Cannot properly execute bash script because of a redirection in an environment where root is the owner

Question

My script is executable and I run it as sudo. I tried many workarounds and alternatives to the ">>" operator but nothing seemed to work properly.

My script:

#! /bin/bash
if [[ -z "$1" || -z "$2" ]]; then
  exit 1
else
  root=$1
  fileExtension=$2
fi
$(sudo find $root -regex ".*\.${fileExtension}") >> /home/mux/Desktop/AllFilesOf${fileExtension}.txt

I tried tee, sed and dd of, I also tried running it with bash -c or in sudo -i , nothing worked. Either i get an empty file or a Permission denied error. I searched thoroughly and read many command manuals but I can't get it to work

Answer 1

The $() operator performs command substitution. When the overall command line is expanded, the command within the parentheses is executed, and the whole construct is replaced with the command's output. After all expansions are performed, the resulting line is executed as a command.

Consider, then, this simplified version of your command:

$(find /etc -regex ".*\.conf") >> /home/mux/Desktop/AllFilesOfconf.txt

On my system that will expand to a ginormous command of the form

/etc/rsyslog.conf /etc/pnm2ppa.conf ... /etc/updatedb.conf >> /home/mux/Desktop/AllFilesOfconf.txt

Note at this point that the redirection is separate from, and therefore independent of, the command in the command substitution. Expanding the command substitution therefore does not cause anything to be written to the target file.

But we're not done! That was just the expansion. Bash now tries to execute the result as a command. In particular, in the above example it tries to execute /etc/rsyslog.conf as a command, with all the other file names as arguments, and with output redirected as specified. But /etc/rsyslog.conf is not executable, so that will fail, producing a "permission denied" message. I'm sure you can extrapolate from there what effects different expansions would produce.

I don't think you mean to perform a command substitution at all, but rather just to run the command and redirect its output to the given file. That would simply be this:

sudo find $root -regex ".*\.${fileExtension}" >> /home/mux/Desktop/AllFilesOf${fileExtension}.txt

Update:

As @CharlesDuffy observed, the redirection in that case is performed with the permissions of the user / process running the script, just as it is in your original example. I have supposed that that is intentional and correct -- i.e. that the script is being run by user 'mux' or by another user that has access to mux's Desktop directory and to any existing file in it that the script might try to create or update. If that is not the case, and you need the redirection, too, to be privileged, then you can achieve it like so:

sudo -s <<END
find $root -regex ".*\.${fileExtension}" >> /home/mux/Desktop/AllFilesOf${fileExtension}.txt
END

That runs an interactive shell via sudo, with its input is redirected from the heredoc. The variable expansions are performed in the host shell from which sudo is executed. In this case the redirection is performed with the identity obtained via sudo, which affects access control, as well as ownership of the file if a new one is created. You could add a chown command if you don't want the output files to be owned by root.