Passing variable from c program to shell script as argument

Question

I am calling a shell script from a \'c\' program and have some variables in c which I would like to pass as arguments to the shell script. I tried using the system() to call

Answer 1

You will have to construct a string which contains the full command line for system to execute. Simplest is probably using sprintf.

char buf[100];
sprintf(buf, "progname %d %s", intarg, strarg);
system(buf);

That's the quick way for starters.

But there's also the power-duo of fork and exec (for unix systems, at least). If your arguments are already separate strings, this can be easier than a really complicated format specification; not to mention calculating the correct buffer size for a complicated format specification!

if (fork() == 0) {
    execl(progname, strarg1, strarg2, (char *)NULL);
}
int status;
wait(&status);
if (status != 0) {
    printf("error executing program %s. return code: %d\n", progname, status);
}

Answer 2

shell script (a.sh):

# iterates over argument list and prints
for (( i=1;$i<=$#;i=$i+1 ))
do
     echo ${!i}  
done

C code:

#include <stdio.h>

int main() { 
  char arr[] = {'a', 'b', 'c', 'd', 'e'}; 
  char cmd[1024] = {0}; // change this for more length
  char *base = "bash a.sh "; // note trailine ' ' (space) 
  sprintf(cmd, "%s", base);
  int i;
  for (i=0;i<sizeof(arr)/sizeof(arr[0]);i++) {
    sprintf(cmd, "%s%c ", cmd, arr[i]); 
  }
  system(cmd);
}

Answer 3

This is not going to print return status of the child process.

The return status is a 16 bit word. For normal termination: byte 0 has a value of zero, and return code is in byte 1 For termination due to an uncaught signal: byte 0 has the signal number and byte 1 is zero.

To print the return status you will need to do something like:

 while ((child_pid =wait(&save_status ))  != -1 )  {
  status = save_status >> 8;
  printf("Child pid: %d with status %d\n",child_pid,status);

Answer 4

This below program have worked for me

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
int main(int argc, char const *argv[])
{
   
char buf[1000]; //change this length according to your arguments length
  
  int i;
  for (i=0;i<argc;i++) {
    if(i==0)
    {
    sprintf(buf, "%s", "sh shell-scriptname.sh");
    sprintf(&buf[strlen(buf)]," ");
    }
    else
    {
        sprintf(&buf[strlen(buf)],argv[i]);
        
        sprintf(&buf[strlen(buf)]," ");
    }

    
  }
  
  //printf("command is %s",buf);
  
    system(buf);
}

My Shell script had arguments like

sh shell-scriptname.sh -a x -b y -c z -d blah/blah/blah

I complied the C program using the following

gcc c-programname.c -o utility-name

To execute

./utility-name -a x -b y -c z -d blah/blah/blah

worked for me