Sxmo: a text-centric mobile user interface

Sxmo, short for "Simple X Mobile", is described on its web site as "a minimalist environment for Linux mobile devices"; it offers a menu-driven interface that is controlled with the phone's hardware buttons. Sxmo enables the user to send SMS messages from a text editor and is entirely customizable with shell scripts. This peculiar mobile user interface significantly differs from the prevailing approach—but it works.

While mobile user interfaces such as Phosh, KDE Plasma Mobile, and Lomiri have some differences between them, they are all rooted in the same philosophy. They center on touch-based interactions and display apps through icons, an approach influenced by the conventional point-and-click paradigm of desktop user interfaces. However, on the desktop, a text-centric approach centering on keyboard input and terminal programs following the Unix philosophy has remained popular among advanced users. Sxmo aims to offer such an environment for mobile devices.

There are two flavors of Sxmo: Xorg or Wayland. The Xorg version is based on a couple of forks of tools from the suckless project, which has "a focus on simplicity, clarity, and frugality". This includes the dynamic window manager dwm, the menu system dmenu, and the simple terminal emulator st. The Wayland version uses some tools inspired by their Xorg counterparts, including the tiling Wayland compositor Sway, the menu system bemenu, and the terminal emulator foot. In practice, both versions work similarly, with some minor low-level differences in configuration.

Sxmo is best supported on the postmarketOS Linux distribution for mobile devices (previously covered here). Pre-built images of postmarketOS with Sxmo for various devices can be found on the download page of the distribution. Alternatively, a custom image can be built by running pmbootstrap init and choosing sxmo-de-sway (for the Wayland version) or sxmo-de-dwm (for the Xorg version) as the interface. I tested Sxmo's Wayland version by installing a custom-built postmarketOS image.

Menu-driven interface

Since interacting with Sxmo deviates significantly from other mobile interfaces, its user guide is required reading, especially to learn about the actions behind the phone's hardware buttons. Most phones have three buttons on the side: volume-up, volume-down, and power. For each of these buttons, Sxmo triggers an action based on whether you tap the button once, twice, or thrice; a long-press can be used instead of tapping three times. This way, the user can start nine different actions solely by using the hardware buttons, just with the thumb. Touch-based input also works.

Sxmo's home screen is merely a background image featuring the current date and time, with a status bar on top that includes a workspace number (only one at first), and, next to it, status icons for the mobile network, WiFi connection, battery, volume, lock state, and time. The global system menu opens when the user taps on the volume-up button or swipes down from the top of the screen. While within a menu, the hardware buttons exhibit a different behavior: the volume-up button navigates to the previous item, while the volume-down button advances to the next item. The power button selects the current item. Tapping an item on the touch screen also selects it.

The global system menu gives access to various scripts and applications. Several menu items open an application in a terminal window upon selection. For instance, scanning for WiFi networks starts the nmcli d wifi list command. Configuring the phone (under the Config submenu) enables setting the brightness, enabling or disabling touch, gestures, and Bluetooth, as well as upgrading packages, among other things.

SMS messages and calls

Sending an SMS message requires typing a phone number or selecting a person from the contact list first. Then the Vim-like text editor vis is opened to compose the message. After the user exits the editor and confirms, the message is sent. An incoming SMS message briefly appears on the home screen; it can also be read later from the global system menu, where it's included in a Notifications menu item, and in the Texts submenu. Upon receipt of a new text message, the phone's LED emits a green light and the vibration motor momentarily triggers.

Calling uses a similar text-centric process. To place a new call, open the Dialer submenu in the global system menu and enter a phone number or choose an entry from the contact list. After selecting the number, Sxmo starts calling, and once the call connects, a menu appears with options to hang up the call, manage audio routing, and more. An incoming call triggers the phone's green LED and the vibration motor, and a menu appears that enables the user to accept or dismiss the call.

