====== Linux : Compression and Archiving ======

===== What is Compression? =====

**Compression** is a process or technique to reduce the size of a data file. This is achieved using specific algorithms that identify patterns in the data to reduce its size.

Compression is divided into two types:

  * **Lossless Compression**: Data is compressed without any loss of information. The decompressed data is identical to the original.
  * **Lossy Compression**: Some information is intentionally removed to reduce file size. The decompressed result may not be identical to the original.

===== What is Archiving? =====

Before exploring compression, it is important to understand **archiving**.

**Archiving** is the process of collecting multiple files or directories into a single file. This archive can then be compressed using compression tools.

==== tar ====

**tar** is a common archiving tool on Linux. Example usage:

<code bash>
tar -cvf archive.tar file1 file2 directory
</code>

In the example above:
  * <code>-c</code>: Create a new archive
  * <code>-v</code>: Verbose mode (shows detailed process)
  * <code>-f</code>: Specifies the archive file name

<WRAP info>
You can add compression options: <code>-z</code> for gzip, <code>-j</code> for bzip2, or <code>-J</code> for xz.
</WRAP>

===== Compression Tools =====

Common compression tools on Linux:

^ Compression Tool ^ Compression Algorithm ^
| **gzip**  | DEFLATE |
| **bzip2** | Burrows-Wheeler |
| **xz**    | LZMA |
| **zip**   | DEFLATE |

==== gzip ====

**gzip** is a widely used compression utility using the **DEFLATE** algorithm.

  * Compress a file:
    <code bash>
    gzip filename
    </code>
    → Produces <code>filename.gz</code>

  * Decompress a file:
    <code bash>
    gunzip filename.gz
    # or
    gzip -d filename.gz
    </code>

  * View compression info:
    <code bash>
    gzip -l filename.gz
    </code>

==== bzip2 ====

**bzip2** uses the **Burrows-Wheeler** algorithm for better compression than gzip.

  * Compress a file:
    <code bash>
    bzip2 filename
    </code>
    → Produces <code>filename.bz2</code>

  * Decompress a file:
    <code bash>
    bunzip2 filename.bz2
    # or
    bzip2 -d filename.bz2
    </code>

  * View compression info:
    <code bash>
    bzcat filename.bz2 | wc -c
    </code>

==== xz ====

**xz** uses the **LZMA (Lempel-Ziv-Markov chain algorithm)**. It offers higher compression ratios but is slower and more resource-intensive.

  * Compress a file:
    <code bash>
    xz filename
    </code>
    → Produces <code>filename.xz</code>

  * Decompress a file:
    <code bash>
    unxz filename.xz
    # or
    xz -d filename.xz
    </code>

  * View compression info:
    <code bash>
    xz -l filename.xz
    </code>

==== zip ====

**zip** is commonly used to compress and archive multiple files.

  * Compress files:
    <code bash>
    zip archive.zip file1 file2 folder1
    </code>

  * Recursively compress a folder:
    <code bash>
    zip -r archive.zip folder1
    </code>

  * Add files to existing zip:
    <code bash>
    zip -u archive.zip file3
    </code>

  * Password-protect a zip:
    <code bash>
    zip -r -e archive.zip folder1
    </code>

  → Produces <code>archive.zip</code>

  * Decompress:
    <code bash>
    unzip archive.zip
    </code>

  * View compression info:
    <code bash>
    unzip -l archive.zip
    </code>

===== Archiving and Compression with tar =====

**tar** supports built-in compression in one command:

==== tar + gzip ====
<code bash>
# Compress
tar -czvf archive.tar.gz directory/
# Decompress
tar -xzvf archive.tar.gz
</code>

==== tar + bzip2 ====
<code bash>
# Compress
tar -cjvf archive.tar.bz2 directory/
# Decompress
tar -xjvf archive.tar.bz2
</code>

==== tar + xz ====
<code bash>
# Compress
tar -cJvf archive.tar.xz directory/
# Decompress
tar -xJvf archive.tar.xz
</code>

<WRAP info>
Explanation of flags:

  * <code>-c</code>: Create archive  
  * <code>-x</code>: Extract archive  
  * <code>-z</code>: Use gzip  
  * <code>-j</code>: Use bzip2  
  * <code>-J</code>: Use xz  
  * <code>-v</code>: Verbose (detailed output)  
  * <code>-f</code>: Specify archive file name
</WRAP>