## Description and Evaluation Criteria of the Project
**Out:** January 20, 2020
**Due:** February 3, 2020
### Project Goals:
1. To dust off your C programming skills.
2. To design and implement a simple shell.
This project consists of designing a C program to serve as a shell interface that accepts user commands and then
executes each command in a separate process. Your implementation will support input and output redirection, as well as
pipes as a form of IPC between a pair of commands. Completing this project will involve using the UNIX `fork()`, `exec()`,
`wait()`, `dup2()`, and `pipe()` system calls.
### overview
A shell interface gives the user a prompt, after which the next command is entered. The example below illustrates the
prompt `mysh:~$ ` and the user’s next command: `cat prog.c`. (This command displays the file `prog.c` on the terminal using the
UNIX `cat` command.)
“`mysh:~$ cat prog.c“`
One technique for implementing a shell interface is to have the parent process first read what the user enters on the
command line (in this case, `cat prog.c`) and then create a separate child process that performs the command. Unless
otherwise specified, the parent process waits for the child to exit before continuing. This is similar in
functionality to the new process creation illustrated in Figure 3.9. However, UNIX shells typically also allow the
child process to run in the background, or concurrently. To accomplish this, we add an ampersand `&` at the end of
the command. Thus, if we rewrite the above command as
“`mysh:~$ cat prog.c &“`
the parent and child processes will run concurrently. The separate child process is created using the `fork()` system
call, and the user’s command is executed using one of the system calls in the `exec()` family (as described in Section
3.3.1). A C program that provides the general operations of a command-line shell
is supplied in the code block bellow (Figure 3.36 in the book). The `main()` function presents the prompt `mysh:~$ ` and
outlines the steps to be taken after input from the user has been read. The `main()` function continually loops as long
as `should_run` equals 1; when the user enters `exit` at the prompt, your program will set `should_run` to 0 and terminate.
“`c
#include
#include
#define MAX LINE 80 /* The maximum length command */
int main(void) {
char *args[MAX LINE/2 + 1]; /* command line arguments */
int should_run = 1; /* flag to determine when to exit program */
while (should_run){
printf(“mysh:~$ “);
fflush(stdout);
/** After reading user input, the steps are:
* (1) fork a child process using fork()
* (2) the child process will invoke execvp()
* (3) parent will invoke wait() unless command included &
*/
}
return 0;
}
“`
This project is organized into several parts:
1. Creating the child process and executing the command in the child
2. Providing a history feature
3. Adding support of input and output redirection
4. Allowing the parent and child processes to communicate via a pipe
### Executing Command in a Child Process
The first task is to modify the `main()` function in code above so that a child process is forked and executes the
command specified by the user. This will require parsing what the user has entered into separate tokens and storing
the tokens in an array of character strings (args in the code). For example, if the user enters the command `ps -ael`
at the `mysh:~$ ` prompt, the values stored in the args array are:
“`
args[0] = “ps”
args[1] = “-ael”
args[2] = NULL
“`
This args array will be passed to the `execvp()` function, which has the following prototype:
“`c
execvp(char *command, char *params[]);
“`
Here, command represents the command to be performed and params stores the parameters to this command. For this
Project, the `execvp()` function should be invoked as execvp(args[0], args). Be sure to check whether the user
included `&` to determine whether or not the parent process is to wait for the child to exit.
### Creating a History Feature
The next task is to modify the shell interface program so that it provides a **history** feature to allow a user to
execute the most recent command by entering `!!`. For example, if a user enters the command `ls -l`, she can then execute
that command again by entering `!!` at the prompt. Any command executed in this fashion should be *echoed* on the user’s
screen, and the command should also be placed in the history buffer as the next command. Your program should also
manage basic error handling. If there is no recent command in the history, entering `!!` should result in a message
“No commands in history.”
### Redirecting Input and Output
Your shell should then be modified to support the `>` and `<` redirection operators, where `>` redirects the output
of a command to a file and `<` redirects the input to a command from a file. For example, if a user enters
```mysh:~$ ls > out.txt“` the output from the ls command will be redirected to the file `out.txt`. Similarly, input can be
redirected as well. For example, if the user enters “`mysh:~$ sort < in.txt``` the file `in.txt` will serve as input to the
`sort` command. Managing the redirection of both input and output will involve using the `dup2()` function, which
duplicates an existing file descriptor to another file descriptor. For example, if fd is a file descriptor to the
file `out.txt`, the call ```dup2(fd, STDOUT FILENO);``` duplicates fd to standard output (the terminal). This means that
any writes to standard output will in fact be sent to the `out.txt` file. You can assume that commands will contain
either one input or one output redirection and will not contain both. In other words, you do **not** have to be concerned
with command sequences such as ```sort < in.txt > out.txt.“`
### Communication via a Pipe
The final modification to your shell is to allow the output of one command to serve as input to another using a
`pipe`. For example, the following command sequence “`mysh:~$ ls -l | less“` has the output of the command `ls -l`
serve as the input to the less command. Both the `ls` and `less` commands will run as separate processes and will
communicate using the UNIX `pipe()` function described in Section 3.7.4. Perhaps the easiest way to create these
separate processes is to have the parent process create the child process (which will execute `ls -l`). This child will
also create another child process (which will execute `less`) and will establish a pipe between itself and the child
process it creates. Implementing pipe functionality will also require using the `dup2()` function as described in the
previous section. Finally, although several commands can be chained together using multiple pipes, you can assume
that commands will contain only one pipe character and will not be combined with any redirection operators.
### Evaluation
You need to submit the C source code to your shell, and a Makefile that compiles the shell in the Moodle. The TA will
use your Makefile to compile and run your program in the course Virtual Machine (VM).
[Here is a quick tutorial on makefile](http://www.cs.colby.edu/maxwell/courses/tutorials/maketutor/), in case you are
not familiar. The TAs then grade your project according to the following table:
| Item | Mark |
| ————- |:————-:|
| Accurate Makefile and no error in compilation | 2 |
| Simple shell commands | 2 |
| History feature | 2 |
| Handle redirection | 2 |
| Handle pipe | 2 |
| **Sum** | **10** |