Security Hardening: Complete rewrite with modern Bash practices

[liusc45]

BREAKING CHANGE: Script now requires Bash 4.0+ and environment variables

Security Improvements

• Removed hardcoded MySQL credentials (use MYSQL_USER, MYSQL_PASSWD, MYSQL_DB env vars)
• Implemented secure temporary file handling with umask 077
• Added command injection prevention with proper quoting
• Created idempotent cleanup trap for EXIT signal
• Added error trap with line number and command context
• Set explicit secure PATH with absolute paths

Code Quality

• Enforced Bash 4.0+ requirement with runtime check
• Implemented set -Eeuo pipefail with safe IFS
• Used readonly for constants and local for function variables
• Fixed infinite loop condition ([ 1 = 1 ] -> while :)
• Converted pages list to array for safety
• Added curl timeout (5 seconds) to prevent hanging
• Improved PID file handling with validation

New Features

• Added -v/--verbose, -d/--debug, -h/--help flags
• Created dedicated logging functions (log_debug, log_verbose, log_error)
• Implemented ISO 8601 timestamps to prevent log injection
• Added stale PID detection and cleanup
• Enhanced process termination with graceful and force kill

Documentation

• Updated README.md with security best practices
• Added comprehensive troubleshooting guide
• Created systemd service example
• Documented environment variable configuration

Compatibility

• Requires Bash 4.0+ (enforced at runtime)
• Requires CentOS/RHEL with yum package manager
• All temporary files have secure permissions (0600)
• ShellCheck compatible with no warnings