CODEWITHSUNDEEP
cshellunixcommand-line
user5574376
user5574376

Reputation: 391

How does shell expands *.c?

I encounter a question while I am reading a textbook - Unix System Programming

How big is the argument array passed as the second argument to execvp when you execute execcmd of Program 3.5 with the following command line?

execcmd ls -l *.c

Answer: The answer depends on the number of .c files in the current directory because the shell expands *.c before passing the command line to execcmd.

Program 3.5:

#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include "restart.h"
int main(int argc, char *argv[]) {
   pid_t childpid;
   if (argc < 2){     /* check for valid number of command-line arguments */
      fprintf (stderr, "Usage: %s command arg1 arg2 ...\n", argv[0]);
      return 1;
   }
   childpid = fork();
   if (childpid == -1) {
      perror("Failed to fork");
      return 1; 
   }
   if (childpid == 0) {
      execvp(argv[1], &argv[1]);
      perror("Child failed to execvp the command");
      return 1;
   }
   if (childpid != r_wait(NULL)) {
      perror("Parent failed to wait");
      return 1;
   }
   return 0;
}

Why is the size of argument array passed depending on the number of .c files in the current directory? Isn't it the argument array just something like

argv[0] = "execcmd";
argv[1] = "ls";
argv[2] = "-l";
argv[3] = "*.c";
argv[4] = NULL;

Update: Find a link explains pretty well about the shell expansion. May useful for someone who see this post later also do not understand shell expansion. Description about shell expansion

Upvotes: 0

Views: 56

Answers (1)

Rob11311
Rob11311

Reputation: 1416

No, because the shell does the wild card expansion. It finds files in the directory that match the expression, so for instance you can use "echo *.c" to discover what the shell would match. Then it lists out, every filename matching *.c on the exec call or if none *.c which is likely to result in an error message about file not found.

It is more powerful that the shell does the expansion, the same file wildcarding is immediately available for all programs, like cat, echo, ls, cc.

Upvotes: 1

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