I am trying to write a script for my Ubuntu system to hibernate, whenever the amount of battery left is (say) 5%. This script will run on startup.

I can get the battery left using

upower -d |grep perc

Then I wish to use

sudo pm-hibernate

whenever battery reaches <= 5%. But this requires superuser permission. AND I DON'T WANT TO TYPE PASSWORD AGAIN AND AGAIN AFTER EVERY LOGIN.

One way is to use add the following line in /etc/sudoers

yourlogin ALL=(ALL) NOPASSWD: command_here 

But most people recommend against that.

Another way is to add a custom startup script in /etc/init.d. But I am not sure whether it is a safe choice.

Also, is there any other alternative which would be best for my purpose?


Simpler than adding a script to init.d is to write an Upstart configuration. I'd favour this above all. Create a .conf file in /etc/init (say /etc/init/sleep-on-suspend.conf, containing:

description "Automatic suspend"

start on runlevel [2345]
stop on runlevel [016]

exec /path/to/script

This will be automatically started on reboot.

You could even integrate the script into this file. Instead of the exec line, use:

    while sleep 1; do
    upower -d | awk -F'[ %]*' '/perc/ && ($3 < 5) { exit 1 }' || pm-hibernate
end script
  • I haven't tried this script yet, mostly because I do not completely understand the code. I am reading about the syntax of .conf files, and will try once I feel comfortable with .conf files. – viv1 Feb 5 '15 at 11:17
  • @spiderbat Hopefully you're reading upstart.ubuntu.com/cookbook – muru Feb 5 '15 at 11:18
  • Yeah.Looks helpful. – viv1 Feb 5 '15 at 11:24

Run sudo crontab -e -u root and append the line @reboot /path/to/script to the bottom of the file, then the root user will automagically run your script as root on login. And a friendly reminder to make sure nobody apart from you and root can access the script, otherwise people could run any command as root.

  • This one works for me. But instead of keeping it running always, a better option would be to run the script after every 5 minutes or so(as mentioned in @tripleee's answer) – viv1 Feb 5 '15 at 11:38
  • I agree, yes. However, it depends on how the OP writes their script. If it is light and well written, it could be fine. Also, any future people coming here by Google might find the solution I mentioned to be best for their specific futuristic needs. – John Cave Feb 5 '15 at 17:47
  • I read about upstart configuration and found it to be more elegant choice.Your answer is certainly helpful, and I want to upvote it(if not accept), but unfortunately I don't have enough reputations for the same. – viv1 Feb 6 '15 at 4:45

This script will run on startup.

If you really mean this then you are already talking root privileges.

What does the rest of your script look like? We can't really judge the security or lack of same from isolated commands.

One alternative would be to run it, say, every 5 minutes out of root's crontab. If you are scared of security consequences, you could create a dedicated user who has limited privileges except within power management.

Add this to a new file:

*/5 * * * * root /usr/local/sbin/hibernate-on-power-low

(where maybe root could be replaced with a dedicated user if you like) and install this in /etc/cron.d/hibernate-maybe. Obviously, the script which does the real work would have to be installed in /usr/local/sbin/hibernate-on-power-low and properly audited for security and efficiency (probably using a direct kernel interface would be more robust and efficient than grep) and obviously, if you run with adequate privileges already, you don't need sudo to run pm-hibernate.

  • I like the idea of creating a user( or rather a group) with privileges to power management. – viv1 Feb 5 '15 at 11:26

Another alternative is using sudo -S (which is not safe either).

-S switch of sudo will read the password from STDIN. So, you can use it like:

echo 'password' | sudo -S <command>
  • This would require storing my password in a file(or rather in the script) which I do not consider safe.Thanks for the input anyway. – viv1 Feb 5 '15 at 11:20
  • No, you don't need to store the password in a separate file. Just echo the password to sudo. Anyway its not safe. – heemayl Feb 5 '15 at 11:21

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