Sudo and shell redirection

One of our students was stuck using sudo to change a file. This spawned a lengthy email discussion on how the shell works and what sudo actually does.

Here is the crux of the problem:

This works:
$ sudo touch /etc/aaaa
This does not:
$ sudo echo ondemand > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governer 

And here is my response:

The shell redirection takes precedence and runs first as you.

As you have seen this is all done by root:
$ sudo touch /etc/aaaa

But this does not work:
$ sudo echo ondemand > scaling_governer

The reason is because the shell is doing this:
 as root (echo ondemand)
 as the user (> scaling_governer)

Try this:
$ sudo sh -c 'echo "1" > hi'

The 'sudo sh -c' recipe can be used any time you want to use sudo and shell redirection. Another way if things are confusing is to write a shell script and execute the script using sudo.