Question

DONT COPY PASTE THE SOLUTION FROM ANOTHER QUESTION! IT IS NOT THE RIGHT ONE, ANSWER WILL BE REPORTED IF NOT A UNIQUE SOLUTION.

Write a C program called myshell.c, which, when compiled and run, will do what the shell does, namely, it executes in a loop (until user types exit on the keyboard),

prints a prompt on the screen, reads the command typed on the keyboard (terminated by \n),

creates a new process and lets the child execute the user’s command,

waits for the child to terminate and then goes back to beginning of the loop.

If the command typed is exit, then your program should terminate.

Print the total number of commands executed just before terminating your program.

Assume that each line represents one command only, no command will end with & (all commands will be attached commands, no background execution),

user will not type ^c or ^z, all commands are simple commands, etc.

Answer 1

I developed this code last year in operating systems lab it works perfectly for most of the commands if there is any issue just comment once I will look into it. Also, I have tried to explain the code hope it helps

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

#define MAX_INPUT_SIZE 1024
#define MAX_TOKEN_SIZE 64
#define MAX_NUM_TOKENS 64

#define STDOUT 1
#define STDIN 0
#define STDERR 2

char **tokenize(char *line)
{
char **tokens = (char **)malloc(MAX_NUM_TOKENS * sizeof(char *));
char *token = (char *)malloc(MAX_TOKEN_SIZE * sizeof(char));
int i, tokenIndex = 0, tokenNo = 0;

for(i =0; i < strlen(line); i++){

char readChar = line[i];

if (readChar == ' ' || readChar == '\n' || readChar == '\t') {
token[tokenIndex] = '\0';
if (tokenIndex != 0){
tokens[tokenNo] = (char*)malloc(MAX_TOKEN_SIZE*sizeof(char));
strcpy(tokens[tokenNo++], token);
tokenIndex = 0;
}
}
else {
token[tokenIndex++] = readChar;
}
}

free(token);
tokens[tokenNo] = NULL ;
return tokens;
}

char cwd[1024];
int bgprocesses[1024], num_bgp;

void execute(char** args, int bg, int fd){

  
if(strcmp(args[0],"cd")==0){ //comparing the string with cd
if(args[1]==NULL || args[2]!=NULL){
fprintf(stderr, "cd: Wrong number of arguments\n");
//checking if you have passed only one command to cd if more of less than one it gives error.
}
else{
chdir(args[1]); // changes the current working directory of the calling process

}
}
else{

int pid = fork();//creation of a child process
if(pid==0){
//child process
if(fd!=STDOUT){
int copy = dup(STDOUT); //copy of the file descriptor
close(STDOUT);   
dup(fd);
execvp(args[0], args); //execute the file
fprintf(stderr, "%s: No such file or directory\n", args[0]);
//printing the necessary print statements

}
else{
execvp(args[0], args);
fprintf(stderr, "%s: No such file or directory\n", args[0]);
}
}
else{
if(bg==0)
{   
int status;
waitpid(pid, &status, 0); //waiting for child process to end
}
else{

}
}
}

}
//shell
void Shell(){
char cwd[1024];
if (getcwd(cwd, sizeof(cwd)) != NULL); //Determines the path name of the working directory and stores it in buffer
  
else{
perror("Failed to fetch current working directory\n");

exit(1);
}
}

void main(void)
{
char line[MAX_INPUT_SIZE];
char **tokens;
int i;

Shell();

while (1) {   

printf("shell>");   
bzero(line, MAX_INPUT_SIZE); //copies n bytes, each with a value of zero, into string line.
gets(line); //reads a line from stdin and stores it
  
line[strlen(line)] = '\n'; //terminate with newline
tokens = tokenize(line);
char ** lastCommand = tokens;
int fd = STDOUT;
int isSemiColon[66];
for(int i=0; i<66; i++)
isSemiColon[i]=0;

for(int i=0; tokens[i]!=NULL; i++){
if(strcmp(tokens[i],";;")==0){ //comapring the all the tokens to check if there are two semicolons
tokens[i] = NULL;
isSemiColon[i] = 1; // if there is double semi colon set it to 1.
}
}
for(i=0;;i++){
if(isSemiColon[i]){
execute(lastCommand, 0, fd);   
if(tokens[i+1]==NULL){
break;
}
else{
lastCommand = &tokens[i+1];
}
}
else if(tokens[i]==NULL){
execute(lastCommand, 0, fd);
break;
}
else if(strcmp(tokens[i],">")==0){ //again comaping the tokens to search for ">"
printf("> found\n"); //">" print found if enter this loop
tokens[i] = NULL;

if(isSemiColon[i+1]){
fprintf(stderr, "bash: syntax error near unexpected token `;;`");
  
}
else if(tokens[i+1]==NULL){
fprintf(stderr, "bash: syntax error near unexpected token `newline`");
}
else{
  
fd = open(tokens[i+1], O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR);
if(fd>=0){
execute(tokens, 0, fd);
  
}
else{
fprintf(stderr, "%s: Is a directory\n", tokens[i+1]);
}

}
}
}
// Freeing the allocated memory
for(i=0;tokens[i]!=NULL;i++){
free(tokens[i]);
}
free(tokens);

}

}