Mặt nạ mạng là một chuỗi 32-bit mà giấu đi một phần mạng lưới của một địa chỉ IP, do đó chỉ có một phần máy chủ (máy tính) của địa chỉ vẫn còn. Ví dụ, mặt nạ mạng trông giống như một địa chỉ IP, nhưng trên thực tế ẩn 24 bit đầu tiên của địa chỉ IP để chỉ cuối cùng 8 | Advantages 1128708 home postgres base 16976 1136024 home postgres base 140 home postgres global 82024 home postgres pg_xlog 12 home postgres pg_clog 1218232 home postgres The most difficult part of this process is to keep track of each user s usage and their files both the name and the size. Why is this difficult you may ask Shell scripting languages have very primitive support for complex logic operations and data structures. We would need to come up with an alternative mechanism to temporarily store this individual data and then process and sort it once we finish going through all the users directories. The final part of the application would involve sending an e-mail both to the offending users and the administrator using the mail or sendmail commands. You may have noticed that we glossed over the section regarding sed and awk here. There s a reason for this -shell scripting languages allow us to develop scripts that invoke shell commands easily and without much effort. In addition the development cycle is rather simple we use an editor to modify and debug the script and then simply execute it from the command line. On the other hand shell scripts fall apart when we need to implement any type of complex logical operations or store relational or structured data since shell scripting languages were not designed for this purpose. On the flip side how would we fare if we developed this application using Perl We would retain some of the advantages of shell scripting but also have access to more advanced language features. Since Perl allows us interact with the shell we can use the df command to get the disk usage report and then use pattern matching expressions to easily parse and extract the percentage values. If we find a partition that exceeds the threshold value we can use the internal readdir function along with a for loop to iterate through all of the main directories on that filesystem. Perl integrates well with the underlying operating system to abstract .
