if, then, else, fi

Use

if [[ -e "$1" ]]; then
  printf 'exists\n'
else
  printf 'missing\n'
fi

What It Does

if runs one branch when a test or command succeeds and another branch when it fails. then, else, and fi are part of the same Bash compound command.

Command Success

if curl -fsS https://example.org >/dev/null; then
  printf 'up\n'
else
  printf 'down\n'
fi

Bash does not require a numeric comparison. It branches on exit status: 0 means success, nonzero means failure.

File Test

if [[ -f ~/src/pages/index.md ]]; then
  printf 'homepage source exists\n'
fi

[[ ]] is Bash’s safer test syntax for strings, files, and patterns.

One-Line Form

if [[ -d ~/src ]]; then printf 'source exists\n'; fi

Use this only when it remains readable. Multi-line form is better for learning and for scripts.

Watch Out

• Every if needs a matching fi.
• Quote variables inside tests.
• Use elif for another condition; do not stack confusing nested if blocks too early.
• Put ; then on the same line if you write a one-liner.

Docs Pointers

• Run help if and help [[.
• Read conditionals, control flow, quoting, and one-liners.