SYSTEMD-BACKLIGHT@.SERVICE(8)

💖 SYSTEMD-BACKLIGHT@.SERVICE 💖

okay bestie, pull up a chair, because this is the page for systemd-backlight@.service and systemd-backlight — aka the little cuties that live in the systemd manual (section 8, if you're keeping track 🎀) and whose whole job is loading and saving your display backlight brightness at boot and shutdown. yes. your screen glow has a dedicated service. we love a main character.

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✨ NAME ✨

systemd-backlight@.service, systemd-backlight — load and save the display backlight brightness at boot and shutdown 🦋

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🌸 SYNOPSIS 🌸

systemd-backlight@.service

/usr/lib/systemd/systemd-backlight save [backlight|leds]:DEVICE

/usr/lib/systemd/systemd-backlight load [backlight|leds]:DEVICE

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💅 DESCRIPTION 💅

so, systemd-backlight@.service is a service that restores the brightness of a display backlight or LED (think keyboard backlight, bestie) device at early boot, and then saves it right back at shutdown. it's giving "remember my vibe, put it back exactly how i left it" energy. 🌈

the brightness value is stored in /var/lib/systemd/backlight/. that's its little home. periodt.

now for the tea ☕ — when it's restoring the brightness of a display backlight device, systemd-backlight reads the ID_BACKLIGHT_CLAMP udev property. that property takes either a boolean value or a percentage (and the percentage needs to be suffixed with "%", like "30%", no exceptions bestie). here's the breakdown:

• when a percentage is specified, the saved brightness is clamped to a value of at least 1 or the specified percentage of the maximum brightness — whichever is greater. lowkey protective, honestly.
• when it's unset or set to true, the brightness is clamped the same way, with percentage 5%.
• when false, the saved brightness will not be clamped, and gets loaded as is. no cap, literally.

and when it's restoring the brightness of a LED device, systemd-backlight reads the ID_LEDS_CLAMP udev property, which also takes a boolean value or a percentage:

• when a percentage is specified, the saved brightness is clamped to the specified percentage of the maximum brightness.
• when set to true, the brightness is clamped in the same way, with percentage 5%.
• when unset or set to false, the saved brightness will not be clamped, and gets loaded as is.

see how the two are just different enough to matter? iconic behavior tbh 🧚

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👑 KERNEL COMMAND LINE 👑

systemd-backlight understands exactly one (1) kernel command line parameter, and it's this one:

• systemd.restore_state= — takes a boolean argument. defaults to "1". if "0", it does not restore the backlight settings on boot. however — and this is important bestie — settings will still be stored on shutdown. so it remembers, it just doesn't put it back. 😭

added in version 209. 🫶

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💜 SEE ALSO 💜

• systemd(1)

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no cap, that's the whole page. every flag, every property, every percentage — all accounted for. 💖

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mwah 💋 that was systemd 262.1, and this page is dated SYSTEMD-BA...T@.SERVICE(8) — go forth and glow, bestie. ✨🎀🦋