When running a mongod with a keyfile given on the command line, both full file paths and file paths relative to the working directory are acceptable input for the keyFile argument. That is, both keyFile /home/usr/keys/myKeyFile and keyFile myKeyFile should both work, assuming myKeyFile exists in the working directory.

However, if one enables fork along with keyFile only the full directory path starting from root is acceptable. Using a local filepath from the working directory will result in

error getting file myKeyFile: No such file or directory

and cause mongo to fail on startup.