C Homework Help

C Homework Help

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Shell Creation with Variable Support using C Homework Help.

In this assignment, you will create a shell that allows a user to run programs in the foreground and allows the user to store variables that are remembered by the shell.

To run a program, the user will simply type in the name of the program. Assume our prompt is "bw: " and the program we are running is "program1", it would look like the following: bw: program1 To create a variable, the user types in the variable name followed by an = and then the value wrapped in quotes. Below are some examples: bw: x = "24" bw: name = "Yellow Jacket" If the user wants to see a variable, they can use a built in command that your shell provides support for called echo which takes a single argument which is the variable name: bw: echo name

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Yellow Jacket You should use structs to store the variables. Please note that you can have an array of structs which would allow you to create something similar to a table.

Your shell only needs to support up to 10 variables with a max length of 50 characters per variable. Each variable can be stored as characters and does not need to be converted to a type such as an int or float. You do not need to handle an error condition such as a user entering another variable when the table is full. But, you should show an error if a variable does not exist. Below is an example: bw: echo anothername Error: Variable 'anothername' does not exist The shell prompt that you show is up to you.

The test programs for this assignment are up to you. In order to test the jobs functionality, I would create programs that sleep or use an alarm after an extended amount of time.

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Solution: Main: #include <stdio.h> #include <string.h> #include <unistd.h> #include <sys/wait.h> #include <stdlib.h> /** * compile using gcc main.c -o shell * Define a structure type to store variables * * **/ typedef struct vars{ char name[32]; char value[50]; } vars_t; int main(int argc, char* argv[]){ vars_t vars_array[10]; int count = 0; programminghomeworkhelp.com

char name[32]; // Declare an array with length 32 to store variable name char value[50]; // Declare an array with length 50 to store variable value vars_t var;

/** * * Create an infinite loop to read input from user * * **/ while (1) { char command[82]; // Declare an array with length 82 to store command = name + value printf("sh$ : "); // shell prompt fgets(command,82,stdin); // get input from user command[strcspn(command, "\n")] = 0; // replace \n with \x00 if (strstr(command,"=") != NULL)

// look for = in the command programminghomeworkhelp.com

{ strncpy(name,strtok(command," = "),32); // use strtok to tokenize the string with delimitar first part of the token is name strtok(NULL,"\""); // we need to tokenize with ' " ' since variable value are inside the ` " ` strncpy(value,strtok(NULL,"\""),50); if (strlen(name) < 32 || strlen(value) < 50) // check length of variable name and value { if (count < 10) // we will store only 10 variables { strncpy(var.name,name,32); // store to our local variable we use stncpy to make sure there is no bufferoverflow strncpy(var.value,value,50); vars_array[count] = var; count++; } } } else if (strstr(command, "echo") != NULL) // check for echo in the command programminghomeworkhelp.com

{ size_t i = 0; char test = 1; strtok(command," "); // we will tokenize command using space " " first token is echo and char var_name[32]; // second token is the variable name strncpy(var_name,strtok(NULL," "),32); for (; i < count; i++) { if (!strcmp(var_name,vars_array[i].name)) // we will loop through variables arrays and look for { // requested variable printf("%s\n",vars_array[i].value); test = 0; break; } } if (i == count && test) { printf("Error: Variable '%s' does not exist\n", var_name); the variable print error programminghomeworkhelp.com

// if we can't find

} } else{ pid_t pid = fork(); // create a child process int status = 0; char cmd[256] = "./"; // we will be running program from cwd strcat(cmd,command); // concat program name and dir if (pid != 0){ // check parent process or child process waitpid(pid, &status, 0); // wait for the parent process } else{ char *argv[] = {cmd,NULL}; // setting up necessary arguments to call execve char *env[] = {NULL}; execve(cmd, argv, env); // call to execve exit(0); // make sure not to get fork bomb } } } } programminghomeworkhelp.com

Sleep: #include <unistd.h> #include <stdio.h> /** * compile using gcc sleep.c -o sleep * to check if our shell is running program in foreground mode and waiting for child process * * **/ int main(int argc, char **argv){ printf("I am sleeping \n"); sleep(5); _exit(0); }

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