Helpful to verify reports of your own downtime, or to double check a downed website you are trying to access.
One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.
Make sure the search string actually exists in the Exim log; no output is normal if there are no matches.
Also make sure that the grep command can be located -- replace "grep" with "/bin/grep" or whatever the appropriate path is on your server.
Finally, be aware of quoting and shell metacharacter issues, including spaces. If the search pattern is "mail rejected", for example, grep will think "rejected" is a file to search and will return an error without even getting to the Exim log. Test with a simple one-word search string before something complicated.