Mobile terminal

When no menu is active, a single tap on the volume-down button shows or hides the virtual keyboard at the bottom of the screen. This action can also be performed by swiping up and down from the bottom of the screen. One tap on the volume-up button launches an application-specific context menu for the currently focused window of a supported application. Three taps or holding the power button opens the terminal emulator. And three taps (or hold) on the volume-down button terminates the currently focused window.

Although working with terminal applications on a primarily touch-based device might not seem like a good match, Sxmo defines some one-finger swipe gestures that ease the experience. For example, swiping from left to right along the bottom edge sends a Return key to the application. Similarly, swiping from right to left along the bottom sends Backspace. Swiping top to bottom along the right edge sends an arrow-down key, while swiping bottom to top along the right edge sends an arrow-up key. Swiping right to left onto the left edge sends a left-arrow key, while swiping left to right onto the right edge sends a right-arrow key. These gestures allow the user to scroll through the shell history and the current command without the need to open the virtual keyboard and use up precious screen space. The virtual keyboard does not support swipe typing, however.

Plain-text files and shell scripts

Sxmo's configuration is entirely based on plain-text files. For example, contact details are stored in a tab-separated file in ~/.config/sxmo/contacts.tsv. This file has two columns: first the phone number, followed by the contact name. Contacts can be added either by manually editing the file or from the Contacts entry in the global system menu. Similarly, the ~/.config/sxmo/block.tsv file lists phone numbers and corresponding contact names that the user wishes to block.

Sxmo's behavior is defined in the ~/.config/sxmo/profile file, as well as in dozens of hooks, which are shell scripts with a specific name. For example, when the phone is receiving an incoming call, the shell script in /usr/share/sxmo/default_hooks/sxmo_hook_ring.sh is executed with the first argument ($1) set to the contact name or incoming number. This script can be overridden by the ~/.config/sxmo/hooks/sxmo_hook_ring.sh file if the user wants to change the default behavior. User scripts can also be added to the global system menu for custom functionality. The postmarketOS wiki hosts a tips and tricks page for Sxmo, featuring some helpful advice and configuration snippets. For a deeper exploration under the hood, Sxmo's system guide offers an excellent source of information.

Contributing to Sxmo

Much of Sxmo's core functionality is built upon shell scripts. While this allows complete customization, it requires some Linux and shell-scripting knowledge. The developers actively invite contributions, especially for device profiles, which are shell scripts that Sxmo loads early on to set some attributes that ensure the devices work well with Sxmo.

Sxmo was originally designed for the PinePhone. Meanwhile, support for a few other devices has been added, including the Librem 5 and the Fairphone 4, but also the 15-year-old Nokia N900 with a physical keyboard and the Kobo Clara HD e-reader. Users trying to run Sxmo on an unsupported device can find some instructions to create a device profile in the sxmo-utils repository linked above. The result is a shell script that exports some environment variables for the touch-screen device, display output, screen scale, buttons, and more.

In the last two years, Sxmo has had eight releases, slowing down a bit during the last year. The most recent was Sxmo 1.16.1 from June 3, which was a minor release. It had some tiny improvements in the control of the suspend functionality along with initial device profiles for PINE64's PineTab 2 tablet and Xiaomi's Redmi Note 4 phone. There's a ticket list tracking bugs and feature improvements, but it doesn't seem very active. The sxmo-devel mailing list does get patches regularly for various repositories of the project.

With Sxmo's focus on delivering a minimalist environment, it is no surprise that the project doesn't intend to switch to systemd, as postmarketOS has done. In the FAQ about the switch, the postmarketOS developers make it clear that Sxmo will be sticking with OpenRC for its init system.

Conclusion

Sxmo is even less suited for most users than postmarketOS. However, for tech-savvy, seasoned Linux users who want to have a lightweight and completely scriptable interface on their phone, Sxmo provides a different path from the conventional mobile user interfaces. It takes some time to adjust to the hardware buttons to control the phone's menu-based interface, but after some time, it works remarkably well.