USER MANUAL FOR THE SHELL:
To run the shell, go to the location of the shell by typing 'cd ', once in the same directory of myshell.py, type 'python3 myshell.py' to run the shell.
OPERATION AND COMMANDS OF THE SHELL:
This shell can do many things. It supports a variety of commands and background execution of programs. This manual will outline how to use every component of this shell.
cd command: This command allows you change your current working directory. 'cd ' will change your current working
directory to . Typing 'cd' will print your current working directory.'cd ..' will move your up or 'back' on directory.
So, if you are /usr/bin/tmp, 'cd ..' moves you to /usr/bin. This command supports i/o redirection.
clr command: This command clears the screen.
dir command: Typing 'dir' will list all files in your current working directory. Typing 'dir ' will list all files in . This command supports i/o redirection.
environ command: This command displays all the environment strings. This command supports i/o redirection.
echo command: This command prints to the terminal. Typing 'echo ' will print to the terminal. This command supports i/o redirection.
help command: Typing 'help ' will display help on .
pause command: This command pauses the terminal until the Enter key is pressed.
quit command: This command terminates the shell. helpmore command: This command prints this manual. NOTE: This command will not work unless 'readme.txt' and 'myshell.py' are in the same folder.
ENVIRONMENT CONCEPTS:
When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form 'name=value'. Typing 'environ' will list all environment strings. When Bash invokes an external command, the variable '$_' is set to the full pathname of the command and passed to that command in its environment.
I/O REDIRECTION:
Input output redirection is an extremely useful feature. It allows you to take input from a batch file and also allows you to put the output of different commands into batch files. The '>' symbol is used for output (STDOUT) redirection. For example, typing 'dir > output.txt' will put the output of 'dir' into output.txt. > will override everything in output.txt. If you do not wish for the contents of output.txt to be overwritten, using '>>' instead of '>' will append to output.txt rather than overwritting it. The '<' symbol is used for input(STDIN) redirection. Rather than taking input for commands from inside the shell we can take input from batch files. For example, input.txt contains a pathname. Let's call it for simplicity. If we type 'dir < input.txt' the shell will list the files in . 'dir < input.txt' is the exact same as doing 'dir ' in this case.
BACKGROUND EXECUTION:
The shell also supports background execution of programs. A background process executes independently of the shell, leaving the terminal free for other work. To run a process in the background, include an & (an ampersand) at the end of the command you use to run the job. For example, writing 'python3 test.py &' will mean that that program runs as a subprocess.