Problem with (Understanding) function Post: 302993116

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system

SYSTEM(3)						     Linux Programmer's Manual							 SYSTEM(3)

NAME

       system - execute a shell command

SYNOPSIS

       #include <stdlib.h>

       int system(const char *string);

DESCRIPTION

       system()  executes  a  command  specified in string by calling /bin/sh -c string, and returns after the command has been completed.  During
       execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored.

RETURN VALUE

       The value returned is -1 on error (e.g. fork failed), and the return status of the command otherwise.  This latter return status is in  the
       format  specified  in wait(2).  Thus, the exit code of the command will be WEXITSTATUS(status).	In case /bin/sh could not be executed, the
       exit status will be that of a command that does exit(127).

       If the value of string is NULL, system() returns nonzero if the shell is available, and zero if not.

       system() does not affect the wait status of any other children.

CONFORMING TO

       ANSI C, POSIX.2, BSD 4.3

NOTES

       As mentioned, system() ignores SIGINT and SIGQUIT.  This may make programs that call it from a loop uninterruptable, unless they take  care
       themselves to check the exit status of the child. E.g.

	   while(something) {
	       int ret = system("foo");

	       if (WIFSIGNALED(ret) &&
		   (WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
		       break;
	   }

       Do  not	use  system()  from a program with suid or sgid privileges, because strange values for some environment variables might be used to
       subvert system integrity.  Use the exec(3) family of functions instead, but not execlp(3) or execvp(3).	system() will not, in  fact,  work
       properly  from  programs  with  suid  or  sgid  privileges  on systems on which /bin/sh is bash version 2, since bash 2 drops privileges on
       startup.  (Debian uses a modified bash which does not do this when invoked as sh.)

       The check for the availability of /bin/sh is not actually performed; it is always assumed to be available.  ISO C specifies the check,  but
       POSIX.2	specifies  that  the  return  shall always be non-zero, since a system without the shell is not conforming, and it is this that is
       implemented.

       It is possible for the shell command to return 127, so that code is not a sure indication that the execve() call failed.

SEE ALSO

       sh(1), signal(2), wait(2), exec(3)

								    2001-09-23								 SYSTEM(3)