Archiving and compression

Archive versus compress

tar collects multiple paths into one archive; it does not necessarily compress them. The -z flag pipes through gzip, so tar -czf backup.tar.gz project/ both archives and compresses. For extraction, replace -c with -x: tar -xzf backup.tar.gz.

tar flag	Meaning
`-c`	create archive
`-x`	extract archive
`-t`	list contents without extracting
`-z`	gzip compression
`-j`	bzip2 compression
`-J`	xz compression
`-f`	next argument is the archive name
`-v`	print names; optional
`-C dir`	extract into another directory

Always run tar -tzf backup.tar.gz before extracting an archive you did not create, so you can see whether it unpacks into its own folder or scatters files into the current one. --exclude='*.log' leaves matching files out when creating an archive.

flowchart LR
  D["Many files<br/>project/"] -- "tar -cf" --> T["project.tar<br/>one archive"]
  T -- "gzip / xz" --> C["project.tar.gz<br/>smaller"]
  C -- "tar -tzf" --> L["List contents<br/>check first"]
  L -- "tar -xzf -C /tmp" --> X["Extracted<br/>in a safe place"]
  %% step D: You start with a directory of many files.
  %% step T: tar collects them into one file, keeping paths and permissions. No size change yet.
  %% step C: A compressor makes it smaller; tar -czf does both steps at once.
  %% step L: List the archive before extracting so you know what it will create.
  %% step X: Extract into a chosen directory with -C so nothing is overwritten by surprise.

Single-file compressors

gzip big.log compresses one file into big.log.gz and replaces the original by default; gzip -k keeps the original. gunzip reverses the operation. Gzip does not accept a directory directly, so archive a directory tree with tar first.

zcat, zless, and zgrep read .gz files without unpacking them, which is how rotated logs such as syslog.2.gz are usually inspected.

Choosing a format

Format	Trade-off
`gzip` (`.gz`)	Fast and available everywhere
`bzip2` (`.bz2`)	Smaller output, slower
`xz` (`.xz`)	Smallest output, slowest to create
`zip`	One file holds many entries; common when sharing with Windows users

zip -r site.zip site/ and unzip site.zip work on whole directories without tar. Already-compressed data such as JPEG images or video barely shrinks, so compressing it again mostly wastes CPU.

Check before and after

du -sh project/ estimates space before or after archiving; do not treat its output as a guaranteed compressed size. After copying an archive to another machine, compare sha256sum backup.tar.gz on both sides to confirm it arrived intact.