Get grep value using execl

Question

I'm trying to make a program which invokes the ls and grep system calls using exec. Specifically I have to execute ls > tmp; grep -c pattern < tmp in order to count the number of files that fulfill the pattern. As you can see I save the content of ls in tmp file and then I want to use grep to count the files.

Let's supose pattern = txt. I'm trying things like the following code:

char *a = "ls > tmp";
char *b = " -c ";
char *fin = " < tmp";
char *comanda;
if((comanda = malloc(strlen(pattern)+strlen(pattern)+1)) != NULL){
  comanda[0] = '\0';   // ensures the memory is an empty string
  strcat(comanda,b);
  strcat(comanda, pattern);
  strcat(comanda,fin);
} else {
    return -1;
}

ret = execl("/bin/sh","sh","-c",a,NULL);
ret = execl("/bin/sh","sh","-c",comanda, NULL);

But it shows me the following error: ls: cannot access > tmp: No such file or directory. So I don't know how to get the value of grep, because the execl function does not return the value, so how can I achieve the grep value?

You need to run `execl` in a child process created with `fork()`. `execl()` replaces the current process with the program you run, it only returns if it gets an error trying to load the program. — Barmar, Nov 24 '16 at 15:04
That's my actual point, in a child process i will use `execl`, but anyway I get the same error: `ls: cannot access > tmp: No such file or directory` — Mohamed Said Benmousa, Nov 24 '16 at 15:09
I can't reproduce that error unless I change it to `char *a = "ls '> tmp'";` — Barmar, Nov 24 '16 at 15:13
Pattern value is `txt` but this is not the problem. So you actually can run it without error? — Mohamed Said Benmousa, Nov 24 '16 at 15:18
Yes, I run it without error. `pattern` doesn't really matter, because the second `execl` never runs. — Barmar, Nov 24 '16 at 15:20
Where is `grep` in the program? Shouldn't it be `char *b = "grep -c "`? — Barmar, Nov 24 '16 at 15:21
Yes, it should be, but I am actually trying to implement Eliot's code. Basically I'm trying to use `execl("/bin/sh", "sh", "-c", "ls > tmp; grep -c 'pattern' < tmp", (char *)NULL);` now because what I was trying is giving me error. Can we chat? — Mohamed Said Benmousa, Nov 24 '16 at 15:25
I see a problem. You have `strlen(pattern)+strlen(pattern)+1`. It should be `strlen(b)+strlen(pattern)+strlen(fin)+1` — Barmar, Nov 24 '16 at 15:48
I was modifying the code at this moment, so I already had it in mind, but thank you for your help too! — Mohamed Said Benmousa, Nov 24 '16 at 16:15

score 1 · Accepted Answer · edited May 23 '17 at 12:13

1

To get the output of a command, you need to use a pipe.

Have a look at : Connecting n commands with pipes in a shell?

You could just do:

ls | grep -c pattern

If you just want to get files with a specific pattern in filename you might want to use find

find your_path/ -name "*pattern*" | wc -l

Have a look at Grabbing output from exec to get the output of execl

Here is an example, replace the 4th argument of execl with whatever you want :)

(execl("/bin/sh", "sh", "-c", "ls > tmp; grep -c 'pattern' < tmp", (char *)NULL);)

#include <unistd.h>
#include <string.h>

int main()
{
   int fd[2];
   pipe(fd);

   if (fork() == 0)
   {
      close(fd[0]);

      dup2(fd[1], 1);
      dup2(fd[1], 2);
      close(fd[1]);

      execl("/bin/sh", "sh", "-c", "find your_path -name '*pattern*' | wc -l", (char *)NULL);
   }
   else
   {
      char buffer[1024] = {0};

      close(fd[1]);

      while (read(fd[0], buffer, sizeof(buffer)) != 0)
      {
         write(1, buffer, strlen(buffer));
         memset (buffer, 0, sizeof(buffer));
      }
   }
   return 0;
}

edited May 23 '17 at 12:13

Community

1
1

answered Nov 24 '16 at 13:46

Eliot B.

162
11

The problem is that I have to use `ls > tmp; grep -c pattern < tmp` as a requeriment. So I will have a look to the pipes. – Mohamed Said Benmousa Nov 24 '16 at 14:01
Well, if you don't have to use `execl`, use `system` which enables you to have redirections. Otherwise, play with `fork` and `dup2`. – Eliot B. Nov 24 '16 at 14:07
I can't use `system`, this is another requeriment too. – Mohamed Said Benmousa Nov 24 '16 at 14:09
He's not passing `>` as an argument, he's calling `/bin/sh` and passing it a command line that contains the redirection. Why isn't the shell parsing the command? – Barmar Nov 24 '16 at 15:03
Yep, you're write, I didn't remark the sh -c at first. – Eliot B. Nov 24 '16 at 15:11
And when I try his code, I don't get the error. He must not have posted his actual code. – Barmar Nov 24 '16 at 15:15
Okay, with your code I could find a solution, thank you very much! – Mohamed Said Benmousa Nov 24 '16 at 15:31

score 0 · Answer 2 · answered Nov 24 '16 at 15:53

You're not allocating the correct amount of space for comanda, because you don't add the sizes of all the variables correctly. So if the size is too small, you'll write outside the array bounds when you do all the strcat, and this will cause undefined behavior.

You don't need the temporary file, you can just pipe from ls to grep. I've also added quotes around the pattern in case it contains special characters.

#include <string.h>
#include <unistd.h>
#include <stdlib.h>

int main() {
    char *a = "ls | grep -c '";
    char *fin = "'";
    char *pattern = "foo";
    char *comanda;
    if((comanda = malloc(strlen(a) + strlen(pattern) + strlen(fin) +1)) != NULL){
        strcpy(comanda,a);
        strcat(comanda,pattern);
        strcat(comanda,fin);
    } else {
        return -1;
    }
    int ret = execl("/bin/sh","sh","-c", comanda, (char*)NULL);
    perror("execl"); // Can only get here if there's an error
}

Get grep value using execl

2 Answers2