devsetup1-10  Exercise 2: Run the Provisioning Script Twice
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STEP 1: SAVE AND SYNTAX-CHECK
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Save the script from the chapter as ~/machine-setup/bootstrap.sh.

    cd ~/machine-setup
    chmod +x bootstrap.sh
    bash -n bootstrap.sh          # no output means the syntax is valid
    # optional, if you install it: sudo apt install shellcheck ; shellcheck bootstrap.sh

STEP 2: RUN IT TWICE
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    ./bootstrap.sh 2>&1 | tee run1.log
    ./bootstrap.sh 2>&1 | tee run2.log

Run it as your normal user, from the desktop session, not with sudo in front.

WHAT THE SECOND RUN SHOULD SHOW
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    grep -c 'already installed' run2.log      # one line per apt_install section
    grep -E 'Setting up|Unpacking|Get:' run2.log
                                              # should print nothing

Every apt_install line reads "already installed: ...", the sources step does
not edit the file again (the grep finds non-free), the nvm directory exists so
nothing is cloned, and VS Code is already installed so its section is skipped.
Two things still happen by design: "apt-get update" runs, and "nvm install
--lts" reports the version is already installed. The extension loop re-checks
extensions and reports they are already installed.

If the second run installs anything, a check is wrong. Read the log to see
which package name the script asked about, and compare it with
dpkg-query -W -f='${Status}' NAME (the name may differ from the command).

STEP 3: ADD A NEW TOOL, CORRECTLY
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Example: add tmux. Insert a new section before the final echo lines:

    echo "== 8. Extra tools =="
    apt_install tmux

Rules for a correct addition:
  - use apt_install for anything from apt, so it is skipped when present
  - keep the section number and a one-line heading, so the log reads well
  - for a tool NOT from apt, guard it with have TOOL or a directory test, as
    the nvm section does, and never run an installer unconditionally
  - never write a password or token into the script

Run it again: only the new section does any work.

WHY THIS WORKS AS AN ANSWER
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Idempotence (safe to repeat) is the property that makes a provisioning script
trustworthy, and running it twice is the direct test. Logging both runs gives
evidence to compare rather than a feeling that it "looked fine".