Screen Usage

Practical screen usage notes

List of more important screen commands to effectively operate screen

From terminal you can see the Screen Name with echo $STY. Relevant if you are having multiple screen sessions.

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.

rs

#!/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

Below are the steps and command examples to execute commands across screens

• Setup host file - in this case we are calling it webs.txt

webs.txt

server80
server81
server82
server83

• Initialize the screen

./rs webs.txt init

Output:

Initializing Screens ...
Processing server80, Sequence 1
Processing server81, Sequence 2
Processing server82, Sequence 3
Processing server83, Sequence 4

• SSH into each host

./rs webs.txt ssh

Output: The same as above in init

• Run a custom command (in this case sudo su - serviceacc)

../rs webs.txt "sudo su - serviceacc"

Output: The same as above in init

• Just hit enter

./rs webs.txt ""

• Tail the last 3 lines of the output on each session

./rs webs.txt tail

Output: Will display Host Sequence and tail lines from the session

• Quit all screen sessions (opposite of init)

./rs webs.txt quit

Output: Quitting Screens …

The hard-coded version of the script. Notice how you can pass passwords if required as well.

#!/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

GNU Screen

Screen User Manual