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fsck(1M) is a multi-pass filesystem check program. Each filesystem pass invokes a different phase of the fsck program. After initialization, fsck performs successive passes over each filesystem, checking blocks and sizes, pathnames, connectivity, reference counts, and the map of free blocks (possibly rebuilding it), and performs some cleanup.
At boot time fsck is normally run with the -y option, non-interactively. (fsck can also be run interactively by the administrator at any time.) fsck can also be run non-interactively to ``preen'' the filesystems after an unclean halt. While preening a filesystem, it will only fix corruptions that are expected to result from an unclean halt. These actions are a subset of the actions that fsck takes when it is running interactively. When an inconsistency is detected, fsck generates an error message. If a response is required, fsck prints a prompt and waits for a response. When preening, most errors are fatal. For those that are expected, the response taken is noted.
The error conditions are organized by the phase of the fsck program in which they can occur. The error conditions that can occur in more than one phase are discussed below under ``fsck_ufs initialization phase''