IPC::Run and subprocess interaction

xhudik has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks, I'm trying to write a simple perl script that would execute program (e.g. cat), and then make some interaction with the program. I'm using IPC::Run since it is able to emulate terminal (pty) which is needed for the interaction (some putting and getting data from the subprocess). My program:

#!/usr/bin/perl -w
use IPC::Run qw(start pump finish timeout);

my $tok_program = "/bin/./cat"; 


my ($TOK_IN, $TOK_OUT, $TOK_ERR);
my $TOK = start [$tok_program], '<pty<', \$TOK_IN,'>pty>',\$TOK_OUT, '
+2>', \$TOK_ERR , debug => 3 or die "Error: $?;\n"; 


while (my $line = <STDIN> ) {
    chomp($line);
    $TOK_IN = $line;
     
    $TOK_OUT = '';
    $TOK_ERR = '';
    
    print "Input2pump:$TOK_IN\n";
    pump $TOK while length $TOK_IN;

    print "out:$TOK_OUT\n"; 
}
[download]

Basicaly, it should execute cat program and then post each input to the "cat" and wait for the answer. However, the output is not what it was supposed to be :). (The output can differs from time ti time - sometimes is completely empty, sometimes just 1 entry is empty,...) Obviously, I'm missing something but no idea what Any help will be appreciate!

testing file a:
1. line
2. line
The command:
./wrap < a
And the output is:

IPC::Run 0000 0123---7---- [#1(20574)]: debugging fd is 3
IPC::Run 0000 0123---7---- [#1(20574)]: ** starting
IPC::Run 0000 0123---7---- [#1(20574)]: opening pty '0'
IPC::Run 0000 012345-7---- [#1(20574)]: pty() = ( 4, 5 )
IPC::Run 0000 012345-7---- [#1(20574)]: '/bin/./cat' is absolute
IPC::Run 0000 012345-7---- [#1(20574)]: kid to read 0 from pty '0'
IPC::Run 0000 012345-7---- [#1(20574)]: kid 1 to read 0 from SCALAR vi
+a pty '0'
IPC::Run 0000 012345-7---- [#1(20574)]: kid 1 to write 1 to SCALAR via
+ pty '0'
IPC::Run 0000 012345-7---- [#1(20574)]: kid 1 to write 2 to SCALAR
IPC::Run 0000 012345678--- [#1(20574)]: pipe() = ( 6, 8 ) 
IPC::Run 0000 012345678--- [#1(20574)]: kid 1[]'s 0 is my 4
IPC::Run 0000 012345678--- [#1(20574)]: kid 1[]'s 1 is my 4
IPC::Run 0000 012345678--- [#1(20574)]: kid 1[]'s 2 is my 6
IPC::Run 0000 012345678--- [#1(20574)]: child: `'/bin/./cat'`
IPC::Run 0000 012345678--- [#1(20574)]: opening sync pipe 
IPC::Run 0000 01234567890- [#1(20574)]: pipe() = ( 9, 10 ) 
IPC::Run 0000 01234567890- [#1(20574)]: fork() = 20575
IPC::Run 0000 0123456789-- [#1(20574)]: close( 10 ) = 0
IPC::Run 0000 01234567890- [#1(20575) cat]: Cleaning up parent's ptty 
+'0'
IPC::Run 0000 0123-567890- [#1(20575) cat]: closing stdin, out, err
IPC::Run 0000 ---3-567890- [#1(20575) cat]: open fds: 6 8 4 9 10 5
IPC::Run 0000 ---3-5-7890- [#1(20575) cat]: close( 6 ) = 0
IPC::Run 0000 ---3-5-78-0- [#1(20575) cat]: close( 9 ) = 0
IPC::Run 0000 0--3-5-78-0- [#1(20575) cat]: dup2( 5, 0 ) = 0
IPC::Run 0000 01-3-5-78-0- [#1(20575) cat]: dup2( 5, 1 ) = 1
IPC::Run 0000 0123-5-78-0- [#1(20575) cat]: dup2( 1, 2 ) = 2
IPC::Run 0000 0123-5-78-0- [#1(20575) cat]: dup2( 8, 2 ) = 2
IPC::Run 0000 0123---78-0- [#1(20575) cat]: close( 5 ) = 0
IPC::Run 0000 0123---7--0- [#1(20575) cat]: close( 8 ) = 0
IPC::Run 0000 0123---7--0- [#1(20575) cat]: execing /bin/./cat
IPC::Run 0000 0123---7--0- [#1(20575) cat]: exec()ing '/bin/./cat'
IPC::Run 0000 0123456789-- [#1(20574)]: read( 9 ) = 0 but true chars '
+'
IPC::Run 0000 012345678--- [#1(20574)]: close( 9 ) = 0
IPC::Run 0000 01234-67---- [#1(20574)]: close( 8 ) = 0
Input2pump:1. line
IPC::Run 0000 01234-67---- [#1(20574)]: ** pumping
IPC::Run 0000 01234-67---- [#1(20574)]: fds for select: ----b-r-----
IPC::Run 0000 01234-67---- [#1(20574)]: timeout=forever
IPC::Run 0000 01234-67---- [#1(20574)]: selected  ----w-------
IPC::Run 0000 01234-67---- [#1(20574)]: filtering data to fd 4 (kid's 
+stdin)
IPC::Run 0000 01234-67---- [#1(20574)]: writing to fd 4 (kid's stdin)
IPC::Run 0000 01234-67---- [#1(20574)]: write( 4, '1. line' ) = 7
IPC::Run 0000 01234-67---- [#1(20574)]: exiting _select(): io occured 
+and break_on_io set
out:
Input2pump:2. line
IPC::Run 0000 01234-67---- [#1(20574)]: ** pumping
IPC::Run 0000 01234-67---- [#1(20574)]: fds for select: ----b-r-----
IPC::Run 0000 01234-67---- [#1(20574)]: timeout=forever
IPC::Run 0000 01234-67---- [#1(20574)]: selected  ----b-------
IPC::Run 0000 01234-67---- [#1(20574)]: filtering data to fd 4 (kid's 
+stdin)
IPC::Run 0000 01234-67---- [#1(20574)]: writing to fd 4 (kid's stdin)
IPC::Run 0000 01234-67---- [#1(20574)]: write( 4, '2. line' ) = 7
IPC::Run 0000 01234-67---- [#1(20574)]: filtering data from fd 4 (kid'
+s stdout)
IPC::Run 0000 01234-67---- [#1(20574)]: reading from fd 4 (kid's stdou
+t)
IPC::Run 0000 01234-67---- [#1(20574)]: read( 4 ) = 7 chars '1. line'
IPC::Run 0000 01234-67---- [#1(20574)]: exiting _select(): io occured 
+and break_on_io set
out:1. line
[download]

