How to get out of the Perl channel as it appears?

Question

The following code works in order:

open( PIPE, '-|', 'ant' ); for( <PIPE> ) { print; } 

However, he does not do what I want. Since Ant builds can take 5 minutes, I would like to see the output line by line. Instead, I get all the input at the end of the process.

By looking at it with the Perl debugger, Perl waits for the 'for' statement until Ant completes. Why is this?

Answer 1

Just for completeness (the problem was solved, as indicated in the comment to Uri's answer), the problem arose because of the for expression, which evaluates the <> operator in the context of the list ( see ), which is equivalent to the following:

 foreach $line (@lines = <PIPE>) { print $line; } 

In the context of the list, the <> operator tries to read all the lines from its input to asign in the list - and the input comes from the process, it will be blocked until the process ends. Only after that he will enter the body of the cycle.

Alternative syntax

 while( <PIPE> ) { print; } 

equivalent instead

 while( $line = <PIPE> ) { print $line; } 

ie, it consumes every line from the input in each iteration of the loop, and this is what you want to do in this scenario.

Answer 2

I believe the problem is buffering. You can make the file descriptor hot (unbuffered) as follows:

 select PIPE; $| = 1; # the pipe is just a coincidence. that the variable name select STDOUT; # select it back 

This should disable buffering for PIPE. I read about something similar in Learning Perl, 5th. See this . You should probably also make sure that buffering is not happening anywhere else.

EDIT: OP posted that replacing for to solve the problem

I think now I understand. while gets each line and starts the loop, while for acts like foreach and first gets all the lines (needs a list context) and then iterates over the lines.

Answer 3

Found. The problem is the expression for.

The following code works as expected

 open( PIPE, '-|', 'ant' ); while( <PIPE> ) { # replacing 'for' with 'while' print; } 

I do not know the reason, but now it works.