File System Check Utility - `fsck`

fsck stands for “file system check” and is a fundamental tool in UNIX-like operating systems, including Linux, for checking and repairing file systems.

Overview

Purpose: The primary purpose of fsck is to inspect file systems for inconsistencies resulting from crashes, hardware issues, or unexpected shutdowns. If permitted, it can also attempt to rectify these inconsistencies, restoring the file system to a consistent state.

When is it Run: While you can manually run fsck on unmounted file systems, it’s typically run automatically during system startup before file systems are mounted (with exceptions for the root file system, which can be checked while mounted as read-only).

Key Concepts

Multiple Tools: fsck itself is a front-end tool that dispatches control to file system-specific checkers. For example, e2fsck is the checker for ext2, ext3, and ext4 file systems, while xfs_repair serves for XFS file systems. When you run fsck, it determines the type of the file system and runs the appropriate checker.

Inconsistencies: Some common issues fsck checks for include:

1. Orphaned inodes (file system entries that aren’t associated with any file or directory)
2. Cross-linked files (two files that claim to occupy the same disk blocks)
3. Blocks claimed by multiple files
4. Directory discrepancies, such as size mismatches or invalid entries

Interactive Mode: By default, when fsck encounters an inconsistency, it will enter an interactive mode, prompting the user for what action to take. Depending on the nature and severity of the inconsistency, fsck might offer solutions like deleting a corrupted file or reconstructing a damaged inode.

Automatic Repairs: fsck can also be run in a non-interactive mode, where it attempts to automatically fix detected issues without user intervention. This mode is typically used during system boot-ups.

Force Check: Even if the system believes a file system is clean, you can force an fsck check. This can be useful if you suspect underlying hardware issues or if the system has exhibited strange behavior.

Running on Mounted File Systems: It’s crucial not to run fsck on a mounted file system (unless it’s mounted read-only), as this can lead to data corruption. Before checking, the file system should be unmounted or, in the case of the root file system, the system should be booted into a special mode like single-user mode.

Practical Use

Manual Invocation: A system administrator might run fsck manually if:

1. A file system error is suspected.
2. After an unexpected shutdown or system crash.
3. As part of regular maintenance.

During Boot: Linux systems often run fsck during boot. The frequency and conditions under which these checks happen can usually be configured. For instance, some systems are set to run fsck every N number of boots or after X number of days since the last check.

While fsck is a powerful tool, it’s also one that should be approached with caution. A misused or mistimed fsck command can result in data loss. Always ensure backups are in place, especially when performing manual file system checks or repairs.