Question

I’m writing a simple shell that accepts some standard commands like cd and ls in C. I’m trying to implement a feature where the user can enter a “;” in between commands so that a bunch of commands can be written on the same line and be executed separately. So if I input “cd Desktop; ls” the shell should cd to Desktop and print the what’s in the directory. The problem is it only executes the first command. Here’s my main method:

char input[1024];

while(1)
{
    printf("%s ", prompt);
    fgets(input, 1024, stdin);

    char delims[] = ";";
    char *result = NULL;
    result = strtok( input, delims );

    while( result != NULL )
    {
        printf("%s\n", result);

        char * copy = malloc(strlen(result) + 1); //Create a copy of the input token
        strcpy(copy, result);

        format(copy);

        if(programs)
        {
            handle();
            cleanup(programs);
            programs = NULL;
        }
        free(copy);
        result = strtok( NULL, delims );
        cmdno++;
    }
}

First I try to break up the input into tokens based on “;” and then feed the token to the format() method which looks like this:

int format(char input[])
{
    input = strtok(input, "\n");
    ...
}

I know that strtok makes changes to the original string, which is why I create a copy of the token first before passing it to format. Is what I’m doing correct??

Answer 1

You can’t mix multiple strtok calls. Here’s what’s happening:

• You start splitting input so strtok takes note and stores stuff internally
• You take a break from splitting input
  • You start splitting copy so again strtok takes note, thereby destroying the previous info
• At this point strtok only knows about the copy business and doesn’t know anything about the original input.

The main problem is that strtok doesn’t know that you’re doing two things at the same time. From its point of view, you simply started processing a different string before finishing the first string.

---

Possible solutions:

• Use strtok_r if you have it. It’s not standard C (but it is standard POSIX). The r stands for reentrant
• Use your own splitting function (strchr / looping etc)
• Change your program logic such that you don’t need to split copy before finishing with input

About that last point:

• Keep an array of char * and fill it with strtok without pausing to split sub-tokens. So each element should be a different command
• When you’re done with the ";" split, start processing each of the array elements