perl exec system

• system LIST

• system PROGRAM LIST

  Does exactly the same thing as exec LIST,except that a fork is done first,and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST,or if LIST is an array with more than one value,starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument,the argument is checked for shell metacharacters,and if there are any,the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms,but varies on other platforms). If there are no shell metacharacters in the argument,it is split into words and passed directly to execvp,which is more efficient.

  Beginning with v5.6.0,Perl will attempt to flush all files opened for output before any operation that may do a fork,but this may not be supported on some platforms (see perlport ). To be safe,you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles.

  The return value is the exit status of the program as returned by the wait call. To get the actual exit value,shift right by eight (see below). See also "exec" . This is not what you want to use to capture the output from a command,for that you should use merely backticks or qx// ,as described in ""`STRING`"" in perlop . Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason).

  Like exec ,system allows you to lie to a program about its name if you use the system PROGRAM LIST syntax. Again,see "exec" .

  Since SIGINT and SIGQUIT are ignored during the execution of system ,if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value.

      @args = ("command","arg1","arg2");
  system
  (@args) == 0
  
  	 or die
   "system @args failed: $?"

  You can check all the failure possibilities by inspecting $? like this:

      if ($? == -1) {
  print
   "failed to execute: $!/n";
  
      }
  
      elsif ($? & 127) {
  printf
   "child died with signal %d,%s coredump/n",
  
  	    ($? & 127),($? & 128) ? 'with' : 'without';
  
      }
  
      else {
  printf
   "child exited with value %d/n",$? >> 8;
  
      }

  Alternatively you might inspect the value of ${^CHILD_ERROR_NATIVE} with the W*() calls of the POSIX extension.

  When the arguments get executed via the system shell,results and return codes will be subject to its quirks and capabilities. See ""`STRING`"" in perlop and "exec" for details.

• exec LIST

• exec PROGRAM LIST

  The exec function executes a system command and never returns -- use system instead of exec if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).

  Since it's a common mistake to use exec instead of system ,Perl warns you if there is a following statement which isn't die ,warn ,or exit (if -w is set - but you always do that). If you really want to follow an exec with some other statement,you can use one of these styles to avoid the warning:

      exec
   ('foo')   or print
   STDERR "couldn't exec foo: $!";
  
      { exec
   ('foo') }; print
   STDERR "couldn't exec foo: $!";

  If there is more than one argument in LIST,calls execvp(3) with the arguments in LIST. If there is only one scalar argument or an array with one element in it,which is more efficient. Examples:

      exec
   '/bin/echo','Your arguments are: ',@ARGV;
  exec
   "sort $outfile | uniq";

  If you don't really want to execute the first argument,but want to lie to the program you are executing about its own name,you can specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST. (This always forces interpretation of the LIST as a multivalued list,even if there is only a single scalar in the list.) Example:

      $shell = '/bin/csh';
  exec
   $shell '-sh';		# pretend it's a login shell

  or,more directly,

      exec
   {'/bin/csh'} '-sh';	# pretend it's a login shell

  When the arguments get executed via the system shell,results will be subject to its quirks and capabilities. See ""`STRING`"" in perlop for details.

  Using an indirect object with exec or system is also more secure. This usage (which also works fine with system()) forces interpretation of the arguments as a multivalued list,even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.

      @args = ( "echo surprise" );
  
  exec
   @args;               # subject to shell escapes
  
                                  # if @args == 1
  exec
   { $args[0] } @args;  # safe even with one-arg list

  The first version,the one without the indirect object,ran the echo program,passing it "surprise" an argument. The second version didn't--it tried to run a program literally called "echo surprise",didn't find it,and set $? to a non-zero value indicating failure.

  Beginning with v5.6.0,Perl will attempt to flush all files opened for output before the exec,you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles in order to avoid lost output.

  Note that exec will not call your END blocks,nor will it call any DESTROY methods in your objects.

  conclusion:

Both Perl's exec() function and system() function execute a system shell command. The big difference is that system() creates a fork process and waits to see if the command succeeds or fails - returning a value. exec() does not return anything,it simply executes the command. Neither of these commands should be used to capture the output of a system call. If your goal is to capture output,you should use the backtick operator :

$result = `PROGRAM`;