environment
Shell and environment configuration
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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.
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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.
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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.
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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.
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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.
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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.