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Unix shell

halt1001 asked
Last Modified: 2010-05-18
Im doing an assignment, programming in C, in Unix.
The task is, stimulating a shell.
It should accept any command entered.

I have done it, and so far it could run commands entered from the prompt.

However, the problems are:

1. No error returning back when an invalid command entered.

2. Another task is, the pipe operator. I dont know how to do this. Any1 know
how to make it accept pipes like in normal shell? For example:

command1 paratemeters | command 2 ....

Thanks for reading.


Here's the code at the moment:

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

int main();
void parent(int pid);
void child();

char command[100];

int main() {
 int ret;

 do {

      if ( strncmp(command,"logout\n",8) == 0 )
      if ( strncmp(command,"\n",1) == 0 )

      if( (ret=fork()) > 0 )
      else if ( ret == 0 )
           perror("fork() failed");
     } while (1);


void parent(int pid)
    int childs_exit_val;

 ** The parent only invokes the wait() if the command is a foreground
 ** i.e. there is no '&' at the end of the line

void child()
     char *argv[100]; /* 100 is arbitrarily long enough for this assignment
     char *tmp;
     int cnt,i;

     cnt = -1;
            tmp=strtok(command, "\n");
            tmp=strtok(tmp, " ");
            while ( tmp != NULL )
                    argv[++cnt]= tmp;
                    tmp=strtok(NULL," ");

     if ((strncmp(argv[0],"cd",2)==0)&&(strlen(argv[0])==2))

     for (i=0; i<=cnt; i++) {
          printf("argv[%d] = '%s'\n", i, argv[i]);


     execvp(argv[0], argv);

 ** If we arrive here, an error must have occurred otherwise a different
 ** program image would be active.
    exit(1); /* must be careful to exit a child process */

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