Restoring one system file from Time Machine backup
Unfortunately, you're not going to be able to recover that file using Time Machine. There are a number of files and directories that Time Machine does not back up—primarily cache and temporary files. This includes the files in the /tmp directory. The directory itself (minus contents) is backed up, because it is necessary for a reinstall.

The complete list of exclusions can be found in /System/Library/CoreServices/backupd.bundle/Contents/Resources/StdExclusions.plist



ᔥ Devin Lane: Time Machine Exclusions