I have a Samsung Galaxy S2 and a PC running Debian 8. How can I turn on USB debugging using command-line on my PC without touching the phone?
If the question is simply "using the command-line"
Let it be
simple (root possibly required):
settings put global development_settings_enabled 1 settings put global adb_enabled 1
And then you need to
echo "mtp,adb" > /data/property/persist.sys.usb.config
If you want to do this without touching your phone
Then I'm feeling interesting because in most cases you won't gain access to command line via PC before enabling ADB, so it's impossible to enable this prior to touching your phone.
It is in any case possible but made extremely difficult. Please also bear in mind that if your bootloader hasn't been unlocked yet, unlocking it will wipe all data on your phone, making this alternate way mostly useless except for rescuing a brick.
1. Actually you only need either 'system' or 'shell' privilege, but 'system' is impossible to access without rooting and 'shell' is granted only if you're already logging in via adb, where it makes no sense to enable adb while it's already active.