Cross-platform file compression

Command Line – zip

© Photo by Alexander Ross on Unsplash

© Photo by Alexander Ross on Unsplash

Article from Issue 231/2020
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If you use file compression regularly, zip belongs in your Linux toolbox.

File compression in Linux is usually handled by the native commands tar, gzip, or bzip2. However, an additional alternative is zip [1], a popular cross-platform command supported by a variety of scripts and utilities. If you are dealing with someone using another operating system, zip is often the ideal choice among these compression tools.

Admittedly, on Linux, zip has fallen out of favor, because for a time it did not support 64-bit computing and could not handle files large than 2MB. Today, though, zip, gzip, and bzip2 are broadly similar in functionality and structure. All three have similar options, although not always the same name for every option. All three, too, have a history of providing alternate command names for some functions, such as unzip and ungzip, that duplicate standard options – presumably to make the commands easier to remember.

Despite these similarities, neither zip nor gzip recognizes the other's extensions, although both can use files created by the other if the extension is changed. zip can use the option --bzip to use bzip options, which can produce somewhat better compression rates, at least in theory, on binary files.

Zip Basics

If you have worked with other compression programs, zip is easy to start using. The only unusual feature is that a new archive name follows the command and the options and is followed by a space-separated list of the files to archive:

zip OPTIONS NEW-ARCHIVE.zip FILES

Files are not deleted when being added to an archive (Figure 1).

Figure 1: zip's command structure is simple, but has an unusual order.

If you choose, the options can include the option --recurse-patterns (-R), which automatically includes subdirectories. You can also strike a balance between the speed and efficiency of compression with -NUMBER, with   indicating no compression, 1 the fastest but least compression, and 9 the slowest but greatest compression. The highest compression is somewhere between 2:1 or 3:1 for text, while binary files are usually considerably less, perhaps 3:2 or 4:3.

However, these options are only the beginning. You can use --exclude (-x) to list files that should not go into the archive or --include (-i) to specify that only certain files are included. For security, you might also want to add --password (-P) STRING or --encrypt (-e), although the security is somewhat weak by modern standards (see zipcloak below). Still another option is --entry-comments (-c), which lets you annotate each file in an archive with a single-line comment that can be read using the zipnote utility (Figure 2). As the options grow, you are well-advised to add --test (-T) to ensure that nothing unexpected happens.

Figure 2: zip includes the ability to add single-line comments about each file.

zip's options really come into their own once an archive is created. You can use --delete (-d) or unzip to remove files from an existing archive and --grow (-g) to add files. Alternatively, you can use --copy-entries (-U) to create a new archive consisting of files in an existing one. If you are creating archives for backups, you can use --update (-u), --filesync (-FS), or --freshen (-f) to keep the backups current. Should an archive become corrupt, you can try to repair it using --fix (-F). However, many of zip's options require considerably more information. Consequently, the most that I can do in this article is indicate the possibilities.

Zip Utilities

A series of small scripts and utilities have sprouted up around zip. These utilities' usefulness is sometimes limited by their inability to handle files larger than 2GB. This limitation extends to larger files made with recent versions of zip, including those created from a desktop environment. However, even with these limitations, the utilities can sometimes be useful, especially if you are working with text files. Some are installed alongside zip, while one or two have to be installed separately.

zipcmp

Like diff with text files, zipcmp compares the files in zip archives. The command is so simple that it does not include a separate man page or help option. All you need to do is enter the command, followed by the two files to compare. The first file is marked by a minus sign and the second by a plus sign (Figure 3).

Figure 3: zipcmp compares the files in an archive.

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