# Brck -- a simple bit rot checker

Brck is a simple bit rot checker for legacy file systems. It records and
compares the modification times and cryptographic hash sums of regular files.
When a file's hash changed but it's modification time did not, then Brck
reports the file as corrupted so you have a chance to restore the file from
backup. You do have backups, right? 😉

Brck's original source code is hosted [here][brck-repo].

## Installation

Brck is available on [crates.io][brck-cratesio]. You can install it using
Rust's package manager [Cargo][cargo].

    $ cargo install brck

## Usage

Without any options, Brck will record all regular files in current working
directory recursively and write those records to a gzipped `.brck` file.

    $ brck

When you run the same command again, Brck will compare the recorded files
against the current file system and report any corrupted files.

    $ brck
    corrupted: ./test_file
    Error: Found 1 denied difference

Brck is quiet by default. Increase the verbosity using `-v` or request a
summary using `-s`. Alternatively, enable the JSON output using `-J` and
perform your own post-processing, e.g., using [jq][]:

    $ brck -vvJ | jq -sr 'group_by(.type)[] | [.[0].type, length] | @tsv' 2>/dev/null
    added   4
    corrupted       1
    unchanged       1219

See the built-in help for all supported options:

    $ brck --help

## Features

- Parallel sequential processing: Brck processes files in parallel, yet outputs
  and records files in-order

- One-pass: Brck reads your files only once

- Constant memory: Brck's memory footprint is independent of the number of
  processed files

- Relative paths: Brck records relative paths such that you can move the
  containing directory around

- Quiet by default: Without any options, Brck prints denied differences to
  standard output, and errors to standard error; nothing else

- Human readable or machine readable, newline-delimited JSON output, at your
  option

- Graceful shutdown on the first interrupt signal, forceful immediate shutdown
  on the second

- [Pledged][pledge] and [unveiled][unveil] on OpenBSD

## Limitations

- Brck doesn't follow symlinks.

- Brck doesn't track hardlinks. Performance will be suboptimal in the presence
  of large hardlinked files because Brck hashes each copy individually.

- Brck doesn't track reflinks. Modern copy-on-write file systems are out of
  scope because they should check file content integrity themselves, like
  OpenZFS and Btrfs do.

- Brck may leak file meta data because its `.brck` file may have different
  permissions than the listed files.

- Brck doesn't respect platform-specific temporary directories such as `TMPDIR`
  on UNIX. Instead, it creates its temporary files in the current working
  directory, next to the `.brck` file. This way, Brck can update the files
  atomically on POSIX-conform systems.

- Brck may fail to remove its temporary files, e.g., in the event of a
  segfault.

## See also

- Definition of *bit rot* in the [Jargon File][jargon]
  ([archive][jargon-archive])

- A similar Python program named [Bitrot][python-bitrot]

- A similar Rust program named [Legdur][legdur] ([crates.io][legdur-cratesio])

- A blog post on [Things UNIX can do atomically][rcrowley]

[brck-repo]: https://git.skreutz.com/brck.git/
[brck-cratesio]: https://crates.io/crates/brck
[cargo]: https://doc.rust-lang.org/cargo/
[jq]: https://jqlang.github.io/jq/
[pledge]: https://man.openbsd.org/pledge
[unveil]: https://man.openbsd.org/unveil
[jargon]: http://www.catb.org/jargon/html/B/bit-rot.html
[jargon-archive]: https://web.archive.org/web/20240312124910/http://www.catb.org/jargon/html/B/bit-rot.html
[legdur]: https://git.cyplo.dev/cyplo/legdur
[legdur-cratesio]: https://crates.io/crates/legdur
[python-bitrot]: https://github.com/ambv/bitrot/
[rcrowley]: https://web.archive.org/web/20160304224616/http://rcrowley.org/2010/01/06/things-unix-can-do-atomically