In Eclipse you have already added Tomcat as Server than you do following:
- In the servers tab, double-click on the Tomcat Server. You will get a screen called Overview.
- Click on “Open launch configuration”. Click on the “Common” tab.
- Towards the bottom of the screen you can check the “Output File” checkbox and then specify a file that can be used to log your console (catalina.out) output.
- Finally, restart the Tomcat server.
Velislav Krastev Blog 
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