Question

This program is a simple version of the linux shell using execvp by forking using C

Currently this program of the shell assumes user enters no spaces before and no extra spaces between strings.

Using exec, an executable binary file (eg: a.out) can be converted into a process. An example of using exec is implementing a shell program or a command interpreter. A shell program takes user commands and executes them.

• int execvp(const char *file, char *const argv[]);

  • Same as execv except that we don’t have to give the full path name of the command. Uses the $PATH environment variable to find the path.

static char* args[] = {"cp", "cat.txt", "test1.txt", NULL}; execv("cp", args);

Only code void cmdToArray().

Program should be able to execute ls -l.

<cmd> ls -l
<cmd> cp file1.txt file2.txt

#include <stdio.h>
#include <string,h>// strcmp
#include <stdlib.h>// exit
#include <unistd.h>// fork, exec
#include <sys/wait.h>// wait

#define MAXARGS 20
#define ARGLEN 256

void execute(char *cmd, char *arglist[]);
void cmdToArray(char *cmd, char *arglist);

int main(void)
{
int pid;
char *arglist[MAXARGS+1]; // array of pointers
char cmd[ARGLEN]; // read stuff here

while (1)
{
printf("cmd> ");
fgets(cmd, ARGLEN, stdin);

// remove newline from cmd
cmd[strlen(cmd) - 1] = '\0';

// exit if user types exit
if (strcmp(cmd, "exit") == 0)
{
exit(0);
}
else if (*cmd != '\0')
{
// TODO: break cmd into individual strings as elements of arglist
cmdToArray(cmd, arglist);
execute(arglist);
}
}

return 0;
}

void cmdToArray(char *cmd, char *arglist[]) //code here
{
// cmd = "*cp file1.txt file2.txt"
// char cmd[25] = ('c', 'p', ' ', 'f', ')
// arglist[0] = cmd;
// arglist[1] = &cmd[3];
// arglist[2] = &cmd[9];
// arglist[3] = "NULL";
// arglist = (**char)malloc(sizeof(char*) * cmd);
// arglist[0] = (char *)malloc(sizeof(char) *MAXARGS * string);
// int length = strlen(cmd);
/* for (int i = 0; i < length; ++i)
{
if (cmd[i] != ' ')
{
arglist[argc] = &cmd[i];
++argc;
}
else if(cmd[i] == ' ')
{
cmd[i] = '\0';
}
} */

}

void execute(char *arglist[])
{
int pid, exitstatus;

pid = fork(); // make new process
if (pid == -1)
{
perror("fork failed");
exit(1);
}
else if (pid == 0)
{
execvp(arglist[0], arglist);
// exec cannot return
fprintf(stderr, "Cannot execute %s\n", arglist[0]);
exit(1); // exec failed
}
else
{
int got_pid = wait(&exitstatus);
if (got_pid == pid)
{
printf("The child we were waiting for died\n");

if (WIFEXITED(exitstatus))
{
printf("child process %d exited with value %d\n", pid, WEXITSTATUS(exitstatus));
}
}
else if (got_pid == -1)
{
perror("wait failed");
}
}
}

Answer 1

Please look at my code and in case of indentation issues check the screenshots.

-----------------main.c----------------

#include <stdio.h>
#include <string.h>// strcmp
#include <stdlib.h>// exit
#include <unistd.h>// fork, exec
#include <sys/wait.h>// wait
#define MAXARGS 20
#define ARGLEN 256

void execute(char *arglist[]);
void cmdToArray(char *line, char *arglist[]);

int main(void)
{
   int pid;
   char *arglist[MAXARGS + 1];   // array of pointers
   char cmd[ARGLEN];   // read stuff here

   while (1)
   {
       printf("cmd > ");
       fgets(cmd, ARGLEN, stdin);

       // remove newline from cmd
       cmd[strlen(cmd) - 1] = '\0';

       // exit if user types exit
       if (strcmp(cmd, "exit") == 0)
       {
           exit(0);
       }
       else if (*cmd != '\0')
       {
           // TODO: break cmd into individual strings as elements of arglist
           cmdToArray(cmd, arglist);
           execute(arglist);
       }
   }

   return 0;
}

void cmdToArray(char *line, char *arglist[])   //splits the line into words and stores in arglist array
{
   while (*line != '\0')                                         // if not the end of line .......
   {   
       while (*line == ' ' || *line == '\t' || *line == '\n')   //checks if we are near whitespaces
           *line++ = '\0';                                    // replace white spaces with \0 NULL character
       *arglist++ = line;                             // save the argument position (start position of word)
       while (*line != '\0' && *line != ' ' && *line != '\t' && *line != '\n')    //skips characters within a word
           line++;                                             //skip till end of next argument (end position of the word)
   }
   *arglist = '\0';                                         //mark the end of argument list
}

void execute(char *arglist[])
{
   int pid, exitstatus;

   pid = fork();   // make new process
   if (pid == -1)
   {
       perror("fork failed");
       exit(1);
   }
   else if (pid == 0)
   {
       execvp(arglist[0], arglist);
       // exec cannot return
       fprintf(stderr, "Cannot execute %s\n", arglist[0]);
       exit(1);   // exec failed
   }
   else
   {
       int got_pid = wait(&exitstatus);
       if (got_pid == pid)
       {
           printf("The child we were waiting for died\n");

           if (WIFEXITED(exitstatus))
           {
               printf("child process %d exited with value %d\n", pid, WEXITSTATUS(exitstatus));
           }
       }
       else if (got_pid == -1)
       {
           perror("wait failed");
       }
   }
}

--------------Screenshots-------------------

------------------Output-----------------

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