For loop giving error while creating file

I have a issue with for loop. Will brief it out for you.

$ for i in `cat serverss`;do ssh -q $i sudo echo "a b c" > /tcb/files/auth/x/xyz;done
ksh: /tcb/files/auth/x/xyz: cannot create

I am getting above error whenever i try to create or modify a file. serverss is the file file which has list of servers.We have sudo to root on those systems.

The problem is when i run echo "a b c" > /tcb/files/auth/x/xyz on individual servers it runs perfectly. What is the issue in my for loop which throws me the above error.

Solution

There are two issues here:

You're opening /tcb/files/auth/x/xyz locally, not on the remote system.
sudo echo foo > bar does not provide extra privileges to help in opening bar.

This latter is because something > output opens output before it runs something. Thus, sudo hasn't even been started when /tcb/files/auth/x/xyz is opened!

Instead, consider:

ssh -q "$i" 'echo "a b c" | sudo tee /tcb/files/auth/x/xyz >/dev/null'

The first problem is solved because the redirection is inside the string passed to ssh to be run by a remote shell. (If you pass ssh a bunch of separate arguments, it just combines them with spaces to form such a string; it's better to form the string yourself, and thereby have more control).

The second problem is solved by forcing /tcb/files/auth/x/xyz to be opened by tee, after tee has had its privileges escalated by sudo.