Ubuntu: When using sudo with redirection, I get 'permission denied'



Question:

When using sudo to allow edits to files, I regularly get 'permission denied'.

For example, my mouse is jittery and sluggish, so I want to disable polling:

sudo echo "options drm_kms_helper poll=N">/etc/modprobe.d/local.conf  

I'm prompted for a password, and then get:

bash: /etc/modprobe.d/local.conf: Permission denied  

So I tried to do a temporary change to disable polling by using:

sudo echo N> /sys/module/drm_kms_helper/parameters/poll  

Yet again the system responded with:

bash: /sys/module/drm_kms_helper/parameters/poll: Permission denied  

Any ideas?


Solution:1

Output redirection (via the > operator) is done by the shell, not by echo. You have to login as root

sudo -i  

Then you can use redirection

echo N> /sys/module/drm_kms_helper/parameters/poll  

Otherwise you can run bash string with sudo

sudo bash -c "echo N> /sys/module/drm_kms_helper/parameters/poll"  


Solution:2

The output redirection is done by the shell from which the command has been invoked. So, breaking everything into bits, here what is happening*:

  • shell invokes sudo echo "options drm_kms_helper poll=N", which executes sudo command with echo "options drm_kms_helper poll=N" command line

  • sudo asks for a password, opens superuser shell and invokes echo "options drm_kms_helper poll=N", which runs echo command passing it "options drm_kms_helper poll=N"

  • echo, running with root privileges, prints the string to its standard output.

  • echo command terminates, superuser shell exits, sudo terminates

  • the shell from which the command has been invoked collects the output and tries to redirect it to /etc/modprobe.d/local.conf, which is writeable only by root. It gets "permission denied" error.

For the ways to fix this see @shantanu answer.


(*) - while the above sequence helps to understand why the command fails, in reality things happen somewhat out-of-order: the original shell notices the redirection and tries to open the file for writing before invoking the sudo ... command. When opening the file fails the shell doesn't even invoke the command which was supposed to write to the file (thanks to @PanosRontogiannis for pointing this out).

Here's a quick test:

$ touch ./onlyroot.txt  $ sudo chown root:root ./onlyroot.txt  $ sudo bash -c "whoami | tee who.txt" > onlyroot.txt  bash: onlyroot.txt: Permission denied  

In the test above the whoami | tee who.txt was going to create a file named who.txt containing the word "root". However, when the output redirection fails in the calling shell, "who.txt" file is also missing because the command was not invoked.


Solution:3

Adding to Shantanu's answer:

... Or you could use a tee command like this:

sudo tee /sys/module/drm_kms_helper/parameters/poll <<<10  

or if its a command's output:

echo 10 | sudo tee /sys/module/drm_kms_helper/parameters/poll  


Solution:4

An approach I haven't seen mentioned here is to simply execute the entire commandline in its own shell. The sudo manpage itself gives an example of this approach:

To make a usage listing of the directories in the /home partition. Note that this runs the commands in a sub-shell to make the cd and file redirection work.

$ sudo sh -c "cd /home ; du -s * | sort -rn > USAGE"  


Solution:5

Another option is to use a temporary file. This is useful in a bash script.

temp=$(mktemp)  echo "Hello, world!" > $temp  sudo cp $temp /etc/wherever  


Solution:6

sudo dd of=

To append as you want:

echo inbytes | sudo dd of=outfile oflag=append conv=notrunc  

or to recreate the file from scratch:

echo inbytes | sudo dd of=outfile  

Advantages:

  • nicer than tee since no /dev/null redirection
  • nicer than sh since no subshell
  • dd has many powerful options, e.g. status=progress to see transfer progress

Works because sudo forwards stdin to the command.


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