How to Use the 'compress' Command (with Examples)

The Unix compress command is a utility that provides data compression by using the Lempel-Ziv-Welch (LZW) compression algorithm. It reduces the size of given files and provides several options to handle various compression tasks efficiently. While it’s not as commonly used today due to the popularity of more advanced compression tools like gzip and bzip2, it remains useful in specific use cases, especially in environments relying on traditional Unix systems.

Use Case 1: Compress Specific Files

Code:

compress path/to/file1 path/to/file2 ...

Motivation: Compressing specific files is useful when you want to reduce the storage space they occupy or when you want to prepare files for network transfer, as smaller files take less time to transfer. This command allows you to select exactly which files you want to compress without having to process entire directories.

Explanation:

• compress: The command itself, which initiates the compression process.
• path/to/file1 path/to/file2 ...: These are the paths to the files you intend to compress. Each file you specify will be individually compressed, reducing its size on disk.

Example Output:

path/to/file1:   50.0% -- replaced with path/to/file1.Z
path/to/file2:   48.5% -- replaced with path/to/file2.Z

Use Case 2: Compress Specific Files, Ignore Non-existent Ones

Code:

compress -f path/to/file1 path/to/file2 ...

Motivation: Sometimes, you may want to compress a set of files, but not all of them might exist. This scenario is common in scripts that handle files dynamically. Using compress -f ensures that the command proceeds without errors for files that are absent, focusing on those that are present.

Explanation:

• -f: The -f (force) option instructs compress to suppress errors for missing files.
• path/to/file1 path/to/file2 ...: The list of files to potentially compress. The command skips over any that do not exist.

Example Output:

path/to/file1:   55.0% -- replaced with path/to/file1.Z
compress: path/to/file2 not found

Use Case 3: Specify the Maximum Compression Bits (9-16 Bits)

Code:

compress -b 12 path/to/file

Motivation: Different data types or sizes might benefit from varying compression techniques or bit settings. By specifying the number of compression bits, you can optimize the compression level based on file characteristics or system requirements, which can lead to better compression ratios.

Explanation:

• -b 12: This option sets the bit limit used during compression. The number 12 specifies that a 12-bit approximation will be used. Valid values are between 9 and 16.
• path/to/file: This is the file to be compressed using the specified bit limit.

Example Output:

path/to/file:   45.2% -- replaced with path/to/file.Z

Use Case 4: Write to stdout (No Files Are Changed)

Code:

compress -c path/to/file

Motivation: Outputting to stdout is useful when the compression operation is part of a pipeline or when you want to preview compressed data without altering original files. This can be particularly useful for processing streams of data in complex Unix commands.

Explanation:

• -c: The option that tells compress to output the compressed data to standard output (stdout) rather than replacing the input file with its compressed version.
• path/to/file: The file whose compressed form will be sent to stdout.

Example Output: This will output a stream of compressed data directly to the terminal, which may appear as gibberish due to its compressed nature.

Use Case 5: Decompress Files (Functions Like uncompress)

Code:

compress -d path/to/file

Motivation: In cases where file decompression is needed, providing the -d option allows compress to function similarly to uncompress, which is beneficial when you want a single tool that can handle both compression and decompression tasks seamlessly.

Explanation:

• -d: Switches the function of the command to decompression.
• path/to/file: The file you want to decompress, commonly identified by the .Z extension as a result of a previous compress operation.

Example Output:

path/to/file.Z:    uncompressed to 101.5%

Use Case 6: Display Compression Percentage

Code:

compress -v path/to/file

Motivation: It’s often helpful to receive feedback on how effective the compression was in terms of space savings. By displaying the compression percentage, you can measure efficiency and decide whether further optimizations are necessary.

Explanation:

• -v: The verbose option, which provides additional information about the operation, including the percentage of compression achieved.
• path/to/file: The file that will be compressed, with feedback given on how much the size was reduced.

Example Output:

path/to/file:   52.3% -- replaced with path/to/file.Z

Conclusion:

The compress command is a versatile utility in Unix systems, catering to various compression and decompression needs. Although it’s somewhat dated, the command still serves useful purposes, especially in legacy systems or environments confined to traditional Unix utilities. Each use case explored above demonstrates the command’s flexibility in handling files efficiently across different scenarios.