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sudo & redirect output

These are suggestions given by powertop to save energy:

echo 1500 > /proc/sys/vm/dirty_writeback_centisecs

echo 1 > /sys/module/snd_hda_intel/parameters/power_save

However, I cannot execute them from terminal even as sudo. Ideas?


Your command doesn't work because the redirection > file is done by the current shell prior to the execution of the command, so before the sudo is in effect.

There is a command, called tee, that writes to a file and to stdout what it receives on its stdin: this is handy to write something to a file without redirection. If the file can be modified only by root, it is sufficient to prepend sudo to tee.

echo 1500 | sudo tee /proc/sys/vm/dirty_writeback_centisecs

Another way to obtain the desired result, mantaining the redirection, is to move the redirection to a subshell executed by root, through sudo:

sudo sh -c 'echo 1500 > /proc/sys/vm/dirty_writeback_centisecs'

Lastly, you can enter a root shell in various ways, and you remain root until you exit explicitly that shell:

sudo su
sudo -s
sudo bash

For more information see the manuale pages of sudo, su and of course of bash.

  • I always forget about tee :-X +1 tomorrow ;) and dago: accept this one please instead of mine. it is shorter and a better answer.
    – Rinzwind
    Jun 23 '11 at 18:52
  • Gentlem, as a beginner thanks but still a load of questions. I know hoe to make a script executable but what is the difference to do sudo su before and how? wHAT is the sudo tee or sudo sh - c
    – dago
    Jun 23 '11 at 19:45
  • I will give more information in the answer
    – enzotib
    Jun 23 '11 at 19:47

Instead of creating a cronjob for setting values as described by @Seppo Erviälä, you can make use of the configuration files of procps and sysfsutils.


Values for /proc/sys can be set in the file /etc/sysctl.conf. To make echo 1500 > /proc/sys/vm/dirty_writeback_centisecs persistent, add a line with:

vm.dirty_writeback_centisecs = 1500

To load the changes in /etc/sysctl.conf to the current session, run:

sudo sysctl -p


For setting values in /sys the package sysfsutils needs to be installed. The configuration file is located at /etc/sysfs.conf. To make echo 1 > /sys/module/snd_hda_intel/parameters/power_save persistent, add a line with:

module/snd_hda_intel/parameters/power_save = 1

To apply the values in /etc/sysfs.conf to the current session, run:

sudo /etc/init.d/sysfsutils restart
  • Thanks for info, this seems much more proper way to do this Jun 24 '11 at 11:11

Do a sudo -s or a sudo su first:

enter image description here

From a script:

# more powersave
echo 1500 > /proc/sys/vm/dirty_writeback_centisecs
echo 1 > /sys/module/snd_hda_intel/parameters/power_save
# ./powersave 

Best practice would be to ussue the sudo -s before activating the script (hence the #).

  • Thanks, of course you earned a voting up and solved, however, how can I use this in a script together with other power saving measures? BTW what is the difference between sudo su and command and sudo command?
    – dago
    Jun 23 '11 at 18:34
  • Toss the lines in a script, make it executable and run sudo su before activating the script.
    – Rinzwind
    Jun 23 '11 at 18:40
  • ... or run each line with "sudo -i " immediately before the echo statements
    – fossfreedom
    Jun 23 '11 at 18:41
  • worked on my test program - also see @enzotib answer for an alternate
    – fossfreedom
    Jun 23 '11 at 18:51
  • @Rinzwind Using a root shell is not a "best practice" in Ubuntu. To allow writing to privileged locations using redirection operators, see askubuntu.com/q/20578/6969
    – Lekensteyn
    Dec 18 '11 at 9:07

EDIT: See answer by @Lekensteyn for more proper way to edit /proc/sys and /sys defaults.

Running these suggestions from command line will only enable them for your current session and they will reset to defaults after a reboot. If you want to enable these suggestions each time your system starts you should make them into a script:

echo 1500 > /proc/sys/vm/dirty_writeback_centisecs
echo 1 > /sys/module/snd_hda_intel/parameters/power_save

You can place this script somewhere convinient eg. /root/power_save.sh.

Then you want to make sure it runs with root privileges each time your system starts. This can be done with sudo crontab -e which opens list of time based tasks for root. Add line:

@reboot /root/power_save.sh

Don't forget to make your script executable:

sudo chmod u+x /root/power_save.sh

This way these power saving options will be enabled for all users, even before login, and no password is needed to authorize them each time.

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