Safe Atomic File Update

Update a configuration file so a power loss mid-write can never leave a truncated file behind. Write to a temporary file, sync it, then rename over the original.

Constraints

• The temporary file must be on the same filesystem as the target.
• Must sync before the rename.
• Preserve the original file's permissions.
• Clean up the temporary file on any failure.
• Exit non-zero if the target directory is not writable.
• POSIX sh with a trap for cleanup.