Very specifically for learning about GNU/Linux and Unix, I highly recommend the book Classic Shell Scripting by Arnold Robbins and Nelson Beebe (O'Reilly Media, 2005).

ISBN: 9780596005955

I recently wrote the following about it in a post:

This book is extremely readable and gives a very good introduction to the various standard Unix shell commands (grep, sed, awk, tr, sort, to name but a few) and how to tie them together to do useful things. It’s very suitable if you have some experience with the command line at the level of individual commands but now want to see how to do construct more interesting pipelines and scripts. It includes an introduction to regular expressions. The fact that the book is already 20 years certainly means that some explanations and approaches are outdated, but since shell programming is at the core about text processing, almost all contents of the book are still highly relevant today.

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