Assignment 4

Author: ankan17

myshell.c

@@ -0,0 +1,162 @@
+/* In this assignment, you start writing a command interpreter (shell).
+
+The shell will give a prompt for the user to type in a command from the set of
+commands. Take the command, execute it and give the prompt back to the user for
+the next command. Your program should be able to do the following:
+
+  1. Give a prompt "myshell>" for the user to type in a command
+  2. Implement the following built in commands:
+    a) cd <dir>     :   Changes the directory to dir
+    b) pwd          :   Prints the current directory
+    c) mkdir <dir>  :   Creates the directory dir
+    d) rmdir <dir>  :   Removes the directory dir
+    e) ls           :   Lists the files and directories in the present directory
+    f) exit         :   Exits out of the shell
+  3. Any other command typed at the prompt should be executed as if it is the
+  name of the executable file. For example - typing "a.out" should execute the
+  file a.out. This file can be in the current directory or any of the other
+  directories specified by PATH environment variable.
+  4. The executable file should be executed after creating a new process and
+  executing the file onto it. The parent process should wait for the file to
+  finish execution and then go on to read the next command from the user. The
+  executable can take any number of command line parameters.
+*/
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#define PROMPT "myshell>"
+#define BUFFERSIZE 150
+
+char** split_by_space(char* string) {
+  char** tokens = (char**)malloc(sizeof(char*) * 40);
+  int i = 0;
+  char *token = strtok(string, " ");
+  while (token != NULL) {
+    tokens[i++] = token;
+    token = strtok(NULL, " ");
+  }
+  tokens[i] = NULL;
+  return tokens;
+}
+
+int main() {
+  char command[BUFFERSIZE];
+  int status;
+  pid_t pid;
+
+  while (1) {
+
+    // Give the prompt
+    printf("%s ", PROMPT);
+
+    // Read the command
+    fgets(command, BUFFERSIZE, stdin);
+    command[strlen(command) - 1] = '\0';
+
+    // Exit out of the program if the command is "exit"
+    if (strcmp(command, "exit") == 0)
+      break;
+
+    // If the command is "cd", use chdir() system call in the parent process
+    else if (strstr(command, "cd") != NULL) {
+      char dirname[BUFFERSIZE/3];
+      strcpy(dirname, command + 3);
+      int ret = chdir(dirname);
+      if (ret)
+        printf("%s: No such directory\n", dirname);
+    }
+
+    // If the command is "cd", use getcwd() system call in the parent process
+    else if (strcmp(command, "pwd") == 0) {
+      char cwd[BUFFERSIZE];
+      getcwd(cwd, BUFFERSIZE);
+      printf("%s\n", cwd);
+    }
+
+    // If the command is "mkdir", use mkdir() system call and give 755 permission
+    else if (strstr(command, "mkdir") != NULL) {
+      char dirname[BUFFERSIZE/3];
+      strcpy(dirname, command + 6);
+      int status = mkdir(dirname, S_IRWXU | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH);
+      if (status)
+        printf("%s: Directory could not be created due to some error!\n", dirname);
+    }
+
+    // If the command is "rmdir", use rmdir() system call
+    else if (strstr(command, "rmdir") != NULL) {
+      char dirname[BUFFERSIZE/3];
+      strcpy(dirname, command + 6);
+      int status = rmdir(dirname);
+      if (status)
+        printf("%s: Directory could not be removed due to some error!\n", dirname);
+    }
+
+    // If the command is ls, use readdir() system call and filter out entries like
+    // ., .. and any hidden files beginning with . to match the format used by ls
+    else if (strcmp(command, "ls") == 0) {
+      struct dirent *dp;
+      DIR *dirp = opendir(".");
+      int count = 0;
+      if (dirp != NULL) {
+        while ((dp = readdir(dirp)) != NULL) {
+          if (dp->d_name[0] != '.') {
+            if (++count % 4)
+              printf("%-16s", dp->d_name);
+            else
+              printf("%-16s\n", dp->d_name);
+          }
+        }
+        closedir(dirp);
+        if (count % 4 != 0)
+          printf("\n");
+      } else {
+        printf("Could not get the files of the current directory\n");
+      }
+    }
+
+    // For any other command, spawn a child process and execute the process in it
+    else {
+      if ((pid = fork()) == 0) {
+        char **tokens = split_by_space(command);
+        int file_in_cur_dir = 0;
+
+        // Check if the file is present in the current directory
+        struct dirent *dp;
+        DIR *dirp = opendir(".");
+        if (dirp != NULL) {
+          while ((dp = readdir(dirp)) != NULL) {
+            if (strcmp(tokens[0], dp->d_name) == 0) {
+              file_in_cur_dir = 1;
+              break;
+            }
+          }
+          closedir(dirp);
+        }
+
+        // If the file is present in the current directory, add the current
+        // directory to the PATH. This only lasts in the child process.
+        if (file_in_cur_dir) {
+          const char* PATH = "PATH";
+          char* path = getenv(PATH);
+
+          char newpath[BUFFERSIZE*10];
+          getcwd(newpath, BUFFERSIZE);  // New path starts with the current directory
+          strcat(newpath, ":");
+          strcat(newpath, path);        // Concatenate ":" followed by old path
+
+          setenv(PATH, newpath, 1);     // Set the PATH environment variable
+        }
+        execvp(tokens[0], tokens);
+      }
+      else {
+        waitpid(pid, &status, 0);
+      }
+    }
+  }
+}