How to enable activity monitoring in tmux

Long-running commands in a background tmux window can finish without drawing attention when the current window stays quiet. Activity monitoring marks a window after new output arrives while another window is selected, so the status line shows which background job needs attention before switching back.

Tmux treats monitor-activity as a window option. Setting it globally makes windows that inherit the global option raise an activity alert, and visual-activity controls whether tmux also shows a message for that alert instead of only marking the window list.

Activity alerts are cleared when the flagged window becomes current. Enable the setting in sessions where background output means a job finished, a log changed, or a build printed an error. Noisy commands can keep raising alerts, so verify the change by forcing output in a temporary background window before relying on it for real work.

Steps to enable tmux activity monitoring:

Open the tmux config file used by the running server.
```
$ vi ~/.tmux.conf
```
tmux also reads $XDG_CONFIG_HOME/tmux/tmux.conf or ~/.config/tmux/tmux.conf when those paths are used instead of ~/.tmux.conf.
Add the activity monitoring settings.
```
set-window-option -g monitor-activity on
set-option -g visual-activity on
```
monitor-activity marks windows after background output. visual-activity makes tmux show an activity message as well as the status-line marker.
Save the file and return to the shell prompt.
Reload the config into the active tmux server.
```
$ tmux source-file ~/.tmux.conf
```
No output means tmux accepted the file and applied the settings to the running server. Use the same config path here when you edited an XDG tmux.conf file instead of ~/.tmux.conf.

Related: How to reload the tmux config
Confirm that global window activity monitoring is enabled.
```
$ tmux show-options -gw monitor-activity
monitor-activity on
```
The -w flag checks the window-option layer, and -g checks the global value inherited by windows without a local override.

Confirm that activity messages are enabled.

$ tmux show-options -g visual-activity
visual-activity on

Create a temporary background window that produces output.
```
$ tmux new-window -d -n activity-test 'sleep 5; echo activity check; sleep 300'
```
The -d flag keeps the current window selected, so the temporary window is in the background when its output appears.

Related: How to create a window in tmux
Check the activity flag on the temporary window.
```
$ tmux display-message -p -t :activity-test '#{window_activity_flag}'
1
```
1 means tmux recorded activity for that window. The window list also shows a # activity marker and uses the activity style until the window becomes current.
Remove the temporary activity test window.
```
$ tmux kill-window -t :activity-test
```