How do I know if tmux is running?

The most straightforward way to tell if you're in a tmux session is by observing visual cues in your terminal or by using simple command-line checks.

Visual Indicators

When you are inside a tmux session, you will typically see a distinctive status line at the bottom of your terminal window.

Often, a green bar along the bottom of your Linux command-line window indicates you are inside a tmux session. This bar usually displays information like the session name, current window name, and current time, providing an immediate visual confirmation.

Command-Line Methods

Beyond visual cues, several command-line approaches can definitively tell you if tmux is running and if you are currently attached to a session.

Check for Active `tmux` Sessions

You can query tmux directly to see if any sessions are running on your system.

tmux ls (or tmux list-sessions): This command lists all active tmux sessions. If tmux is running and has active sessions, it will display them with details such as session ID, number of windows, and creation time.
```
tmux ls
```
- Output if tmux sessions exist:
```
0: 1 windows (created Sat Jan 20 10:00:00 2024) [200x60]
my_session: 2 windows (created Sat Jan 20 10:30:00 2024) [180x50]
```
- Output if no tmux server is running or no sessions:
```
no server running on /tmp/tmux-1000/default
```
tmux attach: While primarily used to attach to a session, if you run tmux attach and it connects you to an existing session without prompting to create a new one, it confirms tmux was already running and had at least one session.

Examine Environment Variables

When you are inside a tmux session, a specific environment variable is automatically set.

echo $TMUX: This command will display the value of the TMUX environment variable.
```
echo $TMUX
```
- Output if in a tmux session:
```
/tmp/tmux-1000/default,12345,0
```
  (The exact path and numbers will vary, but a non-empty string indicates you are in a session.)
- Output if not in a tmux session:
  (An empty line or no output indicates you are not in a tmux session.)

Inspect Running Processes

You can also check if the tmux server process is active on your system, which indicates tmux is running in the background, ready to host sessions.

ps aux | grep tmux: This command lists all running processes and filters for those containing "tmux."
```
ps aux | grep tmux
```
- Output if tmux server is running:
```
youruser    12345  0.0  0.0  123456  7890 ?        Ss   Jan20   0:00.00 tmux -L default
youruser    67890  0.0  0.0   54321  1234 pts/0    S+   10:45   0:00.00 grep tmux
```
  (The presence of a tmux -L default or similar line indicates the tmux server is active.)
- Output if tmux server is not running: Only the grep tmux process will be shown, or nothing if grep isn't running too fast.

Summary of Methods

Method	Description	Outcome if `tmux` is running
Visual Cue	Look for a distinctive status bar.	Often a green bar along the bottom of the terminal displaying session, window, and time info.
`echo $TMUX`	Check the `TMUX` environment variable.	Returns a non-empty string (e.g., `/tmp/tmux-PID/session-name`).
`tmux ls`	List active `tmux` sessions.	Displays a list of sessions (e.g., `0: 1 windows (created...)`).
`ps aux \\| grep tmux`	Check for running `tmux` processes.	Shows `tmux` server processes in the output, even if no active session is attached.

Why Use Tmux?

tmux is a powerful terminal multiplexer that significantly enhances productivity for command-line users. A key advantage of tmux is its ability to run commands on a VM, logout, and then reconnect to your running programs later. This means your long-running processes, such as data analyses, server applications, or compilation jobs, continue to execute even if your network connection drops or you close your local terminal window, allowing you to seamlessly reattach to your work at any time.