Shell basics — pipes and redirection
This chapter starts from the three streams of standard input, standard output, and standard error, and explains when to use pipes vs. redirection. It covers the difference between overwriting `>` and appending `>>`, why standard error doesn't ride pipes, and combinations with grep.
Shell scripting and environment variables — export and process inheritance
This chapter explains the difference between shell variables and environment variables from the angle of whether they're inherited by child processes. It covers what export does, correct use of the env command, the difference between tilde expansion and absolute paths, and how to choose a way to run a script.