Aujourd'hui j'ai découvert tldr (faites: sudo apt install tldr). C'est marrant. Par exemple faites: tldr tar. Ça donne ceci:  
(EDIT: il y a aussi https://cheat.sh , par exemple https://cheat.sh/tar )

tar
Archiving utility.
Often combined with a compression method, such as gzip or bzip.

- Create an archive from files:
  tar cf {{target.tar}} {{file1 file2 file3}}

- Create a gzipped archive:
  tar czf {{target.tar.gz}} {{file1 file2 file3}}

- Extract an archive in a target folder:
  tar xf {{source.tar}} -C {{folder}}

- Extract a gzipped archive in the current directory:
  tar xzf {{source.tar.gz}}

- Extract a bzipped archive in the current directory:
  tar xjf {{source.tar.bz2}}

- Create a compressed archive, using archive suffix to determine the compression program:
  tar caf {{target.tar.xz}} {{file1 file2 file3}}

- List the contents of a tar file:
  tar tvf {{source.tar}}

- Extract files matching a pattern:
  tar xf {{source.tar}} --wildcards {{"*.html"}}