Resilience to loss of connection

A brief introduction to running processes immune to loss of connection.

What happens when the connection is lost? #

The connection to a Bash session on the CCB cluster may be lost for a number of reasons:

• Loss of internet connection anywhere between your computer and the CCB cluster.
• Power loss causing an unexpected outage of CCB systems.
• Loss of connection to the University VPN (if applicable).

When the connection to a Bash session is lost, the HUP (hangup) signal is sent to the Bash session. The HUP signal prompts the Bash session to terminate commands that are currently running, potentially resulting in lost work for users.

How to run commands immune to loss of connection? #

nohup #

The nohup command can be used to run a command that is immune to the HUP signal.

The process launched by the nohup command will ignore any HUP signal, and continue running until the command completes.

By default, the standard output of the process launched by the nohup command is redirected to a file nohup.out in the working directory, unless the command explicitly redirects the standard output to another file.

By default, the standard error of the process launched by the nohup command is not saved. However, the command can explicitly redirect the standard error to a file using the operator 2>.

The & symbol is commonly added at the end of nohup command, to launch those commands in the background.

For instance:

nohup wc -l file.txt > stdout.txt 2> stderr.txt &

In particular:

• The nohup command is used to make the command immune to the HUP signal; the command will continue to run even in the event of network connection loss problems.
• The wc command is used to count the number of lines in the file file.txt.
• The standard output of the command is redirected to the file stdout.txt.
• The standard error of the command is redirected to the file stderr.txt.

tmux #

The tmux command launches an open-source terminal multiplexer for Unix-like operating systems.

It allows multiple terminal sessions to be accessed simultaneously in a single window. It is useful for running more than one command-line program at the same time.

To start your first tmux session on the CCB cluster, type tmux in the Terminal application, while logged into the CCB cluster:

tmux

The green banner at the bottom indicates that the user is now in a tmux session. At this point, users can then work in that terminal session like any other Bash session.

However, a benefit of the tmux session is that it will continue to exist and keep running commands alive even if the connection between the client and the CCB cluster is lost, or users close the Terminal application.

For instance:

• Close the Terminal application.
• If prompted, click ‘Terminate’ to force-quit the application.
• Open a new Terminal application.
• Log into the CCB cluster.
• Type tmux attach (or, in abbreviated form, tmux a).

tmux attach

The very same tmux session will be attached to the new Bash session and users can pick up their work where they left off.

Sometimes, users may wish to effectively terminate one tmux session and start another (e.g., to reset the state of their Bash session). To do so, terminate the Bash session running in the current tmux session using any of the logout methods described in the section Log out.

For instance:

logout

Do so until you fully exit the tmux session (i.e., the green banner at the bottom of the Terminal application has disappeared). You should see a the message [exited], indicating that you have exited the tmux session.

Then, start a new tmux session using the tmux command:

tmux

A ‘Tmux Cheat Sheet & Quick Reference’ is available at https://tmuxcheatsheet.com/.

For more details, search search “tmux tutorial” in a search engine or YouTube.

Emacs #

Emacs is a family of text editors that are characterized by their extensibility.

The command emacs launches the application.

emacs

The Emacs editor can be configured to persist in the event of a connection loss, independently of tmux sessions.

To do so, the Emacs daemon – a program that runs continuously as a background process – must run in the Bash session.

To automatically start the Emacs daemon in every new Bash session, add the following lines to the ~/.bashrc file.

export ALTERNATE_EDITOR=""
alias emacs="emacsclient --tty"

Then, launch the Emacs application.

emacs

At this point, users can then work in that Emacs session without fear of losing work in the event a connection loss.

For instance:

• Type anything in the Emacs application.
• Close the Terminal application.
• If prompted, click ‘Terminate’ to force-quit the application.
• Open a new Terminal application.
• Log into the CCB cluster.
• Type emacs.

The very same Emacs session will be restored in the new Bash session and users can pick up their work where they left off.

We do recommend regularly saving your work in the Emacs text editor (i.e., saving the contents of the buffer to a file).

A ‘GNU Emacs Reference Card’ is available at https://www.gnu.org/software/emacs/refcards/pdf/refcard.pdf.

For more details, search search “emacs tutorial” in a search engine or YouTube.