Linux Basic — Input Validation Loops

Input Validation Loops

Interactive scripts must validate everything. A robust loop re-prompts until valid input or a retry limit is reached.

---

Pattern: retry with default

max_retries=3
retries=0
while true; do
    read -rp "Port [8080]: " port
    port=${port:-8080}
    if [[ "$port" =~ ^[0-9]+$ ]] && (( port >=1 && port <= 65535 )); then
        break
    fi
    ((retries++)) || true
    if (( retries >= max_retries )); then
        echo "too many attempts" >&2; exit 1
    fi
    echo "Please enter a valid port (1-65535)" >&2
done

# use $port

Always normalise the input (${var:-default}), validate, and copy to a safe variable.

---

Trim whitespace

trim() { printf '%s' "$1" | sed -E 's/^[[:space:]]+//; s/[[:space:]]+$//' ; }
input=$(trim "$input")

Use read -r to avoid backslash magic.

---

Safe yes/no with defaults

confirm() {
    local prompt default reply
    prompt="$1"; default="$2"
    while true; do
        read -rp "$prompt" reply
        reply=${reply:-$default}
        case "${reply,,}" in
            y|yes) return 0;;
            n|no)  return 1;;
            *) echo "Please answer yes or no.";;
        esac
    done
}

if confirm "Delete all files? [y/N] " "n"; then
    echo "deleting"
fi

case "${reply,,}" lowercases the answer (Bash-only). Use tr '[:upper:]' '[:lower:]' for POSIX.

---

Limit retries and provide an escape

• Offer a q to quit
• Use a timeout to resume with a default
• Log invalid attempts for audit if needed

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