Skip CRC Check in Unrar to Speed Up Extraction

This article explains how the unrar utility handles checksum verification, why a native switch to disable CRC calculation does not exist, and how you can use the -kb switch to bypass CRC error interruptions. It also covers practical command options to reduce extraction overhead and maximize decompression speed.

Can You Disable CRC Calculation in Unrar?

Technically, unrar does not have a command-line switch to turn off CRC calculation entirely.

In the RAR compression format, CRC32 or BLAKE2 checksums are calculated concurrently as data streams through the decompression pipeline. Because the checksum calculation is integrated directly into the unpacking routine and heavily optimized, it incurs virtually zero noticeable CPU overhead on modern hardware. Consequently, the RAR development team never implemented a switch to disable calculation for speed purposes.

Skipping CRC Failure Deletions with -kb

If your objective is to skip CRC checks so the extraction process does not halt or discard files when a checksum mismatch occurs, use the -kb (Keep Broken) switch.

By default, when unrar detects a CRC error, it marks the file as corrupt and deletes it from your destination directory upon completion. The -kb switch overrides this behavior:

unrar x -kb archive.rar
  • x: Extracts files with full paths.
  • -kb: Tells unrar to keep damaged or broken unpacked files, bypassing the automatic removal triggered by failed CRC checks.

How to Actually Speed Up Extraction

Because checksum calculations are not the bottleneck during extraction, you can achieve substantial speed improvements by addressing terminal output and disk I/O bottlenecks.

1. Suppress Terminal Output with -idq or -inul

Printing file lists and extraction progress to the terminal consumes significant CPU cycles, especially on archives containing thousands of small files. Suppressing output speeds up execution considerably:

unrar x -kb -idq archive.rar
  • -idq: Enables quiet mode, suppressing progress percentages and non-essential messages.
  • -inul: Completely disables all console output, redirecting all processing power solely to extraction.

2. Overwrite Files Without Prompting

Prevent the utility from pausing to request permission when overwriting files:

unrar x -kb -idq -o+ archive.rar
  • -o+: Overwrites existing files automatically.

3. Optimize Disk Target

Decompression speed is primarily constrained by storage write speeds. Direct your output to a fast NVMe SSD or a RAM-backed temporary filesystem (such as /dev/shm on Linux) to maximize extraction throughput:

unrar x -kb -idq archive.rar /dev/shm/extracted/