The xtrabackup binary exits with the traditional success value of 0 after a backup when no error occurs. If an error occurs during the backup, the exit value is 1.
In certain cases, the exit value can be something other than 0 or 1, due to the command-line option code included from the MySQL libraries. An unknown command-line option, for example, will cause an exit code of 255.
When an error happens in the main() function of xtrabackup.c, when xtrabackup is preparing to perform the backup, the exit code is -1. This is usually because of a missing or wrong command-line option, failure to open a file or directory that the user specified as a command-line option, or similar. This behavior is changed in xtrabackup 1.4 and later, so it always returns 0 or 1. (However, the MySQL libraries might still exit with a code of 255.)