What is an operating system?

• Practically speaking, the operating system manages:

• Processes user application programs as well as essential services, such as the cron service running as crond and known as the cron daemon;
• Resources the allocation of processor time, memory, and I/O devices among the various processes which use them;
• File System including all I/O devices and some things made to look like devices (such as /dev/kmem or /proc in Linux);
• Security Services to protect against inadvertent as well as malicious damage to the file system and other resources (login is only the beginning).