environment

Shell and environment configuration

crontab

Tested crontab examples: schedule syntax, @reboot/@daily shortcuts, per-user crontabs, and the PATH gotcha that breaks jobs that work fine in your shell.
env and printenv

Tested env and printenv examples: reading a variable, running one command with a changed environment, and what -S fixes on a shebang line.
which

Tested which examples: how PATH order decides the answer, the shell script Debian ships in place of the GNU program, and the deprecation that was reversed.
Environment variables and PATH

Why a program you can see and execute is still not found, how the environment is copied into each new process, and where PATH is really set on Debian.
Terminal, shell and tty

What a terminal, a tty, a shell and a console each actually are, why Ctrl-D is not a signal, and why Ctrl-S appears to freeze everything.
Add a directory to your PATH permanently

How to put a directory on PATH for good on Debian, why ~/.local/bin needs no configuration at all, and what to do for any other directory.
command not found

The shell says this for a command that is missing, one that is not on PATH, one it remembers in the wrong place, and a script whose interpreter is gone.
Works in the shell, fails in cron

cron hands a job five variables and a PATH of /usr/bin:/bin, runs it from your home directory under dash, and sends the failure somewhere you are not looking.
which vs type vs command -v

which searches PATH and nothing else, so it reports that cd does not exist. What type and command -v answer instead, and which one belongs in a script.