What makes the differences between `sudo CMD` and `sudo su` followed by `CMD`?

Question

I have an SSH server with key only auth and I want to add a key for a new user.

I log in with one user(kidmose), I try to append the key to a new user's(remote) authorized_keys and it fails:

kidmose@hech-remote-control:~$ sudo cat .ssh/id_rsa.pub > /home/remote/.ssh/authorized_keys
-bash: /home/remote/.ssh/authorized_keys: Permission denied

I sudo su first and things work:

kidmose@hech-remote-control:~$ sudo su
[sudo] password for kidmose: 
root@hech-remote-control:/home/kidmose# cat .ssh/id_rsa.pub > /home/remote/.ssh/authorized_keys

Bonus info:

kidmose@hech-remote-control:~$ ll /home/remote/.ssh/authorized_keys
-rw-r--r-- 1 remote remote 409 Oct 16 07:14 /home/remote/.ssh/authorized_keys
kidmose@hech-remote-control:~$ uname -a
Linux hech-remote-control.egki 3.13.0-36-generic #63-Ubuntu SMP Wed Sep 3 21:30:07 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

So what is the (relevant) difference between the two approaches?

Answer 1

In the first case

 kidmose@hech-remote-control:~$ sudo cat .ssh/id_rsa.pub > /home/remote/.ssh/authorized_keys

• your bash (try to) open redirection to the file /home/remote/.ssh/authorized_keys
• then execute sudo cat .ssh/id_rsa.pub

As you don't have access, the command failed.

Answer 2

In the first case the only command run with super user privileges is cat. The redirection to /home/remote/.ssh/authorized_keys is executed as normal user.

In the second case you switch to the root account and all commands are executed with the privileges of the super user.