Some operating systems include the command interpreter in the kernel. Others,
such as Windows and UNIX, treat the command interpreter as a special program
that is running when a job is initiated or when a user first logs on (on interactive
systems)

[shell也是一个程序,这个程序为用户提供系统功能的接入]

In one approach, the command interpreter itself contains the code to
execute the command. For example, a command to delete a file may cause
the command interpreter to jump to a section of its code that sets up the
parameters and makes the appropriate system call. In this case, the number of
commands that can be given determines the size of the command interpreter,
since each command requires its own implementing code.
An alternative approach—used by UNIX, among other operating systems
—implements most commands through system programs. In this case, the
command interpreter does not understand the command in any way; it merely
uses the command to identify a file to be loaded into memory and executed.
Thus, the UNIX command to delete a file
rm file.txt
would search for a file called rm, load the file into memory, and execute it with
the parameter file.txt. The function associated with the rm command would

be defined completely by the code in the file rm. In this way, programmers can

add new commands to the system easily by creating new files with the proper
names. The command-interpreter program, which can be small, does not have
to be changed for new commands to be added.

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