[Linux] Compress and Decompress Files with the gzip and gunzip Commands

Overview

The gzip and gunzip commands are the standard tools for handling the .gz file format in Linux. gzip is used to compress files, and gunzip is used to decompress (extract) them. It is important to note that unlike the ZIP format in Windows, these commands do not have the ability to “bundle” multiple files into a single archive. If you need to compress an entire directory into one file, you must use the tar command instead.

Specifications (Arguments and Options)

Syntax

gzip [options] filename
gunzip [options] filename

Note: gunzip is exactly the same as gzip -d.

Main Arguments and Options

Option	Description
-c / --stdout	Writes the result to standard output. Use this if you want to keep the original file.
-d / --decompress	Decompresses the file (same as gunzip).
-f / --force	Forces compression or overwriting if a file with the same name already exists.
-l / --list	Displays the compression ratio and the size of the original file.
-r / --recursive	Processes all files inside a directory individually.
-S <suffix>	Changes the file extension from .gz to a custom string.
-1 to -9	Sets the compression level. -1 is the fastest (low compression), while -9 is the highest (slowest). The default is -6.

Basic Usage

By default, the gzip command replaces the original file with a new file ending in .gz. When you decompress it, the .gz file is replaced by the original file.

Command

# Create a test file
touch app.log

# Compress the file (app.log is replaced by app.log.gz)
gzip app.log

# Verify the file
ls -l app.log*

# Decompress the file (app.log.gz is replaced by app.log)
gunzip app.log.gz

Execution Result

-rw-r--r-- 1 user user 35 Jan 20 12:00 app.log.gz

Practical Commands

Compress All Files Inside a Directory Recursively

Using the -r option tells the command to go through all folders and compress every file it finds “individually.” This does not combine the files into a single archive file.

# Compress all files inside the logs directory individually
gzip -r ./logs/

Check Information for a Compressed File

You can see the original file size and the compression ratio without having to decompress the file first.

# Display compression information
gzip -l app.log.gz

Example Output:

         compressed        uncompressed  ratio uncompressed_name
                 65                 120  48.3% app.log

Compare Sizes Using Different Compression Levels

You can test the trade-off between compression speed (-1) and file size (-9). The default level is -6.

# Copy a file for testing
cp /bin/ls test_file

# Fast compression (Level 1)
cp test_file test_1 && gzip -1 test_1

# Highest compression (Level 9)
cp test_file test_9 && gzip -9 test_9

# Compare the resulting file sizes
ls -l test_*

Compress or Decompress While Keeping the Original File

By using the -c option to send the result to standard output and redirecting it to a new file, you can keep the source file intact.

# Decompress while keeping the .gz file
gzip -dc data.csv.gz > data_restored.csv

# Compress while keeping the original text file
gzip -c config.txt > config.txt.gz

Customization Points

• Changing Extensions: You can change the default .gz extension using -S. However, this is not recommended as it makes file management more difficult.
• Viewing Text Files: To quickly read the content of a compressed text file without decompressing it, use the zcat or zless commands.

Important Notes

• Loss of Original Files: As mentioned, gzip deletes the source file after compression. Always use the -c option with redirection if you need to keep the original file.
• No Archiving Feature: Running gzip on a directory name will result in an error. To compress an entire folder into one file, use tar (e.g., tar czf dir.tar.gz dir/).
• Overwriting: If a file with the same name exists at the destination, the command will ask for confirmation. Use -f to force the action if you are using this in a script.

Applications

Automatically Compress Old Log Files

You can search for specific files, such as log files that haven’t been modified in over 30 days, and compress them to save disk space.

# Find and compress all .log files modified more than 30 days ago
find /var/log/app -name "*.log" -mtime +30 -exec gzip {} \;

Summary

The gzip and gunzip commands are fundamental for managing file compression in Linux. They are specifically designed to reduce the size of individual files and are frequently used for log rotation and data transfer tasks. By understanding that these commands replace original files and do not archive directories, you can effectively use them as part of your server management toolkit.