A simple shell within linux, maintainning environment variables and implement basic tools such as cd,help

Command:

• ·cd [directory]: Change the current working directory to [directory directory parameter]. If there is no directory parameter, the current directory is displayed; if the directory does not exist, an error message is output.
• ·clr: Clear the screen.
• ·dir [directory]: List the contents of the directory [directory].
• ·echo [comment]: Multiple spaces and tabs in the comment will be reduced to one space and displayed on the screen.
• ·exec: execute executable files, such as scripts, C language source files, etc.
• ·quit: Quit the shell, support parameter form.
• ·exit: Exit the shell, no parameters are supported.
• ·environ: List all environment variables.
• ·help: Display the user manual, and use the more command to filter.
• ·pwd: Display the current path.
• ·time: Display the current time.
• ·shift: todo..

I/O redirection pipeline Program environment Background program execution

it also support executable application in /user/bin