File Streams in UNIX

• Standard In, Standard Out, Standard Error
• standard file input/output (I/O)
• files: ordered stream of bytes that can be identified by the operating system
• stream can be from any input process:
• keyboard entry
• reading from disk file
• to any output process
• on-screen display
• writing to disk file

The difference between the file streams

• standard file in, standard in, or STDIN
• standard file out, standard out, or STDOUT
• standard file error, standard error, or STDERR

Universal to all Unix implementations

• STDIN
• byte stream associated with keyboard entry
• when you type something while Perl program is running, your entry becomes the contents of STDIN
• STDOUT
• byte stream sent to monitor
• "instead of writing to a file, write to the monitor"
• STDERR
• error messages sent to monitor or file (log)
• can use STDIN, STDOUT, AND STDERR as file handles in Perl
• don't have to open to name file handles

$GetLine = <STDIN>

· Perl will wait for you to type something

· followed by end-of-line character on keyboard

print STDERR "Bad user input.\n"

Code examples

print STDOUT "Enter a number: ";

$number = <STDIN>;

print STDOUT "The number is $number\n";

example of your input: 9 is really "9\n"

may want to do this:

chop($number = <STDIN>);

same as:

$number = <STDIN>;

chop($number);

while ($tickets_sold < 10000) {

$available = 10000 - $tickets_sold;

print "$available tickets are available. How many

would you like: ";

$purchase = <STDIN>;

chomp($purchase);

$tickets_sold += $purchase;

}

print "Sorry, no more tickets left.\n";

exit;

BTW...

die "Cannot open file: $!\n."

contains a string of most recent system error

exit versus die?

Piping data in and out of a program

• pipe acts like a conduit for data going from one source to another
• in Unix, use "pipes" to give a stream of bytes to a program
• use pipes to take a stream of bytes from a program via STDIN, STDOUT

• uptime program: how long a system has been on (and its load)
• uptime sends results to STDOUT
• so capture it with Perl's open statement
• pipe data from uptime to a file hanle this way:

open(INPUT, 'uptime |');

open tells Perl

run uptime command

takes STDOUT and feed to INPUT file handle

uptime outputs single line

read this way

$uptimeIn = <UPIN>;

• which directories exist under /tmp directory
• piping to a file handle
• pipe on right of Unix command

open(INPUT, 'ls -l /tmp | ');

while(<INPUT>) {

$aLine = $_;

chomp(aLine);

if(substr($aLine, 0, 1) eq 'd') {

$lastSpc = rindex($aLine, ' ');

$dirName = (substr($aLine, $lastSpc + 1));

print "$dirName is a directory\n";

}

}

• rindex returns last position (rightmost) of last match for a string

reminder name/value pairs separated by ?

lookfor=john?hometown=milwaukee?age=10

name=wood&address=510+brookside+drive&age=under+11&state=OR

lookfor=john #name/value pair

510+brookside+drive #plus is a space

...drive&age #& separates each pair from other pair

lookfor=john #equals separates name/values

sub Parse_Form {

if ($ENV{'REQUEST_METHOD'} eq 'GET') {

@pairs = split(/&/, $ENV{'QUERY_STRING'});

} elsif ($ENV{'REQUEST_METHOD'} eq 'POST') {

read (STDIN, $buffer,

$ENV{'CONTENT_LENGTH'});

@pairs = split(/&/, $buffer);

if ($ENV{'QUERY_STRING'}) {

@getpairs =split(/&/, $ENV{'QUERY_STRING'});
push(@pairs,@getpairs);

}

} else {

print "Content-type: text/html\n\n";

print "<P>Use Post or Get";

} foreach $pair (@pairs) {

($key, $value) = split (/=/, $pair);

$key =~ tr/+/ /;

$key =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;

$value =~ tr/+/ /;

$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;

$value =~ s/<!--(.|\n)*-->//g;

if ($formdata{$key}) {

$formdata{$key} .= ", $value";

} else {

$formdata{$key} = $value;

}

}

}

Hashes or associative arrays

List:

@names

access fist name:

$names[0]

Consider that you would like to store a pair of values:

employee numbers and their names

ID	Name
742	Jim Blank
334	Tina Blue
431	Lisa Blum

(742, 'Jim Blank', 334, 'Tina Blue', 431, 'Lisa Blum')

Locate items by their keys

$anEmployee = $IDList{"334"};

$anEmployee is now "Tina Blue"

Create an associative array

%IDList = (742, 'Jim Blank', 334, 'Tina Blue', 431, 'Lisa Blum')

difficult to read pairs?

Perl 5+ can do this:

%IDList = (742 => 'Jim Blank',

334 => 'Tina Blue',

431 => 'Lisa Blum')

if keys are more than one value, use quotes on keys

%IDList = () # begins an empty assoc array.

Add records:

$IDList{566} = "Gin Rummy";

Delete records:

delete($IDList{"Gin Rummy"});

delete returns value of key just deleted

%asDay = ("mon", "Monday", "tue", "Tuesday",

"wed", "Wednesday", "thu", "Thursday",

"fri", "Friday", "sat", "Saturday",

"sun", "Sunday");

# This prints the string "Friday"

print "$asDay{'fri'} \n";

# This also prints the string "Friday"

print "$asDay{fri} \n";

# This prints the string "Wednesday"

$sWhichDay="wed";

print "$asDay{$sWhichDay} \n";

# This adds another day to the associative array

$asDay{"frog"}="Froggyday";