Here, the output is written but the expected output is:
1. line
2. line
or, the output without debug info:

Input2pump:1. line
out:
Input2pump:2. line
out:
[download]

Here, there is nothing in the output

Thank you, Tomas

Comment on IPC::Run and subprocess interaction Select or Download Code

Replies are listed 'Best First'.
Re: IPC::Run and subprocess interaction by ikegami (Patriarch) on Jul 14, 2011 at 19:08 UTC
Two problems. `cat` still buffers its output when its STDOUT is connected to a terminal. Like Perl's default, it uses block buffering when its STDOUT isn't connected to a terminal and line buffering when it is. You'll have to send a newline to get it to react. You're program is too fast! `cat` never has a change to run. You need to wait for output from `cat`. `#!/usr/bin/perl -w use strict; use IPC::Run qw( start pump finish timeout ); my @tok_program = 'cat'; my ($TOK_IN, $TOK_OUT, $TOK_ERR); my $TOK = start \@tok_program, '<', \$TOK_IN, '1>pty>', \$TOK_OUT, '2>', \$TOK_ERR or die "Error: $?;\n"; while (my $line = <STDIN> ) { # Send input. $TOK_IN = $line; pump $TOK while length $TOK_IN; # Wait for output. pump $TOK while $TOK_OUT !~ /\n\z/; print "out: $TOK_OUT"; $TOK_OUT = ''; } finish $TOK or die "returned $?";` [download] No reason to make STDIN a pty. Added missing call to `finish`. `>pty>` changed STDOUT and STDERR. Switched to `1>pty>`. Expect also uses a pty, and it might be better suited to control interactive application.	[reply] [d/l] [select]
Re^2: IPC::Run and subprocess interaction by xhudik (Initiate) on Jul 15, 2011 at 07:52 UTC
Thanks ikegami. You are right. Hmm, so my basic mistake was not to put the second pump (for reading back to $TOK_OUT). During my Internet browsing I didn't find any example with 2 pumps. Just for the sake of curiosity. $TOK_OUT doesn't seem to be an ordinary variable. If you run the following program: #!/usr/bin/perl -w use IPC::Run qw(start pump finish timeout); my $tok_program = "/bin/./cat"; my ($TOK_IN, $TOK_OUT, $TOK_ERR); my $TOK = start [$tok_program], '<', \$TOK_IN, '1>pty>',\$TOK_OUT, '2>', \$TOK_ERR , debug => 0 or die "Error: $?;\n"; while (my $line = <STDIN> ) { # Send input. $TOK_IN = $line; pump $TOK while length $TOK_IN; # Wait for output. pump $TOK while $TOK_OUT !~ /\n\z/; my $msg = $TOK_OUT; chomp($msg); print "out:$TOK_OUT;;MESSAGE=$msg;;END\n"; $TOK_OUT = ''; } finish($TOK) or die "returned: $?"; [download] You will get: `out:1. line ;;ENDSAGE=1. line out:2. line ;;ENDSAGE=2. line` [download] Instead of: `out:1. line ;;MESSAGE=1. line;;END out:2. line ;;MESAGE=2. line;;END` [download] Do you know why? Once more - thank you!	[reply] [d/l] [select]
Re^3: IPC::Run and subprocess interaction by ikegami (Patriarch) on Jul 15, 2011 at 18:16 UTC
During my Internet browsing I didn't find any example with 2 pumps. Only one is needed. This would do: `$TOK_IN = $line; pump $TOK while $TOK_OUT !~ /\n\z/; print "out: $TOK_OUT"; $TOK_OUT = '';` [download] This one is in the docs. $TOK_OUT doesn't seem to be an ordinary variable. Change `chomp` to `s/\r?\n\z//`.	[reply] [d/l] [select]
Re^4: IPC::Run and subprocess interaction by xhudik (Initiate) on Jul 18, 2011 at 15:24 UTC
Re^5: IPC::Run and subprocess interaction by ikegami (Patriarch) on Jul 18, 2011 at 17:27 UTC
Re: IPC::Run and subprocess interaction by runrig (Abbot) on Jul 14, 2011 at 18:23 UTC
My guess: 'cat' is buffering the input because there are no linefeeds. If you don't chomp the lines, you get the expected output at the expected time. Update: Nevermind. I am likely wrong.	[reply]