Don’t delete production

The shell does exactly what you tell it, right now, with no undo. Most famous shell disasters come down to a few small mistakes, and a few habits stop nearly all of them.

Here’s a line from a real kind of cleanup script. What could go wrong?

rm -rf "$BUILD_DIR"/*

If BUILD_DIR is empty or unset, Bash happily runs rm -rf /*. A typo in a variable name does it too, because Bash treats unknown variables as empty. Two defenses:

• ${BUILD_DIR:?} stops the script with an error if the variable is empty.
• set -u makes any unset variable an error.

And a stray space is just as bad: rm -rf / tmp/build deletes / and tmp/build.

• Preview: put echo in front, or run ls with the same pattern first.
• Quote every variable: "$file".
• Use set -euo pipefail in scripts, and -- before file names that might start with a dash.
• Check where you are: cd /some/dir && rm -r * won’t delete anything if the cd fails, but cd /some/dir; rm -r * deletes from wherever you were.
• Run ShellCheck on every script. It catches most of what this lesson covers.
• Be careful with curl ... | bash: you’re running code you haven’t read. Download it, read it, then run it.