Screen Usage

Practical screen usage notes

Screen Commands

List of more important screen commands to effectively operate screen

When NOT in screen

Command Description
screen -D Power Detach - Close everything but keep the screens running in background. This is when you are done working!
screen -R When loggin back into the system do this to get back to the screens
screen -d -r Detach and Reattach. This is used when there are some screens stuck as attached but you are actually detached

When in screen

Command Description
Ctrl-a c Create new screen terminal
Ctrl-a d Detach from screen termial and go to main non screen terminal
Ctrl-a w List all screen terminals
Ctrl-a :at “#” stuff “echo hello world\n” Send commands to other screen windows at #. Ref
C-a h, C-a C-h Save the scroll back buffer http://web.mit.edu/gnu/doc/html/screen_17.html
C-a H Begins/ends logging of the current window to the file `screenlog.n'. Saves Color codes https://stackoverflow.com/questions/18127728/differences-between-screenlog-and-hardcopy-on-gnu-screen

When in or not in screen

Command Description
screen -D -RR Power Detach, Reattach or create new screen if there is no screen running

Sending commands to screen terminals

Reference - Scroll to scripting

Script to send commands to screen. steps

  • Create two sessions
    • One GNU screen session where all the scripting is directed to.
    • Another sessions (can be screen if needed) where all the commands are sent from.
  • Opening a named screen session called thenag as below

In Target Session, create a screen session named thenag. If you want a different name you will also have to update the script to reflect this new name.

screen -S thenag

In the Command (From) Session, open a regular or optionally a screen session. Create and run this script as needed.

Run the script with the name of the host file that basically contains a list of servers, the command to send and the sleep time (default 0.2 seconds) between screens. The 1st time you want to run an init command followed by a ssh command. These are special commands that will create screen windows and start an SSH session to each host. When you are done you can send the quit command to terminate the screen session.

#!/bin/bash
#
if [ $# -lt 2 ]; then
  echo "Usage: $0 <Host file> <Command> [sleep time]"
  echo "Host file contains the list of hosts to open screen session for"
  echo "Command is the command to execute on each screen session"
  echo "First time use init as Command to construct the screens"
  exit 1
fi
#
# Get a list (say server names)
HOSTFILE=$1
# Create a counter for the screen windows
HOSTCMD=$2
# Number of seconds to sleep
if [ -z "$3" ]; then
  SLEEPTIME="0.2"
else
  SLEEPTIME=$3
fi
#if ! [[ "$SLEEPTIME" =~ ^[0-9]+$ ]]; then
#  SLEEPTIME=0
#fi
#echo $SLEEPTIME
#exit
# Define a name for the screen session.
SCRNAME='thenag'
NOW=$(date +"%Y-%m-%d-%H-%M-%S")
TMPFILE="/tmp/${SCRNAME}-${NOW}.txt"
#
if [ ! -f "$HOSTFILE" ]; then
  echo "File: $HOSTFILE does not exist"
  exit 1
fi
#
if [ "$HOSTCMD" == "init" ]; then
  echo "Initializing Screens ..."
fi
#
 
if [ "$HOSTCMD" == "quit" ]; then
  echo "Quitting Screens ..."
  screen -S $SCRNAME -X quit
  exit
fi
 
declare sn integer
sn=0
cat $HOSTFILE | while read ServerName
do
  let sn++
  echo "Processing $ServerName, Sequence $sn"
  case $HOSTCMD in
    init)
      # This creates a blank screen window for each instance
      screen -S $SCRNAME -X screen
      # Sleep may be required as sometimes commands get missed!
      sleep $SLEEPTIME
      screen -S $SCRNAME -p $sn -X stuff "echo $ServerName \n"
      ;;
    ssh)
      screen -S $SCRNAME -p $sn -X stuff "ssh ${ServerName}\n"
      sleep $SLEEPTIME
      ;;
    tail)
      screen -S $SCRNAME -p $sn -X hardcopy $TMPFILE
      sleep $SLEEPTIME
      sed -e :a -e '/^\n*$/{$d;N;};/\n$/ba' $TMPFILE
      tail -3 $TMPFILE
      rm $TMPFILE
      echo "--------------------------------"
      ;;
    *)
      screen -S $SCRNAME -p $sn -X stuff "${HOSTCMD} \n"
      sleep $SLEEPTIME
  esac
 
done

Original Script for reference

#!/bin/bash
#
# Get a list (say server names)
HOSTFILE=$1
# Create a counter for the screen windows
declare sn integer
sn=0
cat $HOSTFILE | while read ServerName
do
  let sn++
  echo "$ServerName - $sn"
  # This creates a blank screen window for each instance
  screen -S thenag -X screen
  # Sleep may be required as sometimes commands get missed!
  sleep 1
  screen -S thenag -p $sn -X stuff "echo $ServerName \n"
  screen -S thenag -p $sn -X stuff "ssh ${ServerName}\n"
  sleep 1
  screen -S thenag -p $sn -X stuff "ls -ld /tmp/files \n"
  screen -S thenag -p $sn -X stuff "sudo /bin/bash \n"
  sleep 1
  # You can even send password!!!
  screen -S thenag -p $sn -X stuff "sudopasswordgoeshere\n"
  sleep 1
  screen -S thenag -p $sn -X stuff "cd /tmp/files \n"
  screen -S thenag -p $sn -X stuff "./run_install \n"
done
# Let the install proceed on all screens
sleep 30
 
# Check on the installs
sn=0
cat $HOSTFILE | while read ServerName
do
  let sn++
  echo "$ServerName - $sn"
  screen -S thenag -p $sn -X stuff "echo $? \n"
  # Display last three lines of screen terminal
  screen -S thenag -p $sn -X hardcopy /tmp/s1.txt
  tail -3 /tmp/s1.txt
  rm /tmp/s1.txt
  echo "--------------------------------"
done

Screen Resources


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