How do you spawn another process in C?

If you want to perform more complicated operations, like reading the output of the external program, you may be better served by the popen system call. For example, to programmatically access a directory listing (this is a somewhat silly example, but useful as an example), you could write something like this:

#include <stdio.h>

int main()
{
  int entry = 1;
  char line[200];
  FILE* output = popen("/usr/bin/ls -1 /usr/man", "r");
  while ( fgets(line, 199, output) )
  {
    printf("%5d: %s", entry++, line);
  }
}

to give output like this

1: cat1
2: cat1b
3: cat1c
4: cat1f
5: cat1m
6: cat1s
...

One solution is the system function defined in stdlib.h

int system(const char *string);

system api example

If you need to check/read/parse the output of your external command, I would suggest to use popen() instead of system().