An Operating System Structure

• Three software layers are typically used to describe the Unix or Linux system:

• The kernel layer: This runs the hardware and allocates resources, sharing them where necessary. The file system is often a separate process, as are other parts of the kernel and various service daemons.

• The shell and other basic utilities layer: When you type in a command, it's to the shell at the command-line interface (or CLI). Some commands are built-in to the shell (e.g., cd for change directory) and others are programs or filters (like grep, global regular expression print). All will use system calls to get kernel services, and some will use X Window services for graphics.

• The applications layer: Where programs like firefox and vi (actually vim in our lab) will run, also using system calls to request services from the kernel.