Question

The code I have is here. Essentially, the custom shell needs to take echo, cd and quit as the commands and fork a child for the commands. There are no errors when it compiles but it is not running. It doesn’t echo the argument when I give, say “echo hello”…it goes into the right functions and all but I cannot pinpoint the error. I am guessing I am making a mistake with the execlp function. Can someone please help me out?

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

int main()
{

    char command[256];
    char * parsedCmd;
    char * argument;

    //char *sep[]=" ";

    while (1<2)
    {
        printf("\nOS Assignment 1@user: ");
        fgets(command, sizeof(command), stdin); 

        parsedCmd=strtok(command, " ");
        argument=strtok(NULL, " ");

        //printf("\n%s\n", argument);

        if (strncmp("quit", command, 4)==0)
            break;
        if (strncmp("cd", parsedCmd, 2)==0)
        {
            printf("\nExecuting cd\n");
            execCD(argument);   
        }
        if (strncmp("echo", parsedCmd, 4)==0)
        {
            printf("\nEchoing now...\n");
            shellEcho(argument);
        }       
        else
            printf("\nOur shell is simple. Try either cd, echo or quit :) ...\n");  
    }   
}

int execCD(char *receive)
{
    printf("\nExecuting cd as Child...\n"); 
    printf("\nDirectory to cd is %s\n", receive);
    pid_t pid;

    pid=fork();

    if (pid<0)
    {
        fprintf(stderr, "\nFork Failed\n");
        return 1;
    }

    else if (pid==0)
    {
        execlp(receive, "cd", NULL);
    }

    else
    {
        wait(NULL);
        printf("Child Complete");
    }

    return 0;
}

int shellEcho(char *receive)
{
    printf("\nExecuting echo as Child...\n");   

    pid_t pid;

    pid=fork();

    if (pid<0)
    {
        fprintf(stderr, "\nFork Failed\n");
        return 1;
    }

    else if (pid==0)
    {
        execlp(receive, "echo", NULL);
    }

    else
    {
        wait(NULL);
        printf("Child Complete");
    }

    return 0;
}

Answer 1

You have the arguments to execlp backwards. Also, note that by convention, you need to specify “echo” twice: once as the name of the program to execute, and the second time as the 0th argument (which is usually, but not necessarily, the name of the program).

int shellEcho(char *receive)
{
    printf("\nExecuting echo as Child...\n");   
    pid_t pid;
    pid=fork();

    if (pid<0) {
        fprintf(stderr, "\nFork Failed\n");
        return 1;
    } else if (pid==0) {
        execlp("echo", "echo", receive, NULL);
    } else {
        wait(NULL);
        printf("Child Complete");
    }

    return 0;
}

Also, note that “cd” cannot be executed, as it must be implemented by the shell (it is not a separate program that can be run). It’s probably more appropriate to discuss that issue with your teacher rather than pursuing it futher here.