creating a shell like program in C

Question

I am trying to create a simple c program that takes user input, passes it to underlying shell and return the ouput of the user command. For eg: pwd will give the present working directory.

I want to do this using fork() and exec() in an infinite loop but I am facing two problems:

My loop terminates after first run

It only takes first argument. 'ls -ltr' will give me the output of 'ls' and not 'ls -ltr'

int runit(char*);

void main() {

int pid=0;                          
char command[50];
while(1)

{
    int d=0;
    printf("Please enter your command!\n");
    scanf("%s", &command);
    switch (pid = fork()) 
    {
        case 0:                    // a fork returns 0 to the child 
            printf("Child process \n");
            d=runit(command);
            if(d==-1){
            printf("command not found \n");}
            break;

        default:
            wait(5);               // a fork returns a pid to the parent 
            printf("Parent process \n"); 
            break;

        case -1:                   //if something went wrong 
            perror("fork");
            exit(1);

    }
  }                
}

int runit(char* command) {                     //executing the command

    char path[50]="/bin/";

    int d = execl(strcat(path,command),command,NULL,NULL);     

    return(d);

}

Can somebody tell me what I am doing wrong or guide me how to correct this.

P.P · Accepted Answer · 2017-02-16 19:27:14Z

2

This input reading

scanf("%s", &command);

would stop at the first whitespace (unless input failure occurs -- you should check the return value of scanf() in any case). So, when you input ls -ltr, command will have just ls. You need to read lines. For example, use fgets() to read input (and make sure to handle the trailing newline):

fgets(command, sizeof command, stdin);
command[strcspn(command, "\n")] = 0; /* to remove \n if present */

And the next problem is that execl() takes multiple arguments. That means this usage:

int d = execl(strcat(path,command),command,NULL,NULL);

is broken. You need to split the input command into multiple strings and pass each arguments to execl(). A better option to store the individual strings in an array and use execv() instead.

Another thing you need to be aware of is exec*() family functions do not return on success. So, the return code check isn't very useful - instead you can just use perror() to know why it failed if it failed.

answered Feb 16, 2017 at 19:27

P.P

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2 Comments

Peng Shuai Over a year ago

Thanks. This was really helpful. I changed from scanf to gets and modified execl() command and it worked. It works with execv() as well. But which one is better? I prefer execl() just because of the simplicity over execv()

P.P Over a year ago

There's no "better" between execl and execv - they just differ in their arguments. So, use whichever is convenient for your use case.

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