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Section 5.9.  Using Filehandles

 
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5.9. Using Filehandles

Once a filehandle is open for reading, you can read lines from it the same way you can read from standard input with STDIN. So, for example, to read lines from the Unix password file:

    if ( ! open PASSWD, "/etc/passwd") {
      die "How did you get logged in? ($!)";
    }
    while (<PASSWD>) {
      chomp;
       . . .
    }

In this example, the die message uses parentheses around $!. Those are literal parentheses around the message in the output. (Sometimes, a punctuation mark is just a punctuation mark.) As you can see, what we've been calling the "line-input operator" is two components; the angle brackets (the real line-input operator) are around an input filehandle.

A filehandle open for writing or appending may be used with print or printf, appearing immediately after the keyword but before the list of arguments:

    print LOG "Captain's log, stardate 3.14159\n";  # output goes to LOG
    printf STDERR "%d percent complete.\n", $done/$total * 100;

Did you notice that there's no comma between the filehandle and the items to be printed?[*] This looks especially weird if you use parentheses. Either of these forms is correct:

[*] If you got straight A's in freshman English or Linguistics, then when we say this is called "indirect object syntax," you may say "Ah, of course! I see why there's no comma after the filehandle nameit's an indirect object!" We didn't get straight A's; we don't understand why there's no comma, but we omit it because Larry told us to do that.

    printf (STDERR "%d percent complete.\n", $done/$total * 100);
    printf STDERR ("%d percent complete.\n", $done/$total * 100);

5.9.1. Changing the Default Output Filehandle

By default, if you don't give a filehandle to print (or to printf as everything we say here about one applies equally well to the other), the output will go to STDOUT. But that default may be changed with the select operator. Here we'll send some output lines to BEDROCK:

    select BEDROCK;
    print "I hope Mr. Slate doesn't find out about this.\n";
    print "Wilma!\n";

Once you've selected a filehandle as the default for output, it will stay that way. But it's generally a bad idea to confuse the rest of the program, so set it back to STDOUT when you're done.[Section 5.9.  Using Filehandles] Also by default, the output to each filehandle is buffered. Setting the special $| variable to 1 will set the selected filehandle (the one selected at the time the variable is modified) to flush the buffer after each output operation. If you want to ensure the logfile gets its entries immediately, in case you might be reading the log to monitor progress of your long-running program, you could use something like this:

[Section 5.9.  Using Filehandles] In the unlikely case that STDOUT might not be the selected filehandle, you could save and restore the filehandle using the technique shown in the documentation for select in the perlfunc manpage. And as long as we're sending you to that manpage, we may as well tell you that there are two built-in functions in Perl named select, and both covered in the perlfunc manpage. The other select has four arguments, so it's sometimes called "four-argument select".

    select LOG;
    $| = 1;  # don't keep LOG entries sitting in the buffer
    select STDOUT;
    # ...time passes, babies learn to walk, tectonic plates shift, and then . . .
    print LOG "This gets written to the LOG at once!\n";

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