File Compression And Archives

Core Idea

An archive groups files. Compression makes bytes smaller. These are related but not the same. A .tar file is usually an archive. .gz and .bz2 are compression formats. A .tar.gz file is both: tar archive first, gzip compression around it.

Always Identify First

file unknown

Do not guess the decoder from the filename. Filenames can lie, especially in puzzles.

Safe Archive Workflow

mkdir -p ~/extract-work
tar -tf archive.tar
tar -xf archive.tar -C ~/extract-work
find ~/extract-work -maxdepth 2 -type f -print

List before extracting. Extract into a scratch directory so an archive cannot spray files across your current workspace.

Compression Workflow

gzip -d file.gz
bzip2 -d file.bz2
base64 -d encoded.txt > decoded.bin

Decompression tools often replace the compressed file with the decompressed output. Work on copies when experimenting.

Common Confusions

• tar does not mean compression by itself.
• gzip and bzip2 usually handle one byte stream, not a tree of files.
• base64 is encoding, not encryption and not compression.
• file is a strong hint, not an oracle.

Proof Check

Given an unknown file, run file first, choose the next command, and write down why that command fits the reported type.

Docs Pointers

• Run man file, man tar, man gzip, man bzip2, and man base64.
• Read the GNU tar manual section on operations.