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STDIN, STDOUT, STDERR and DATA - Perl file handles

A Typical computer program is run from a single keyboard or other control point, but with access to thousands of files. So it's "cheap" for a programming language to open a connection and provide a buffer to that control centre (often known as "standard in" and "standard out", but impractically expensive to have connections open to all the available files. For this reason, most languages provide a function with a name such as open or fopen to provide a resource through which files can be read and written.

Perl is like this but (being Perl) goes a step further. The following file handles are open by default:
STDIN - Standard Input, for user control input, typically the keyboard
STDOUT - Standard Output,for regular use output, typically the screen
STDERR - Standard Error, for error output; typically defaults to the screen too
DATA - Input for data stored after __END__ at the end of the program

Here's an example program that uses all four of these special file handles:


while (<DATA>) {
chop;
$mname{$_} = ++$mn;
}

# seaweed - - [15/Jul/1998:08:32:38 -0400] "GET / HTTP/1.0" 200 1476

print "Which Host? ";
chop ($host = <STDIN>);

while (<>) {

($day, $month, $year,$page) =
m!^$host\s.*\[(\d{2})/(...)/(....).*(?:POST|GET)\s+(\S+)!;

if ($day) {
if ($page !~ m!^/!) {
print STDERR "YIKES ... $page\n";
}
print STDOUT "$day - $mname{$month} - $year .... $page\n";

}
}
__END__
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STDERR merits a short extra comment. You may wonder why there are two outputs both of which, by default, are routed to the screen. The "trick" is that it's useful to have two separate file handles for use then STDOUT is diverted away from the screen - for example, when STDOUT is diverted to a browser or a file. In such a circumstance, the user typically doesn't want error messages going to the same place, but wants it to go to the server's error log, or to the screen, respectively. You'll find more examples of Perl's file handling in our File Handling module.
(written 2005-03-23, updated 2006-06-05)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P207 - Perl - File Handling
  [12] How many people in a room? - (2004-08-12)
  [114] Relative or absolute milkman - (2004-11-10)
  [616] printf - a flawed but useful function - (2006-02-22)
  [618] Perl - its up to YOU to check your file opened - (2006-02-23)
  [702] Iterators - expressions tha change each time you call them - (2006-04-27)
  [867] Being sure to be positive in Perl - (2006-09-15)
  [1312] Some one line Perl tips and techniques - (2007-08-21)
  [1416] Good, steady, simple example - Perl file handling - (2007-10-30)
  [1442] Reading a file multiple times - file pointers - (2007-11-23)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1841] Formatting with a leading + / Lua and Perl - (2008-10-15)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1861] Reactive (dynamic) formatting in Perl - (2008-10-31)
  [2233] Transforming data in Perl using lists of lists and hashes of hashes - (2009-06-12)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [2818] File open and read in Perl - modernisation - (2010-06-19)
  [2821] Chancellor George Osborne inspires Perl Program - (2010-06-22)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [3326] Finding your big files in Perl - design considerations beyond the course environment - (2011-06-14)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3830] Traversing a directory in Perl - (2012-08-08)
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)


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