Persisting Sessions with tmux and nohup¶

Login nodes enforce a per-user resource limit of 1 CPU core and 8 GB of memory (combined across all your processes). These tools are intended for lightweight tasks such as monitoring jobs, editing files, or launching submissions — not for running compute-intensive work. For any resource-heavy work, submit a batch job or request an interactive compute node.

When you need a lightweight process to survive a disconnection or prevent timeout due to keyboard inactivity, use tmux or nohup.

tmux¶

tmux is a terminal multiplexer that keeps a session alive on the server even after you disconnect. You can return to the session from any new SSH connection.

Starting a session¶

tmux new -s mysession

This creates a new named session called mysession. Naming sessions makes them easier to find later.

Detaching from a session¶

Press Ctrl+b, then d to detach. Your session continues running in the background.

Listing active sessions¶

tmux ls

Reattaching to a session¶

tmux attach -t mysession

If there is only one session, you can simply run tmux attach.

Killing a session¶

From inside the session:

exit

Or from outside:

tmux kill-session -t mysession

Basic key bindings¶

All tmux key bindings begin with the prefix Ctrl+b.

Shortcut	Action
`Ctrl+b` then `d`	Detach from session
`Ctrl+b` then `c`	Create a new window
`Ctrl+b` then `n`	Next window
`Ctrl+b` then `p`	Previous window
`Ctrl+b` then `%`	Split pane vertically
`Ctrl+b` then `"`	Split pane horizontally
`Ctrl+b` then `[`	Enter scroll mode (q to exit)

Note

tmux sessions live on the specific login node you connected to. If you reconnect and land on a different login node, use ssh login0.crc.pitt.edu or ssh login1.crc.pitt.edu to reach the node where your session is running, then reattach.

nohup¶

nohup (no hangup) runs a command so that it is not terminated when your SSH session ends. It is simpler than tmux and best suited for single commands that you want to run unattended.

Basic usage¶

nohup my_command &

The & sends the process to the background. Output that would normally go to the terminal is written to nohup.out in your current directory by default.

To redirect output to a specific file:

nohup my_command > my_output.log 2>&1 &

The shell prints the background process ID (PID) when you run this. Take note of it.

Checking whether the process is still running¶

ps -p <PID>

Or search by name:

pgrep -a my_command

Stopping the process¶

kill <PID>

Choosing between tmux and nohup¶

Scenario	Recommended tool
You need to return interactively to a running session	tmux
You want to watch output as it scrolls	tmux
You are running a single fire-and-forget command	nohup
You want the simplest possible setup	nohup

Important reminders¶

Login node limits of 1 CPU core and 8 GB of memory apply to all your processes combined, including those inside tmux or running via nohup.
Processes that exceed these limits or consume excessive I/O may be terminated without notice.
For any compute- or memory-intensive work, use interactive jobs or batch jobs on dedicated compute nodes.