View source for Perl I/O

== Opening files ==
<source lang="perl">
open CONFIG, "dino.config";       # open for reading
open CONFIG, "<", "dino.config";  # same thing
open BEDROCK, ">", "fred.txt";    # overwrite
open LOG, ">>", "logfile.log";    # append
</source>

== Read a file line-by-line ==
<source lang="perl">
open(FH, $fileName) or die "open $fileName failed: $!";
while (my $line = <FH>){
    chomp $line;   # get rid of end-of-line
    # do something with $line
}
</source>
A simpler alternative:
<source lang="perl">
open(FH, $fileName) or die "open $fileName failed: $!";
while (<FH>){
    chomp;  # acts on the default variable $_
    # do something with $_
}
</source>

== Read a file all at once ==
<source lang="perl">
open(FH, $fileName) or die "open $fileName failed: $!";
chomp(@lines = <FH>);
# process @lines
</source>

== Write to a file that's been opened for writing ==
<source lang="perl">
print LOG "Captain's log, stardate 3.14159\n";  # output goes to LOG
printf STDERR "%d percent complete.\n", $done/$total * 100;
</source>

== Perl's built-in filehandles ==
STDIN, STDOUT, STDERR allow your Perl program to act like a standard Unix command.

Use select to choose an alternate default output filehandle: 
<source lang="perl">
select LOG;
$| = 1;  # don't keep LOG entries sitting in the buffer
select STDOUT;
...
print LOG "This gets written to the LOG at once!\n";
</source>

== Closing files ==
<source lang="perl">
close CONFIG;  # Perl automatically closes open files at program exit
</source>

== Remapping STDERR to a log file ==
<source lang="perl">
open STDERR, ">>", "/var/log/system.log" or die "$!";
</source>

== Example: updating several files ==
<source lang="perl">
#!/usr/bin/perl -w

chomp(my $date = `date`);
$^I = ".bak";

while (<>) {
  s/^Author:.*/Author: Randal L. Schwartz/;
  s/^Phone:.*\n//;
  s/^Date:.*/Date: $date/;
  print;
}
</source>
Setting $^I causes the <> to print the updated lines to the original file name and save the original contents as a backup file.