Inspect gzip Files without decompress: zcat, zless, & zmore

Linux provides several commands to work with compressed files such as zcat, zless, and zmore.

This tutorial will deep dive into the usage of these commands, allowing you to navigate and inspect compressed files without decompressing them.

Here’s a table that summarizes the difference between the 3 tools:

Tool	Description	Primary Use(s)	Interactive	Searching Capabilities
zcat	Displays compressed file content to stdout.	Quick content viewing, Piping data.	No	No (unless piped)
zless	Interactive viewer for compressed files.	Deep dives into file content with scrolling, searching, and line jumps.	Yes	Yes
zmore	Simple pager for compressed files.	Basic page-by-page viewing of gzipped content.	Somewhat	No

 

 

Introduction to zcat

zcat is akin to the standard cat command but is designed to read gzip compressed files.

By default, zcat decompresses the data and writes the results to standard output.

$ zcat file.txt.gz

When you run this command, the content of the compressed file.txt.gz is displayed on the terminal.

If you try to use cat on a gzipped file, you’ll get gibberish since cat doesn’t know how to decompress on the fly.

When to use zcat over decompressing a file

Using zcat has its advantages, especially when working with large compressed files.

• Space-saving: If you’re low on disk space, decompressing a large file might be infeasible. With zcat, you can view the content directly without needing extra space for the decompressed version.
• Time-saving: Decompressing a file takes time. By using zcat, you can directly access the content, skipping the decompression process.

Force the reading of files

zcat provides an option -f to force the reading of files even if they aren’t compressed.

This is useful when you have a mixture of compressed and uncompressed files and you want to read them using a single command.

$ zcat -f *.log

If *.log contains a mix of gzipped and plain text log files, this will decompress the gzipped ones and pass through the plain text ones untouched.

Using pipes to filter the output

You can pipe the output of zcat to other commands for filtering, searching, or other manipulations.

$ zcat file.txt.gz | grep "search_term"

With this command, you’re directly searching within the compressed file for lines that contain “search_term”.

This method is efficient as it allows you to search inside compressed files without needing to decompress them.

 

Introduction to zless

zless is an interactive pager for compressed files. It’s quite analogous to the less command but specializes in reading gzip-compressed files.

$ zless file.txt.gz

This command opens the compressed file.txt.gz in an interactive viewing mode, allowing you to scroll through its content.

zless allows you to interactively inspect compressed content.

Scrolling, searching, jumping to a line number

Inside the zless environment, you have a rich set of controls:

• Scrolling: Use the arrow keys to scroll line-by-line or the space bar to scroll page-by-page.
• Searching: To find a specific term, type / followed by your search term. Press n to move to the next occurrence and N for the previous.
• Jumping to a line number: Type the desired line number, followed by G to jump directly to that line.

These controls mirror those in less, ensuring a seamless experience when transitioning between viewing compressed and uncompressed files.

Configuration and customization

zless allows you to configure your viewing experience:

• Changing the display colors: You can set the LESS_TERMCAP environment variables to customize the color scheme. For instance, to set the search highlights to blue:

export LESS_TERMCAP_so=$(tput setaf 4)

• Custom key bindings: While zless defaults are sufficient for many, you might prefer a custom set of key bindings. By setting the LESSKEY environment variable to a keybindings file, you can define your personal shortcuts.
• Setting default options: If you find yourself always using specific less options, you can make them default for zless by exporting them in the LESS environment variable. For instance:

export LESS='-N -g'

Here, -N displays line numbers, and -g highlights only the last search pattern.

 

Introduction to zmore

zmore offers another way to inspect compressed files, functioning as a simple pager. Unlike zless, zmore offers basic paging capabilities.

$ zmore file.txt.gz

Executing the above will display portions of the compressed file.txt.gz on your terminal. When you reach the end of a displayed portion, press space to view more of the file.

zmore offers a straightforward “more” functionality for gzipped files, hence its naming convention.

Scrolling, searching, and other essential controls

zmore is designed to be a simple tool, and as such, its controls are more limited compared to zless. Here are the primary controls you’ll use:

• Scrolling: Press space to scroll to the next portion of the file.
• Searching: Unfortunately, direct in-app searching isn’t natively supported in zmore like in zless. However, combining it with other tools (e.g., zgrep) can aid in searching.
• Exit: To exit from zmore, simply press q.

While zmore may seem basic, its simplicity can be an asset when you only need a quick glance at a compressed file without the overhead of additional features.

Using with zcat or zgrep

zmore can be paired with other utilities to enhance its capabilities:

• Combining with zcat: For quick viewing and subsequent paging, you can combine zcat with zmore.

$ zcat file.txt.gz | zmore

This allows you to first decompress the file to stdout and then page through it using zmore.

• Searching with zgrep: While zmore lacks native searching, zgrep fills that gap. To search and then page through the results:

$ zgrep "search_term" file.txt.gz | zmore

With this, you’ll see the lines containing “search_term” and can page through them.

 

Choosing the right tool

Selecting the right tool depends on the task at hand:

• Quick Content Viewing: If all you need is to quickly display the entire content of a compressed file, zcat is the tool.
• Interactive Exploration: For a deeper, interactive dive into the contents, where you might want to scroll, search, or jump to specific lines, zless is the best choice.
• Basic Paging: If you’re looking for a simple way to page through a gzipped file without added complexities, zmore is the choice.