execvp() with grep that has quotation marks

Question

I did 'grep' in my own little shell. Other commands, like 'ls -al', 'pwd' ..etc, is working. But When I put 'grep', there was some problems.

char* f_grep[] = {"grep", "-n", "a", "a.txt", NULL};
char* s_grep[] = {"grep", "-n", "'a'", "a.txt", NULL};

execvp(f_grep[0], f_grep); // This works.

execvp(s_grep[0], s_grep); // This doesn't work.

I was wondering why I cannot use quotation marks with grep in execvp().

Thank you.

Answer 1

Normally, the shell removes one level of quotes from the command line arguments. So if you type,

grep -n 'a' a.txt

that's no different than

grep -n a a.txt

because the shell will remove the quotes.

However, when you pass a string with quotes to execvp, the quotes will not be removed. Which means that the example with s_grep is the same as typing

grep -n "'a'" a.txt

The shell will remove the double quotes, and leave the single quotes, which is what's happening with execvp.

---

If you want to experiment with this, you'll need two programs. The first program (called "showme") just echoes its command line arguments:

// showme.c
int main(int argc, char *argv[])
{
    for (int i = 0; i < argc; i++)
        printf("%d: %s\n", i, argv[i]);
}

The second program calls execvp with some arguments for "showme":

int main(void)
{
    char *args[] = { "./showme", "a", "'b'", NULL };
    execvp(args[0], args);
}

The output:

0: ./showme
1: a
2: 'b'