Control Flow

Branch on conditions using test expressions and case statements.

if / elif / else

if [[ $user == "admin" ]]; then
  echo "welcome"
elif [[ -n ${user:-} ]]; then
  echo "hi $user"
else
  echo "anonymous"
fi

Use [[ ... ]] in Bash for safer tests (no glob expansion, better operators).

Test syntaxes

• [[ ... ]] Bash keyword (preferred)
• [ ... ] POSIX test (single bracket)
• test ... command

Common string tests in [[:

• [[ -z $s ]] empty, [[ -n $s ]] non-empty
• [[ $a == $b ]] equality (quote variables)
• [[ $s == foo* ]] glob match; =~ for regex

File tests

[[ -f file ]]   # regular file
[[ -d dir ]]    # directory
[[ -s file ]]   # size > 0
[[ -r file ]]   # readable; -w writable; -x executable

case patterns

case "$1" in
  start|up)   echo starting ;;
  stop|down)  echo stopping ;;
  *)          echo "usage: $0 {start|stop}" ;;
esac

Patterns use shell globs; separate with |.

Short‑circuit operators

cmd || echo "failed"
check && deploy

|| runs the right side if the left fails (non‑zero); && runs on success.

Arithmetic conditions

x=3; y=5
if (( x < y )); then echo less; fi

(( ... )) evaluates arithmetic without $ sigils on variables.

Exit status

Commands return 0 on success, non‑zero on failure. Use $? to read the last status.

Summary

• Prefer [[ for tests; use file and string operators
• Use case for multiple patterns; rely on &&/|| for compact control flow