Written by Alex Taylor | 11/20/2023

gzip

Gzip (GNU zip) is a widely used compression utility for file compression and decompression. It was created as a free software replacement for the UNIX compress utility, adhering to the philosophy of the GNU Project, and is based on the DEFLATE algorithm, which is a combination of LZ77 and Huffman coding.

Gzip is designed for fast, efficient compression, particularly for text files such as HTML, CSS, and source code. Here's an overview of how it works:

  1. LZ77 Algorithm: Gzip begins by using the LZ77 algorithm, which searches for duplicate strings in the input data. When a duplicate string is found, it is replaced with a pointer to the previous string's location and length. This effectively eliminates redundant information, particularly useful in text where such redundancy is common.

  2. Huffman Coding: After LZ77, Huffman coding is applied to the compressed data. Huffman coding is a type of optimal prefix coding that reduces the average code length used to represent the symbols of an alphabet. In the context of gzip, it takes the output from the LZ77 algorithm and encodes it in a way that uses shorter codes for more common elements (such as characters in a text file) and longer codes for less common elements.

  3. File Structure: A gzip file consists of a series of "members" (compressed data sets). Each member has a header with information such as the original file name, a timestamp, and checksums for data integrity. The compression of each member is independent of the others, which allows for concatenation and splitting of gzip files.

  4. Deflate: The combination of LZ77 and Huffman coding is known as the DEFLATE algorithm, which is the core of gzip's compression process. DEFLATE is designed to be fast while still providing good compression ratios.

  5. Checksums: Gzip appends a checksum (CRC-32 or the newer CRC-64-ISO) to the end of the compressed data to ensure that the data can be verified for integrity when it is decompressed.

  6. Portability: One of the key features of gzip is its portability. The gzip utility is available on a wide range of operating systems and has been adopted as a standard compression tool on Unix-like systems, including Linux and macOS.

Gzip is often used in combination with tar, a utility that can combine multiple files into a single archive (a .tar file), which can then be compressed into a .tar.gz or .tgz file. This is commonly used for distributing application source code or for transferring multiple files over the internet more efficiently.

In the context of the web, gzip compression is used by web servers to compress data sent to browsers, significantly reducing the amount of data transmitted and thus improving loading times for websites. Most modern web browsers automatically decompress gzip-compressed content, making it transparent to the user.

Gzip's effectiveness and speed have made it a staple for file compression, and while newer formats like Brotli and Zstandard offer better compression ratios, gzip remains prevalent due to its widespread support and the balance it offers between speed and compression efficiency.