Can't redirect standard output

Question

I'm following along with the examples in Chapter 6 Redirection of The Linux Command Line (2012). I am trying to redirect the standard output of an ls command to a text file. I keep receiving a permission denied error. I'm using Ubuntu 13.04. See below:

ta2tyrants@Vaio:/$ ls -l /usr/bin >> ls-output.txt
bash: ls-output.txt: Permission denied

I tried to run this command using root access and get the same error:

ta2tyrants@Vaio:/$ sudo ls -l /usr/bin >> ls-output.txt
bash: ls-output.txt: Permission denied

What am I doing wrong? Is there a way to get around this?

score 4 · Answer 1 · edited Apr 13 '17 at 12:23

Solution 1: redirect to a file you can write to

Your current working directory is /, which by default can only be modified by root. Type cd ~ to change to your home folder, or cd followed by the path to a directory you can write to, then do it again.

Solution 2: redirect with sh under sudo

Shell redirections are not passed along to programs. This means that even if execute a program under sudo, redirection (like >>) will still be performed under your account. This is what happens:

  sudo         ls -l /usr/bin         >>/ls-output.txt
-------------------------------------------------------
|     execute external program      | redirect stdout |       what your shell sees
| sudo | command to execute as root |                            what sudo sees

If you want to sudo a redirect, you need to run a shell as root. You can do this:

     sudo         sh   -c '      ls -l /usr/bin           >>/ls-output.txt   '
-------------------------------------------------------------------------------
|      execute external program (pass the single-quoted string verbatim)      | your shell
|    sudo       |                command to execute as root                   |    sudo
                | sh | -c | execute external program |    redirect stdout     | root shell

That way, the redirect is part of a sh invocation that is done under sudo.

Solution 3: redirect with tee under sudo

As Oli♦ suggested, you can also pipe your command into tee:

       ls -l /usr/bin          |     sudo        tee /ls-output.txt
-------------------------------------------------------------------------
| execute external program | pipe |      execute external program       | your shell
                                  |  sudo  | command to execute as root |    sudo

Note that the tee is executed under sudo, but not the ls. This is because tee needs root priveleges (to write to /ls-output.txt), and ls does not. If the program generating output did need root privileges, you would need to sudo both of them.

   sudo        ls -l /root              |    sudo        tee /ls-output.txt
--------------------------------------------------------------------------------
|     execute external program      | pipe |     execute external program      | your shell
| sudo | command to execute as root |                                             sudo #1
                                           | sudo | command to execute as root |  sudo #2

Also, if you don't want the output displayed on the screen, you can add >/dev/null to the very end of the line.

Can't redirect standard output

1 Answers1