Difference between sudo -u and su -c

Question

The following is being executed from an account with sudo privileges:

I have a user tomcat created with the following command

sudo useradd -g tomcat -d /usr/local/tomcat -m -s /bin/false tomcat

I also have a user test created with the following command

sudo adduser test

I have a folder temp and perms 777 are set on this folder

chmod 777 temp

Following are my observations:

## This works and I can see temp/a.txt being created
sudo -u tomcat touch temp/a.txt 

## Does not work. 
sudo su -c "touch temp/b.txt" tomcat

## works
sudo -u test touch temp/c.txt

## works
sudo su -c "touch temp/d.txt" test

So my question is what exactly is wrong with tomcat user?

Solution

The tomcat's shell is /bin/false, you can run with su - as

sudo su -s /bin/bash -c "touch temp/b.txt" tomcat