Temporary Files Used by SCCS

When SCCS modifies an s.file (that is, a history file), it does so by writing to a temporary copy called an x.file. When the update is complete, SCCS uses the x.file to overwrite the old s.file. This ensures that the history file is not damaged when processing terminates abnormally. The x.file is created in the same directory as the history file, is given the same permissions, and is owned by the effective user.

To prevent simultaneous updates to an SCCS file, subcommands that update the history create a lock file, called a z.file, which contains the PID of the process performing the update. Once the update has completed, the z.file is removed. The z file is created with mode 444 (read-only) in the directory containing the SCCS file, and is owned by the effective user.