Bash: Forks in the Road

Until today, your scripts have been conveyor belts: top to bottom, no thought. Today they grow opinions. Conditionals let a script look at the world and pick a path — and in a plot twist that will never stop being funny, all that decision-making power rests on a single question inherited from part 05: did the last thing exit zero?

The if

if grep -q "dragon" prophecy.txt; then
  echo "Pack the fireproof cloak."
fi

Anatomy, because the punctuation is load-bearing:

• if … then on one line, separated by a semicolon (or a real line break)
• grep -q — the -q (quiet) flag: don't print matches, just report success or failure. It's a pure yes/no probe
• fi — "if" backwards, because bash closes doors the way it opens them, and the 1980s thought this was charming. It is. You'll miss fi in every other language.

And the else:

if grep -q "dragon" prophecy.txt; then
  echo "Pack the fireproof cloak."
else
  echo "Prophecy is boring. Take snacks."
fi

And elif — "else if", for scripts with ranges of opinions:

count=$(grep -c "dragon" prophecy.txt)
if [ "$count" -eq 0 ]; then
  echo "No dragons. Suspicious, honestly."
elif [ "$count" -eq 1 ]; then
  echo "One dragon. Manageable."
else
  echo "Multiple dragons. Reconsider career."
fi

The brackets: test in a costume

Here's the secret hiding in plain sight: [ … ] is not punctuation. It's a command — an alias for test, a program that checks conditions and exits zero (true) or non-zero (false). That's why the spaces around the brackets are mandatory: bash needs to see it as a command name followed by arguments.

[ "$age" -gt 100 ]     # the command "test", wearing square brackets

Forget a space and bash tries to execute ["$age" as a command name and fails in ways that generate exactly zero useful error messages. This single missing space has consumed more beginner hours than any other bug in this course. Put it in your notes. Tattoo optional.

Which brackets, though? Bash offers two dialects:

• [ … ] — the classic. Portable: works in every shell ever made. Strict, minimal, occasionally pedantic.
• [[ … ]] — the bash-only upgrade. More forgiving and more powerful: no word splitting surprises, pattern matching, and && / || operators inside.

This course uses [[ ]] (you're here for bash), but you'll meet [ ] in every old script on every server, so learn to read both.

The test vocabulary

Numbers, strings, files — three families:

# numbers (mnemonic: 'g'ter, 'l'ess, 'e'qual — the letters are the operators)
[ "$count" -eq 3 ]     # equal
[ "$count" -ne 3 ]     # not equal
[ "$count" -gt 3 ]     # greater than
[ "$count" -lt 3 ]     # less than

# strings
[ "$name" = "Gandalf" ]    # equal (yes, one = here; bash hates consistency)
[ "$name" != "Sauron" ]    # not equal — also a valid security policy
[ -z "$name" ]             # zero length: is the string empty?
[ -n "$name" ]             # non-zero: does it have content?

# files — the three you'll use weekly
[ -f potion.txt ]     # exists and is a regular file
[ -d potions ]        # exists and is a directory
[ -s potion.txt ]     # exists and is non-empty

And the connectors:

[[ -f potion.txt && -s potion.txt ]]    # AND: exists AND has content
[[ "$age" -lt 18 || "$age" -gt 200 ]]   # OR: outside the mortal range
[[ ! -f dragon.txt ]]                   # NOT: dragon file is missing

&& and || also work between commands, which is a lovely piece of shorthand you'll use forever:

mkdir -p backups && cp save.log backups/    # copy only if the folder got made
grep -q "ok" health.log || echo "ALERT: not ok"    # shout only if the check failed

The granddaddy: does the file exist

The single most common conditional in real scripts:

if [[ -f config.env ]]; then
  source config.env
  echo "Config loaded."
else
  echo "No config found. Running on vibes."   # (vibes are not a supported feature)
fi

case: the readable one

When you're comparing one value against many options, chains of elif turn into a staircase. case is the elevator:

read -p "Choose your familiar: " pet
case "$pet" in
  cat)     echo "Aloof. Powerful. Judges your commits." ;;
  owl)     echo "Mail service included. Hooting extra." ;;
  toad)    echo "Moist companion. Low maintenance, high warts." ;;
  dragon)  echo "Ambitious. Please check your insurance." ;;
  *)       echo "Unknown familiar. It chose YOU." ;;
esac

Read it: case VALUE in pattern) body ;; — each pattern tries in order, * is the catch-all "anything else", and esac closes it, because yes, "case" backwards, they really did that consistently.

This pattern is everywhere in real scripts: argument parsing, install menus, "which OS am I on" checks. It's also the most readable spell in bash, which given the competition is not a high bar, but still.

Your quest

• Write weather.sh: a variable holding "rain", and an if/else that packs an umbrella or sunglasses accordingly
• Deliberately remove a space before ]]. Meet the error. Return the space. Never speak of it again.
• Write a check: [[ -d backups ]] || mkdir backups — the "ensure folder" one-liner you'll use in every script from now on
• Port the familiar chooser to case, add your own animal

Next: part 08 — The Loop, where one spell learns to cast itself a thousand times — for, while, and the file-iterating moves that define the shell.

Filed under Wizardy of Bash — part 07 of 15 · Previous: Labelled Boxes · Next: The Loop