Goal: This lab helps you understand the concept of processes, how to create, and how to terminate a process. In addition, you are expected to get familiar with system calls related to processes and file operations.
Introduction: As the first step of the UNIX Shell project, this lab assignment asks you to build a simple SHELL interface that accepts user commands, creates a child process, and executes the user commands in the child process. The SHELL interface provides users a prompt after which the next command is entered. The example below illustrates the prompt sh> and the user's next command: cat prog.c. This command displays the content of the file prog.c on the terminal using the UNIX cat command.
sh> 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 (i.e. cat prog.c), and then creates a separate child process that executes the command. Unless otherwise specified, the parent process waits for the child to exit before continuing. This is similar in functionality to what is illustrated in Figure 1. However, UNIX shells typically also allow the child process to run in the background - or concurrently as well by specifying the ampersand (&) at the end of the command. By rewriting the above command as
sh> cat prog.c &
the parent and child process now run concurrently.
The separate child process is created using the fork() system call and the user's command is executed by using one of the system calls in the exec() family (for more details about the system call, you can use the man command).
Figure 1. Process Creation & Termination see image.
A C program that provides the basic operations of a command line shell is supplied in the file shell.c, which you can download from Canvas. This program is composed of two functions: main() and setup(). The setup() function reads in the user's next command (which can be up to 80 characters), and then parses it into separate tokens that are used to fill the argument vector for the command to be executed. (If the command is to be run in the background, it will end with '&, and setup() will update the parameter background so the main() function can act accordingly. This program is terminated when the user enters < Control>< D> and setup() and invokes exit().
The main() function presents the prompt COMMAND-> and then invokes setup(), which waits for the user to enter a command. The contents of the command entered by the user are loaded into the args array. For example, if the user enters ls -l at the COMMAND-> prompt, args[0] will be set to the string ls and args[1] will be set to the string l. (By "string", we mean a null-terminated, C-style string variable.)
#include < stdio.h>
#include < unistd.h>
#define MAX_LINE 80
/** setup() reads in the next command line string stored in inputBuffer, separating it into distinct
tokens using whitespace as delimiters. setup() modifies the args parameter so that it holds pointers
to the null-terminated strings that are the tokens in the most recent user command line as well as a
NULL pointer, indicating the end of the argument list, which comes after the string pointers that
have been assigned to args. */
void setup(char inputBuffer[], char *args[],int *background)
{
/** full code available in the file shell.c */
}
int main(void)
{
char inputBuffer[MAX_LINE]; /* buffer to hold the command entered */
int background; /* equals 1 if a command is followed by '&' */
char *args[MAX_LINE/+1]; /* command line arguments */
while (1){
background = 0;
printf(" COMMAND->n");
setup(inputBuffer,args,&background); /* get next command */
/* the steps are:
(1) fork a child process using fork()
(2) the child process will invoke execvp()
(3) if background == 0, the parent will wait,
otherwise returns to the setup() function. */
}
}
Figure 2. Outline of shell.c
This lab assignment asks you to create a child process and execute the command entered by a user. To do this, you need to modify the main() function in shell.c so that upon returning from setup(), a child process is forked. After that, the child process executes the command specified by a user.
As noted above, the setup() function loads the contents of the args array with the command specified by the user. This args array will be passed to the execvp() function, which has the following interface:
execvp(char *command, char *params[]);
where command represents the command to be performed and params stores the parameters to this command. You can find more information on execvp() by issuing the command "man execvp". Note, you should check the value of background to determine if the parent process needs to wait for the child to exit or not.