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dago
Asked: 2011-06-24 10:03:19 +0800 CST

Why does echoing these parameters with sudo not work? [duplicate]

  • 772

Possible Duplicate:
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?

command-line

4 Answers

  1. Best Answer

    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.

    • 15

  2. 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.

    /proc/sys

    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

    /sys

    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
    • 9

  3. 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 #).

    • 2

  4. 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:

    #!/bin/dash
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.

    • 2