1.  Introduction

      This document reflects the use of fsck with the 4.2BSD and 4.3BSD file system organization. This is a revision of the original paper written by T. J. Kowalski.

      When a UNIX operating system is brought up, a consistency check of the file systems should always be performed. This precautionary measure helps to insure a reliable environment for file storage on disk. If an inconsistency is discovered, corrective action must be taken. Fsck runs in two modes. Normally it is run non-interactively by the system after a normal boot. When running in this mode, it will only make changes to the file system that are known to always be correct. If an unexpected inconsistency is found fsck will exit with a non-zero exit status, leaving the system running single-user. Typically the operator then runs fsck interactively. When running in this mode, each problem is listed followed by a suggested corrective action. The operator must decide whether or not the suggested correction should be made.

      The purpose of this memo is to dispel the mystique surrounding file system inconsistencies. It first describes the updating of the file system (the calm before the storm) and then describes file system corruption (the storm). Finally, the set of deterministic corrective actions used by fsck (the Coast Guard to the rescue) is presented.