Calling a shell script from a C program

Applications Programming
1

Hi,

I have a shell script which connects to a database and fetches the count of the records from a table. I want to embed this whole script in a C program. Also the count fetched should be available in the C program for further usage.

Please let me know how this can be done.

Thanks

2

Start it using man popen (POSIX), and read from the returned file descriptor as you would from any other.

3

Hi pludi,

can you show me how it's done with a sample snippet please?

4

hello

You need system() function to call scripts or other commands. you can use "wc" command to count the number of records returned by it.

Regards,
Gaurav.

5

Quick example, will copy the contents of the files given on the command line to stdout, prefixed by the filename and the line number (yes, it's UUOC):

#include <stdio.h>

int main(int argc, char **argv)
{
    FILE *pipe = NULL;
    int i;
    char buffer[1024];

    if (argc == 1) {
        return 1;
    }
    for (i = 1; i < argc; i++) {
        sprintf(buffer, "cat -n %s", argv);
        if ((pipe = popen(buffer, "r")) == NULL) {
            perror("popen");
            return 1;
        }
        while (fgets(buffer, 1024, pipe) != NULL)
            printf("%s: %s", argv, buffer);
        if (pclose(pipe) == -1) {
            perror("pclose");
            return 1;
        }
    }
    return 0;
}
6

Hi Gaurav,

below is the system call that I have used to call my shell script named "test"

#include <stdio.h>
#include <stdlib.h>

int main()
{

int i;

i=system("./test 1234");
printf("i=%d\n",i);

}

Here "test" is the shell script that is fetching the count from a table for a particular column("1234") that I am passing as the parameter. When the "test" script alone is run, it is returning the correct count but when trying to check through the C program it's returning some different value. How can I get the exact value from the script in my program above? wc is returning 1 as it's a single line output but I want the actual count value.

Can you help?

7

Because system() does not return the output of a script, but only the exit status. Eg. if instead of "./test" you'd run /bin/true (or /usr/bin/true), you'd always get 0, while with /bin/false you'd always get 1. If you want to get the output of a command, either use a temporary file, or popen as in the example I've given.

8

thanks for the clarification pludi. so, does popen only take a command or can an entire script be passed in. can you show me how my test script fits in your example please?

9

or a fifo.

Regards,
Gaurav.

10

From the man page of man popen (POSIX)

So any program you could call using

sh -c "/path/to/program arg1 arg2 ... argn"

can be called just the same.

Also, please make it a habit of reading the man pages first when available, experiment a bit for yourself, and ask specific questions. We're here to help you, not to do your work for you.