Decisions: if, test, and exit codes

Every command reports success or failure

When any command finishes, it hands the shell an exit code: 0 means success, anything else (1-255) means some kind of failure. The special variable $? holds the exit code of the last command.

This is the shell's version of true/false, and it's what if actually checks. A useful pair to see it with: grep -q pattern file searches quietly (q = print nothing) and only reports through its exit code: 0 if found, 1 if not.

if, then, else, fi

if [ -f notes.txt ]; then
  echo "notes.txt exists"
else
  echo "notes.txt is missing"
fi

Surprise: [ is itself a command (called test) that exits 0 or 1. That's why the spaces inside the brackets are mandatory. Its most useful checks:

Always quote variables inside [ ] ("$age", not $age), or empty values break the test.

if without if: && and ||

Because every command reports an exit code, bash can chain commands on success or failure without writing a full if block:

• a && b: run b only if a succeeded (exited 0)
• a || b: run b only if a failed (exited non-zero)

So mkdir reports && cd reports only enters the directory if it was actually created, and grep -q error app.log || echo "clean" speaks up only when nothing was found. These two operators are everywhere in real scripts and one-liners — they are the practical payoff of exit codes even when you never type $?.

Don't confuse || with the pipe | from unit 5: the pipe moves data between commands regardless of success; || and && decide whether the next command runs at all.