自己的作业自己做

There are always three default files open, stdin (the keyboard), stdout (the screen), and stderr (error messages output to the screen). These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script and sending it as input to another file, command, program, or script.

• '< [Filename]' : redirect the stdin to a file.
• '> [Filename]' : redirect the stdout to cover a file.
• '>> [Filename]' : redirect the stdout to append to a file.

The variable environ points to an array of strings called the 'environment', which can be inherited from parent process. In myshell program, there's a environ var called shell that stores the full path of this myshell program. And for all processes created by myshell, there's a var called parent that points out the path of the parent process.

Background processes run behind the scenes and without user intervention. Append a '&' at the end of a command can turn it to a backgound process.