I wrote a book focusing only on text processing commands (includes exercises as well): https://learnbyexample.github.io/cli_text_processing_coreuti...

You can practice some of the exercises using this interactive TUI app: https://github.com/learnbyexample/TUI-apps/tree/main/CLI-Exe...

See also: https://ratfactor.com/slackware/pkgblog/coreutils, https://maizure.org/projects/decoded-gnu-coreutils/ and https://www.pixelbeat.org/docs/coreutils-testing.html