• $? contains the return code of the most recent command
• PREVCOMM=$?
  if [[ $PREVCOMM -ne 0 ]]; then
  echo "whoa, something failed"
  exit 1
  fi
• old school check to see if a variable is defined:
  if [[ $MYVAR"x" == "x" ]]; then
• variables with interesting characters inside:
  "$MYVAR"
  ${MYVAR}
  "${MYVAR}"
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