Basically in an operating system there are these two elements:
* The kernel (= kernel): which performs the fundamental functions of
the operating system such as memory management, processes, files ...
* The shell: At its base, a shell is simply a macro processor that executes commands.
-The term macro processor means functionality where text and symbols are expanded to create larger expressions.
-A Unix shell is both a command interpreter and a programming language.
Parsing
2. The separations
1. Pipes
3. Command and arguments
4. Protections
5. The <,>, >> redirects
6. Environment variables
The execution
1. Redirects
2. Env, export, unset
3. Exit and $?
4. Pipes / signals / process links
https://medium.com/@nickolasteixeira/want-to-build-a-simple-shell-heres-how-you-do-it-75890647bae8
https://medium.com/dev-genius/lets-build-a-linux-shell-part-i-954c95911501
https://medium.com/better-programming/how-bash-works-8424becc12f3
https://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html
https://man7.org/linux/man-pages/dir_all_by_section.html
https://unix.stackexchange.com/questions/159513/what-are-the-shells-control-and-redirection-operators
https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html
https://www.geeksforgeeks.org/pipe-system-call/?ref=rp
https://www.geeksforgeeks.org/wait-system-call-c/?ref=rp
https://www.geeksforgeeks.org/fork-system-call/?ref=rp
http://www-igm.univ-mlv.fr/~dr/NCSPDF/chapitre11.pdf
https://www.geeksforgeeks.org/making-linux-shell-c/
https://www.quora.com/What-is-the-3rd-parameter-in-the-main-function-char-envp#:~:text=The%20non%2Dportable%2Dbut%2D,As%20always%2C%20I%20recommend%20experimenting
https://chrtophe.developpez.com/tutoriels/minisysteme/#L11
https://stackoverflow.com/questions/6697753/difference-between-single-and-double-quotes-in-bash
https://brennan.io/2015/01/16/write-a-shell-in-c/
https://www.youtube.com/watch?v=5BE9qM79fgM
https://sites.cs.ucsb.edu/~manish/discussion/week2.pdf
https://www.geeksforgeeks.org/chdir-in-c-language-with-examples/
Important
https://theswissbay.ch/pdf/Gentoomen%20Library/Programming/Bash/O%27Reilly%20bash%20CookBook.pdf