cvs commit: perlfaq perlfaq8.pod

cvsuser     02/06/25 14:21:10

  Modified:    .        perlfaq8.pod
  Log:
  * How can I capture STDERR from an external command?
  
  	+ added IPC::Open3 examples from Benjamin Goldberg
  
  * How can I call backticks without shell processing?
  
  	+ added a multiple argument example for open()
  	new to version 5.8
  	+ removed Microsoft-hostile language
  
  * How do I avoid zombies on a Unix system?
  
  	+ updated reference to double fork example which is no
  	longer in perlfunc, although it is in here now :)
  
  Revision  Changes    Path
  1.10      +80 -9     perlfaq/perlfaq8.pod
  
  Index: perlfaq8.pod
  ===================================================================
  RCS file: /cvs/public/perlfaq/perlfaq8.pod,v
  retrieving revision 1.9
  retrieving revision 1.10
  diff -u -w -r1.9 -r1.10
  --- perlfaq8.pod	21 Jun 2002 04:33:43 -0000	1.9
  +++ perlfaq8.pod	25 Jun 2002 21:21:10 -0000	1.10
  @@ -1,6 +1,6 @@
   =head1 NAME
   
  -perlfaq8 - System Interaction ($Revision: 1.9 $, $Date: 2002/06/21 04:33:43 $)
  +perlfaq8 - System Interaction ($Revision: 1.10 $, $Date: 2002/06/25 21:21:10 $)
   
   =head1 DESCRIPTION
   
  @@ -620,6 +620,68 @@
   script's STDOUT and STDERR, unless the system() command redirects them.
   Backticks and open() read B<only> the STDOUT of your command.
   
  +You can also use the open3() function from IPC::Open3.  Benjamin
  +Goldberg provides some sample code:
  +
  +To capture a program's STDOUT, but discard it's STDERR:
  +
  +    use IPC::Open3;
  +    use File::Spec;
  +    use Symbol qw(gensym);
  +    open(NULL, ">", File::Spec->devnull);
  +    my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
  +    while( <PH> ) { }
  +    waitpid($pid, 0);
  +
  +To capture a program's STDERR, but discard it's STDOUT:
  +
  +    use IPC::Open3;
  +    use File::Spec;
  +    use Symbol qw(gensym);
  +    open(NULL, ">", File::Spec->devnull);
  +    my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
  +    while( <PH> ) { }
  +    waitpid($pid, 0);
  +
  +To capture a program's STDERR, and let it's STDOUT go to our own STDERR:
  +
  +    use IPC::Open3;
  +    use Symbol qw(gensym);
  +    my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
  +    while( <PH> ) { }
  +    waitpid($pid, 0);
  +
  +To read both a command's STDOUT and its STDERR separately, you can
  +redirect them to temp files, let the command run, then read the temp
  +files:
  +
  +    use IPC::Open3;
  +    use Symbol qw(gensym);
  +    use IO::File;
  +    local *CATCHOUT = IO::File->new_tempfile;
  +    local *CATCHERR = IO::File->new_tempfile;
  +    my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
  +    waitpid($pid, 0);
  +    seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
  +    while( <CATCHOUT> ) {}
  +    while( <CATCHERR> ) {}
  +
  +But there's no real need for *both* to be tempfiles... the following
  +should work just as well, without deadlocking:
  +
  +    use IPC::Open3;
  +    use Symbol qw(gensym);
  +    use IO::File;
  +    local *CATCHERR = IO::File->new_tempfile;
  +    my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
  +    while( <CATCHOUT> ) {}
  +    waitpid($pid, 0);
  +    seek CATCHERR, 0, 0;
  +    while( <CATCHERR> ) {}
  +
  +And it'll be faster, too, since we can begin processing the program's
  +stdout immediately, rather than waiting for the program to finish.
  +
   With any of these, you can change file descriptors before the call:
   
       open(STDOUT, ">logfile");
  @@ -744,11 +806,20 @@
   
   =head2 How can I call backticks without shell processing?
   
  -This is a bit tricky.  Instead of writing
  +This is a bit tricky.  You can't simply write the command
  +like this:
   
       @ok = `grep @opts '$search_string' @filenames`;
   
  -You have to do this:
  +As of Perl 5.8.0, you can use open() with multiple arguments.
  +Just like the list forms of system() and exec(), no shell
  +escapes happen.
  +
  +   open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
  +   chomp(@ok = <GREP>);
  +   close GREP;
  +
  +You can also:
   
       my @ok = ();
       if (open(GREP, "-|")) {
  @@ -764,12 +835,9 @@
   Just as with system(), no shell escapes happen when you exec() a list.
   Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
   
  -Note that if you're stuck on Microsoft, no solution to this vexing issue
  +Note that if you're use Microsoft, no solution to this vexing issue
   is even possible.  Even if Perl were to emulate fork(), you'd still
  -be hosed, because Microsoft gives no argc/argv-style API.  Their API
  -always reparses from a single string, which is fundamentally wrong,
  -but you're not likely to get the Gods of Redmond to acknowledge this
  -and fix it for you.
  +be stuck, because Microsoft does not have a argc/argv-style API.
   
   =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
   
  @@ -945,6 +1013,9 @@
   ``Signals'' in the Camel.  You may instead use the more flexible
   Sys::AlarmCall module available from CPAN.
   
  +The alarm() function is not implemented on all versions of Windows.
  +Check the documentation for your specific version of Perl.
  +
   =head2 How do I set CPU limits?
   
   Use the BSD::Resource module from CPAN.
  @@ -953,7 +1024,7 @@
   
   Use the reaper code from L<perlipc/"Signals"> to call wait() when a
   SIGCHLD is received, or else use the double-fork technique described
  -in L<perlfunc/fork>.
  +in L<perlfaq8/"How do I start a process in the background?">.
   
   =head2 How do I use an SQL database?