E-mail comments

Use `screen` to detach from work

Within an SSH connection, if you start an interactive program it is likely to stop working when you close your terminal. You can set up your VM so that the program will keep working after you close your connection. You can use a program called screen for that.

Set-up

Create a new VM (you may be able to reuse the existing images and templates that you have created so far; but you can always import a new appliance from the AppMarket).
Log into your VM using SSH.
Let us now create a simple script file called example.sh. You can use your favourite editor. In our example we use nano, like this:

cd
nano example.sh

Write (or copy-and-paste) these lines into the file:

#!/bin/bash
while sleep 2
do
  date
done

Save the file and exit the editor. For example, in nano, you press control-x and then you press Y to confirm that you want to save your changes. Then you are back in the terminal.
Make the script executable:

chmod u+x example.sh

Run the script:

./example.sh

After a while, terminate the script by typing control-c

First detach

Log in to your VM,
run the example script:

./example.sh

observe the output’s progress
log in with a second ssh session
check with ps or pstree that your script runs,

pstree
# look for a line like this:
#  ... -example.sh---sleep

Food for brain:

Is pstree command installed? If not, are you able to figure out how to install it? After all, you are the sysadmin!

Suggestion just in case:

Find the Linux distribution (aka distro) name and version you are running.

Find the package for the distro you are running that provides pstree.

Install the package using the package manager for your linux distro. Tip, try the apt command-line tool.

disconnect the terminal with the running example by closing its window
in the other session, use ps or pstree again to verify the the execution has stopped.

Using `nohup`

Log in to your VM,
use nohup to run the example script in the background:

nohup ./example.sh &

The output will be:

nohup: ignoring input and appending output to ‘nohup.out’

as before, check the process with ps or pstree and have a look at the progress:

tail -f nohup.out

log out and log back in
use ps or pstree to verify that your script is still running
look at the output file(s) to see that output is still continuing

tail -f nohup.out

kill the running script (as there is no natural end to it)

killall example.sh

check the output file again to verify the script is no longer running
remove the output file.

For more information read the manual, it is small and simple but effective.

man nohup

Using `screen`

You will use a program called screen. Similar programs are: tmux, dtach+dtvm.

start screen for the first time

screen

after reading an initial message, you press the Enter key
you see the normal shell prompt
use pstree to see that you are actually in a screen session: look for a line similar to

├─sshd───sshd───sshd───bash───screen───screen───bash───pstree

start your example script

./example.sh

wait a few seconds and observe the output, as expected
detach from the screen session by typing control-a, then d
the view is restored to the situation before you started screen and line says something similar to:

[detached from 14691.pts-0.145]

wait several seconds and reconnect using the -r flag:

screen -r

you see that the output has accumulated while you were detached, the script continued to run.
detach again (control-a plus d)
log out and back in
reconnect to the screen session
find the script still running
terminate the script (control-c)
terminate the shell inside screen (control-d)
you are back with the extra line [screen is terminating]
try screen -r

Multiple `screen` sessions

screen command has a lot of options. Reading the manual page will provide you with a wealth of information.

Examples:

Start additional session

in screen, type a command, so that you can recognize the session
type control-a c (lowercase C)
you see a fresh screen with a new shell
type a different command in the new shell
type control-a control-a (yes, twice)
you see the previous session
type control-a 1 (digit one)
you see the second session
type control-a 0 (digit zero)
you see the first session again, sessions are numbered from zero
add another session (control-a c, as before)

Play with these switching commands between the three sessions using control-a and a digit or the control-a control-a sequences

type control-a w (lowercase W)
during several seconds, a black bar at the bottom shows all sessions (windows) with their number.

Split screen

still in screen, type control-a S (capital S)
the screen is now split into two panes with one the old session and the other blank
switching session still works within the one pane, try it
type control-a TAB
the cursor moved to the blank pane
switch one of your sessions
experiment with switching between panes (control-a TAB) and between sessions in that pane
combine with detaching from screen and re-attaching

Use screen to detach from work