenrc:
hardstatus alwayslastline
hardstatus string "%{= kw}%-Lw%{= BW}%n*%f %t%{-}%+Lw %<"

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Pro Tips for Sysadmins

1. Always name your sessions: screen -S descriptive-name makes management easier
2. Use a good .screenrc: Configure escape key, scrollback, and status bar for efficiency
3. Log important sessions: Ctrl+a H to capture output for audit trails
4. Split for monitoring: Use horizontal splits (Ctrl+a S) to watch multiple logs simultaneously
5. Copy/paste between windows: Great for moving commands between sessions
6. Monitor for silence: Ctrl+a _ alerts you if a process hangs (no output)
7. Use with SSH: Never lose work due to network drops
8. Share sessions: screen -x for pair programming or collaborative troubleshooting
9. Auto-start: Add to .bashrc to always have a screen session available
10. Window 0 is annoying: Configure screen 1 in .screenrc to start at window 1

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Cheat Sheet Summary Table

Session Commands

Key	Action
Ctrl+a d	Detach
Ctrl+a D D	Detach & logout
Ctrl+a ?	Help
Ctrl+a :	Command prompt

Window Commands

Key	Action
Ctrl+a c	New window
Ctrl+a n	Next window
Ctrl+a p	Previous window
Ctrl+a 0-9	Switch to window #
Ctrl+a "	Window list
Ctrl+a A	Rename window
Ctrl+a w	Show windows
Ctrl+a Ctrl+a	Toggle windows
Ctrl+a k	Kill window
Ctrl+a \	Kill all

Split Screen

Key	Action
Ctrl+a S	Split horizontally
Ctrl+a \|	Split vertically
Ctrl+a tab	Next region
Ctrl+a X	Remove region
Ctrl+a Q	Remove all but current

Copy/Paste

Key	Action
Ctrl+a [	Copy mode
Space	Start/end selection
Enter	Copy selection
Ctrl+a ]	Paste
Ctrl+a >	Save buffer
Ctrl+a <	Load buffer

Logging & Monitoring

Key	Action
Ctrl+a H	Toggle logging
Ctrl+a M	Monitor activity
Ctrl+a _	Monitor silence
Ctrl+a x	Lock screen

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Additional Resources

• Man page: man screen
• Info documentation: info screen
• Official GNU Screen manual: https://www.gnu.org/software/screen/manual/
• Configuration examples: /usr/share/doc/screen/examples/
• Alternative: Consider tmux for more modern features (similar functionality)