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All SCCS commands that modify an SCCS file do so by first writing and modifying a copy called x.file. This is done to ensure that the SCCS file is not damaged if processing terminates abnormally. x.file is created in the same directory as the SCCS file, given the same mode (see chmod(C)) and is owned by the effective user. It exists only for the duration of the execution of the command that creates it. When processing is complete, the contents of s.file are replaced by the contents of x.file, whereupon x.file is destroyed.
To prevent simultaneous updates to an SCCS file, the same modifying commands also create a lock-file called z.file. z.file contains the process number of the command that creates it, and its existence prevents other commands from processing the SCCS file. z.file is created with access permission mode 444 (read-only for owner, group, and other) in the same directory as the SCCS file and is owned by the effective user. It exists only for the duration of the execution of the command that creates it.
In general, you can ignore these files. They are useful only in the event of system crashes or similar situations.