screen and tmux, and the ps command

LPIC1-101LINUX

screen and tmux

Think of them as a "Virtual Terminal Server" that lives on your machine (usually a remote server) and stays active even if you lose your internet connection.

1. Persistence (The "Don't Panic" Feature)

The biggest selling point is Session Persistence.

2. Window & Pane Management (The "Multiplexer" Part)

This is where the name comes from. Instead of opening 5 different SSH windows on your desktop, you open one and split it.

Panes: Split your screen horizontally or vertically. You can have your code on the left and your logs running on the right.
Windows: Like tabs in a browser. You can have one "window" for your text editor and another for your database console, switching between them with a keystroke (e.g., Ctrl+b then n).

3. Collaboration (The "Over My Shoulder" Feature)

Two people can attach to the same Tmux session simultaneously.

If I type a command, you see it appear on your screen in real-time.
This is incredibly useful for remote pair programming or troubleshooting a server with a colleague without having to share your actual desktop screen.

4. Practice the following commands

Prefix keys

Tool Prefix

screen Ctrl-a

tmux Ctrl-b

Sessions

Action screen tmux

Start new named session screen -S upgrade tmux new -s upgrade

List sessions screen -ls tmux ls

Detach from session Ctrl-a d Ctrl-b d

Reattach to named session screen -r upgrade tmux attach -t upgrade

Reattach to only session screen -r tmux attach

Reattach to unnamed session screen -r PID tmux attach -t 0 (integer name)

Multiuser / sharing

Action screen tmux

Enable sharing :multiuser on not built-in; relies on same user

(inside session) or socket permissions

Grant another user access :acladd user_name n/a

Join a session (same user) screen -x tmux attach -t upgrade

Panes / regions

Action screen tmux

Split horizontally (stacked) Ctrl-a S Ctrl-b "

Split vertically (side by side) Ctrl-a | Ctrl-b %

Move to next region/pane Ctrl-a Tab Ctrl-b + arrow

Start shell in new region Ctrl-a c (required) automatic

Sometimes, if a server crashes or a process hangs, screen -ls will show a session marked as (Dead) or (Remote detached). These are just leftover socket files. You can clean those up with: screen -wipe

To fully close (terminate) a session, rather than just detach from it: exit works the same way in both tools — it closes the current shell, and if that's the last window in the session, the whole session terminates. This is the most common way to close a session.

ps command

GNU long options have two dashes: ps --tty tty2
Unix-style options have a single dash: ps -ef
Berkley software distribution (BSD) options have no dashes: ps aux

You can customize the columns of the ps output, for example,

$ ps -e -o pid,ppid,stat,cmd

The meaning of ps columns:

column STAT in ps aux: R = running, S = sleeping, D = uninterruptible sleep, T= stopped, Z = zombie
column TIME in both ps -ef and ps aux is exam-relevant (103.5, process monitoring column).
It's the cumulative CPU time the process has actually consumed since it started — not wall-clock time since launch, but actual processor time used (format HH:MM:SS or MM:SS).
Key distinction for the exam:
- TIME = actual CPU seconds consumed (usually stays near 00:00:00 for idle/sleeping processes, since they aren't using the CPU)
- START/STIME = the wall-clock time the process was launched

A Note on "Unkillable" Processes

Keep an eye out for processes in the "D" state (Uninterruptible Sleep) when you run top or ps aux.

The "D" State: These processes are usually waiting on I/O (like a failing hard drive or a hung network mount).
The Catch: Even kill -9 cannot kill a process in the "D" state because it's waiting for the kernel to finish a hardware task. In those rare cases, your only choice really is a reboot.

Contact

Email

hello@unixtips.eu

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