In this assignment, you will write a complete C program that will act as a simplecommand-line interpreter (i.e., a shell) for th
e Linux kernel. In writing your shell, you areexpected to use the fork-exec-wait model discussed in class. In particular, you are toimplement the following:• Loop continuously until the user enters quit, which exits your shell.• Inside the loop, you will print your "minor5" prompt and read in the user’scommand, which may consist of a Linux command with 0 or more options andarguments supported by the command. You are expected to read in and processonly 1 command at a time with no pipelining or redirection.• In a child process, you are to execute the command as given, including alloptions and arguments given. If the command is not valid, rather than display an"exec failed" message as shown in class examples, you will simply print outthe command itself with "command not found" as shown in the SAMPLEOUTPUT and then exit the child process. The parent process should wait for thechild process to finish.If you have any questions about this, please contact your instructor, TAs, or IAsassigned to this course to ensure you understand these directions.SAMPLE OUTPUT (user input shown in bold):$ ./a.outminor5> lsa.out grades.txt rec01.txt testdir phone.txt route.txt who.txtdu.txt rec01.c file1 rec01sol.c minor5.cminor5> ls -a -l -ttotal 144-rwx------ 1 cat0299 cat0299 7835 Oct 14 17:39 a.outdrwx------ 4 cat0299 cat0299 4096 Oct 14 17:39 .-rw------- 1 cat0299 cat0299 2665 Oct 14 17:39 minor5.c-rw------- 1 cat0299 cat0299 33 Oct 5 03:30 du.txt-rw------- 1 cat0299 cat0299 33 Oct 5 01:28 file1-rw------- 1 cat0299 cat0299 333 Oct 5 01:02 route.txt
1 answer:
Answer:
/ to access the input scanf()
// and output printf()
#include <stdio.h>
// to access the functions like
// pipe(), fork(), execvp(), dup2()
#include <unistd.h>
// to access the string functions like
// strtok()
#include <string.h>
// to access function wait()
#include <sys/wait.h>
#include <stdlib.h>
int main()
{
// declare a variable to hold the process id
pid_t p_id;
// declare a variable to hold the index value
int array_index;
// declare the string to hold the user input
// as command
char userIn_Command[128];
// use a continuous loop
while (1)
{
// display the prompt for the user
printf("minor5> ");
// read the input from the user
scanf("%[^\n]", userIn_Command);
// check the condition that whether the user
// inputs a command called "quit"
// If the user inputs quit command then exit from
// the script
if (strcmp(userIn_Command, "quit") == 0)
{
printf("\n");
break;
}
// if there are any usage of pipelining or redirection
// display the error message and exit from the script
if (strchr(userIn_Command, '|') != NULL || strchr(userIn_Command, '>') != NULL ||
strchr(userIn_Command, '<') != NULL)
{
printf("Error: Cannot perform the multiple command operations or directions\n");
break;
}
// declare the variables to hold the process
int p_pids[2];
// create the system call to pipe() from kernal
// and display the error message
if (pipe(p_pids) < 0)
{
printf("Error: Pipe creation failed!\n");
}
// create the child process
p_id = fork();
// condition to check whether the fork() is
// created. If so, display an error message
if (p_pids < 0)
{
printf("Error: fork() failed!\n");
break;
}
// if the child process is created
if (p_id == 0)
{
// close pipe in child process
close(p_pids[0]);
// create duplicate of the standard output
dup2(p_pids[1], STDOUT_FILENO);
// close the pipe in child process
close(p_pids[1]);
// declare a variable to store path
// to execute the command
char *command_path;
// declare an array of string to hold the options
char * args[32];
// tokenize the command by using the delimiter at " "(single space)
char *cmd_token = strtok(userIn_Command, " ");
// store the token value
command_path = cmd_token;
args[0] = cmd_token;
array_index = 1;
// loop until all the options in the command are
// tokenized
while (1)
{
// get the next token
cmd_token = strtok(NULL, " ");
// condition to check whether the token is null
// or not
if (cmd_token == NULL)
{
break;
}
// store the token if it is not null
args[array_index] = cmd_token;
// increment the index
array_index++;
}
// last parameter to the command should be NULL */
args[array_index] = NULL;
/* calling exec function with command path and parameters */
if (strcmp(args[0], "cd") == 0 || strcmp(args[0], "history") == 0 ||
strcmp(args[0], "exit") == 0)
{
printf("%s: Command not found\n", args[0]);
break;
}
if (execvp(command_path, args) < 0 )
{
printf("%s: Command not found\n", args[0]);
break;
}
}
else
{
/* closing writing end of pipe in parent process */
close(p_pids[1]);
/* reading ouput written to pipe in child process and
* writing to console */
while (1)
{
char output[1024];
int n = read(p_pids[0], output, 1024);
if (n <= 0)
{
break;
}
output[n] = '\0';
printf("%s", output);
}
/* closing read end of pipe1 */
close(p_pids[0]);
/* waiting until child process complete its execution */
wait(NULL);
}
/* skipping newline character read while scanf() */
getchar();
}
exit(0);
}
Explanation:
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