How to solve "permission denied" when using sudo with redirection in Bash?

  • 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?

  • shantanu

    shantanu Correct answer

    8 years ago

    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"

    You can't run echo with sudo? the what about the result I got: `[email protected]:~$ sudo echo "Hi" [sudo] password for saji: Hi`

    you can write on file, echo "something" > somewhre. It's using pipe.. That is the problem.

    Ok, if that's the case, then please update your answer to reflect that running echo is a problem in that case only.

    You can't simply run the shell builtin `echo` as sudo, unless you do something like `sudo bash -c 'echo …'`; however, POSIX systems usually supply an external `echo` command such as `/bin/echo` on OS X, which sudo can execute without rigamarole. Thus, the `echo` command you usually run and the `echo` command you run with sudo are probably two different, but similar commands.

    If that is the case, why did the answers to this question suggest echo?

  • 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.

    More likely the shell 'forks' itself and tries to open /etc/modprobe.d/local.conf before trying to 'exec' sudo which means that the first 4 steps your describe never actually happen because the file cannot be opened.

    @PanosRontogiannis: thanks, I've updated the answer

  • You can 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

    If you had a situation where you wanted to append rather than overwrite the target file--that is, to make tee behave like >> rather than >--you would use tee -a.

    +1 logging in as root is a bad idea for manual work, and a *really* bad idea for scripted tasks.

    Also, `sudo tee /sys/module/drm_kms_helper/parameters/poll > /dev/null` if you don't want it printing to `stdout` as well.

  • 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"

    wow thanks. a simple solution

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

    echo "Hello, world!" > $temp
    sudo cp $temp /etc/wherever
  • 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


    • nicer than tee since no /dev/null redirection
    • nicer than sh since no explicit subshell (but an implicit one for the redirection)
    • dd has many powerful options, e.g. status=progress to see transfer progress

    Works because sudo forwards stdin to the command.

    This is good. We think of `dd` as how we overwrite our once-great filesystems, and don't realize it's for mundane tasks too--and that other commands as root also cause great harm if used on on the wrong files/devices. Like `sudo tee`, `sudo dd` will of course also work with here strings, e.g., `sudo dd of=outfile <<<'hello world'`. [Thanks for editing. NB with `sh -c 'cmd'`, `sh` is a subprocess that's a shell, but not really a subshell except in the sense *all* external commands begin as one.]

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Content dated before 6/26/2020 9:53 AM