Bash: Can't su Within while loop

x24804 · 07-18-2010, 04:47 PM

I have a while loop in a file that looks like:

Code:

while IFS=":" read name script
do
        su      
        exit 
        ...
        <snip>
done < /path/to/file

Where I redirect the file into the loop, for some reason, I can't do an su when I redirect a file like that. I get the error, "su: must be run from a terminal." Why is this? How can I fix it? Thanks.

x24804 · 07-18-2010, 05:15 PM

Hey, for anyone else with this problem, I think this happens because su thinks that it's going to get it's input non-interactively from the file. su doesn't allow that. Anyone have any solutions to this? Thanks.

PTrenholme · 07-18-2010, 05:15 PM

If su allowed the password to be read from a re-directed [b]stdin[/] a simple shell script could be written to automate a cracking attempt by non-privileged users on your system. (Admittedly, this is more of a concern if your system had many different users, but UNIX was originally designed to run on a "large" main-frame computer with many (hundreds) users accessing it.)

If you really want your script run with "root" access, just run it as "root" instead of having the script change context in mid-stream.

Note that the same consideration means that sudo will also decline to accept input from a redirected stdin.

If you click the "Report" button at the end of your post and ask the moderators to move this thread to the "security" forum, you should get several additional answers from persons more expert than I am in these matters.

x24804 · 07-18-2010, 05:22 PM

I didn't realize that su was taking the file that I defined as input. I only expected the read command to deal with the file, not su. I wanted it to prompt for the password like it normally does when there's no file redirect.

Thank you.

XavierP · 07-18-2010, 07:47 PM

As requested, moved to Linux-Security

piratesmack · 07-19-2010, 02:53 AM

Not sure if this is what you want, but I sometimes put this at the beginning of scripts I want run as root.

Code:

[ "$UID" = "0" ] || exec su -c "/bin/sh $0 $*"

ntubski · 07-19-2010, 09:12 AM

Quote:

Originally Posted by x24804

I didn't realize that su was taking the file that I defined as input. I only expected the read command to deal with the file, not su.

When you put the redirection after the while ... done it applies to the whole loop. Try this:

Code:

exec 3< /path/to/file

while IFS=: read -u3 name script
do
    ...
done

exec 3<&-

x24804 · 07-20-2010, 01:37 PM

ntubski - Thanks. I ended up just reading and stream editing the file based on the line number; I still needed to su within the while loop.