All Linux systems have a number of options built into their power management at kernel level. Unwary users – eg. like me – tend to neglect their meaning and use. But they are useful options though. Time to explain.
Generally there are three options with varying levels of power saving: shutdown, hibernate and suspend. To revert these there are three options again: restart, thaw and resume (of which ‘thaw’ seems to be used by developers only).
Let us have a closer look at the options.
Power saving options
Shutdown does what it says: it completely shuts down the computer. So, maximum power saving. (Although the motherboard may still be on standby power; to switch it off fully, press and hold the on/off button for 4 seconds, until you hear the harddisk stop spinning.) To get the machine going again you will need a restart – or ‘reboot’, or ‘system start’ – which will go through the full procedure from BIOS, boot loader (eg. GRUB) and X server to desktop manager (eg. KDE, GNOME, Cinnamon, Xfce etc.). Therefore, when on a running computer you click ‘Restart’, the machine will be completely shut down and then restarted from the BIOS onwards.
Hibernate also closes down the computer to standby power state, but the active system state is stored to – the swap partition of – the harddisk. For that reason hibernate is also called ‘suspend to disk’. On reactivating (the ‘thaw’ thingy) and login you will be back exactly where you left without having to go through the whole system start procedure. Reactivating may happen by opening the laptop lid, pressing the on/off button, or touching the keyboard and moving the mouse.
Suspend closes down all subsystems, but keeps memory alive, so the active system state can be kept in RAM. Consequently the precise name is ‘suspend to RAM’ – aka ‘sleep’ by those coming from the remote shores of Mac or Windows. On resume the system will be quickly reactivated from what was saved to RAM, and in a wink you are back right where you left. If you wish so you can require a login password on resume. Minor disadvantage: if for some reason the power is cut off, or the laptop’s battery runs empty, you will lose your saved machine state, including open documents or active files, and need a full restart.
From the commandline
Now you may wish to use commandline commands to suspend or hibernate, eg. to create a keyboard shortcut for these actions. The exact commands depend on your desktop manager and even its version. Below I give the commands to lock and suspend Ubuntu based systems running KDE (eg. Kubuntu or LinuxMint).
Possibly you will need to install a few extras. Just test the command in a terminal and you will be told.
Up to Ubuntu 14.04 the screen can be locked by using qdbus, and suspended through dbus. The full command is rather hard to find out for yourself, but it reads:
qdbus org.freedesktop.ScreenSaver /ScreenSaver Lock && dbus-send --system --print-reply --dest="org.freedesktop.UPower" /org/freedesktop/UPower org.freedesktop.UPower.Suspend
To hibernate replace ‘Suspend’ at the end of the command by ‘Hibernate’ (minding the capitals).
If you only want to lock the screen, without suspending, you might use i3lock, but this does not present a neat login dialog on resume, so it works a bit clumsy.
On Ubuntu 16.04 the suspend command is much simpler:
To also lock the screen, go to System Settings > Desktop Behaviour > Screen Locking and on the ‘Activate’ tab check the box at ‘Lock screen on resume’.
And to hibernate, not surprisingly, type:
Please forget about the pm-suspend or dm-tool commands. They require installing older or conflicting software.
More information (read carefully and critically!):