Usually BIOSes have an option to schedule a time to which to boot at. Is there a Unix/Linux interface to specify the scheduled boot?

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NVRAM WakeUp claims to do it; I've never tried. It may not work on all BIOSes, and if it fails a likely consequence is to overwrite a different critical setting that could make your computer unbootable, so use with caution.

If you only suspend the computer, APM tools can set a wake-up time with apmsleep. I've successfully used my laptop as an alarm clock with this tool. But it can't wake up a powered off computer.

If your computer supports ACPI (all modern ones do), and if ACPI actually works on your OS (that, on the other hand, is not a given), there is a standard interface for specifying a wake-up time. Under Linux, date -u +%s -d 'tomorrow 6:00' >/sys/class/rtc/rtc0/wakealarm should do the trick. There is a good guide on ACPI wakeup on the MythTV wiki.

If you have another powered-on device on the local network, you can send your computer a wake-on-LAN packet. Most modern BIOSes support this (you may need to be enable it in the BIOS settings). The wakeonlan utility can send such packets.

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