The tmux status bar (or status line) is the rectangular box that appears at the bottom of the screen (by default)  when tmux starts. It displays helpful information related to the current tmux session, as well as general system information. By default, the status line is enabled and one line in height (it may be disabled or made multiple lines with the status session option) and contains, from left-to-right: the name of the current session in square brackets; the window list; the title of the active pane in double quotes; and the time and date.

The default is made of three parts: configurable left and right sections (which may contain dynamic content such as the time or output from a shell command, see the status-left, status-left-length, status-right, and status-right-length options in the online documentation), and a central window list.

Toggling Status Line

Turn it off:

$ tmux set-option status off

And, turn it on:

$ tmux set-option status on

You can also bind a key (t in this example) to toggle the status line on and off, as follows:

$ tmux bind-key t set-option status

By default, the window list shows the index, name and (if any) flag of the windows present in the current session in ascending numerical order. It may be customized with the window-status-format and window-status-current-format options. The flag is one of the following symbols appended to the window name:

Finding your current status line settings

tmux show-options -g | grep status

An example of the default configuration you see in tmux with no status styles :

status on
status-interval 15
status-justify left
status-keys vi
status-left "[#S] "
status-left-length 10
status-left-style default
status-position bottom
status-right " "#{=21:pane_title}" %H:%M %d-%b-%y"
status-right-length 40
status-right-style default
status-style fg=black,bg=green

The color and attributes of the status line may be configured, the entire status line using the status-style session option and individual windows using the window-status-style window option. The status line is automatically refreshed at interval if it has changed, the interval may be controlled with the status-interval session option.

The colors available to tmux are:

• black, red, green, yellow, blue, magenta, cyan, white.
• bright colors, such as brightred, brightgreen, brightyellow, brightblue, brightmagenta, brightcyan.
• colour0 through colour255 from the 256-color set.
• default
• hexadecimal RGB code like #000000, #FFFFFF, similar to HTML colors.

You can use [bg=color] and [fg=color] to adjust the text color and background within for status line text. This works on status-left and status-right. For example:

$ tmux set-option status-style fg=white,bg=black

Shell Command Output

You can call the output of applications or shell command in tmux. For example, let’s assume you want to display the system uptime into the tmux status line. The shell command to accomplish this (in number of days) is:

$ uptime | cut -f 4-5 -d ' ' | cut -f 1 -d ','

To add the output of this command to right section of the tmux status line, we would run this command:

$ tmux set-option -ag status-right "#[fg=red,dim,bg=default]#(uptime | cut -f 4-5 -d ' ' | cut -f 1 -d ',') "

Or, add this line to ~/.tmux.conf:

set-option -ag status-right "#[fg=red,dim,bg=default]#(uptime | cut -f 4-5 -d ' ' | cut -f 1 -d ',') "

tmux allows you to call applications and output shell commands to your tmux status bar. In the next sections we will cover a couple of examples that demonstrate how you can style your tmux bar to fit your preferences and your workflow. The two examples we will use are: tmux-mem-cpu-load and powerline.

tmux-mem-cpu-load

tmux-mem-cpu-load is a simple, lightweight program provided for system monitoring in the status line of tmux. It monitors CPU, RAM, and system Load. An example output is:

Example output:

2885/7987MB [|||||     ]  51.2% 2.11 2.35 2.44

 ^    ^          ^         ^     ^    ^    ^
 |    |          |         |     |    |    |
 1    2          3         4     5    6    7

1. Currently used memory.
2. Available memory.
3. CPU usage bar graph.
4. CPU usage percentage.
5. Load average for the past minute.
6. Load average for the past 5 minutes.
7. Load average for the past 15 minutes.

Once installed, you can add the following lines to your ~/.tmux.config:

set -g status-interval 2
set -g status-left "#S #[fg=green,bg=black]#(tmux-mem-cpu-load --colors --interval 2)#[default]"
set -g status-left-length 60

Putting the lines below in ~/.tmux.conf , will produce the tmux status line in the screenshot below.

set-option -g status on
set-option -g status-interval 1
set-option -g status-justify centre
set-option -g status-keys vi
set-option -g status-position bottom
set-option -g status-style fg=colour136,bg=colour235
set-option -g status-left-length 20
set-option -g status-left-style default
set-option -g status-left "#[fg=green]#H #[fg=black]• #[fg=green,bright]#(uname -r)#[default]"
set-option -g status-right-length 140
set-option -g status-right-style default
set-option -g status-right "#[fg=green,bg=default,bright]#(tmux-mem-cpu-load) "
set-option -ag status-right "#[fg=red,dim,bg=default]#(uptime | cut -f 4-5 -d ' ' | cut -f 1 -d ',') "
set-option -ag status-right " #[fg=white,bg=default]%a%l:%M:%S %p#[default] #[fg=blue]%Y-%m-%d"
set-window-option -g window-status-style fg=colour244
set-window-option -g window-status-style bg=default
set-window-option -g window-status-current-style fg=colour166
set-window-option -g window-status-current-style bg=default

Powerline is a statusline plugin for vim, and provides statuslines and prompts for several other applications, including zsh, bash, fish, tmux, IPython, Awesome, i3 and Qtile. Powerline has extensive documentation on integrating powerline into tmux. The easiest way to install powerline in Archlinux is to install the following packages using pacman.

To enable the powerline plugin for tmux, simply add the following line to your ~/.tmux.conf and launch tmux. 

source "/usr/lib/python3.8/site-packages/powerline/bindings/tmux/powerline.conf"

Important: the plugin path above may be different on your system, as python version changes. At the time of authoring this article, python 3.8 is the latest. You can always check the powerline package info to find the location of the tmux plugin.

If everything is configured correctly, the powerline status line will look like the screenshot below.