Standard and File I/Os

 

 

 

Files

 

File manipulations include reading, writing, and appending external data files such as saved Email file, password file, log file, web server error report file, and database file.

 

A password file can be used for access control to protect sensitive information from prying eyes. We can set up several usernames and passwords to direct users to specific areas of a web site based on their authentication.

Perl scripts can be used to access data and document files on disk. It allows a user to read/write text files from the command line and through a web browser interface.

Perl scripts allow articles service automation. That is can be designed to serve articles to a web browser as well as serving news articles based on the time of day.

Perl scripts help automating user comment gathering from users. The web server accepts data from HTML forms through the CGI environmental variables. It can be used to check required fields, display error messages, send results back to the user, and write the results to a log file to keep track of your data. It can also keeps statistics of the number of accesses, operating systems, browser types and number of unique IP addresses that have accessed your site or individual pages.


 

Standard Inputs and Outputs

 

In Perl, three predefined I/O streams associated with terminals:

 

A file handle is a name for an I/O connection between a running program called process and the outside peripherals. Therefore, file handles for these three standard I/O are called STDIN, STDOUT, and STDERR

 

STDIN file Handle for Reading Keyboard Input

 

chop () function

 

chomp() function

 

read() Function




 

Example 1: User input example.

 

#stdinex.pl

#

#OUTPUT:

#     E:\Perl\Exs>perl stdinex.pl

#     Enter your phone number = 4816339

#     Enter Your First Name =Paul

#     Enter Your Last Name =Lin

#     You entered:

#

#     Paul

#     Lin

#     4816339

print "Enter your phone number = ";

$phonenum = <STDIN>;

print "Enter Your First Name =";

$firstname = <STDIN>;

print "Enter Your Last Name =";

$lastname = <STDIN>;

print "You entered: \n\n";

print $firstname;

print $lastname;

print $phonenum;

 

 

Example 2: Chopping the new line character for displaying inputs on a single line.

 

#stdinexchop.pl

#

#OUTPUT:

#

#     E:\Perl\Exs>perl stdinexchop.pl

#     Enter your phone number = 4816339

#     Enter Your First Name =Paul

#     Enter Your Last Name =Lin

#     You entered:

#

#     Paul    Lin      4816339

 

print "Enter your phone number = ";

$phonenum = <STDIN>;

chop($phonenum);

print "Enter Your First Name = ";

chop($firstname = <STDIN>);

print "Enter Your Last Name = ";

chomp($lastname = <STDIN>);

print "You entered: \n\n";

print "$firstname \t$lastname\t $phonenum";

 


Formatted Output

 

The  printf() function uses format specifiers in the format string for printing formats references. It defines the data to be printed with appropriate display format on the standard output device (stdout) or monitor screen.  Some basic characteristics of this function are

·        Writes character string and values of variables, formatted, to “stdout”

·        printf (format_control);

·        Format control string supports the following display format:

 

Format Specifiers

·        %c          -- character

·        %d         -- signed integer, in decimal

·        %o         -- unsigned integer, in octal

·        %u          -- unsigned integer, in decimal

·        %x          -- unsigned integer, in hexadecimal (a-f or A-F)

·        %f           --  floating-point number, in fixed decimal notation(default 6 digits)

·        %e         --  floating-point number, in scientific notation

·        %g         --  floating-point number, in %e or %f form

·        %s          --- string

·        %%        --- for printing a literal % symbol

 

It is possible to use flags between the % sign and conversion character to modify the printing format.

 

Flags         Meaning

    -                      Left justified printing

 +                     Print either + or - sign

0                      Zero causes 0’s to be used for padding

 

Width (precision)       Meaning

%10d, %10s              At least 10-char

%.7s                           At most 7-char

%14.8s                       14-char in total, use only 8-char

%-7d                           Left justify, min. field width 7

%8.2f                          Field width 8, precision 2

%05d                          Padding with zeros

%-10.5e                     Left justified e format

 

An example

printf "%20S %4d %8.2f", $name, $pidnum, %amount;

 

 


 

File Processing Functions

 

Perl supports a rich set of file I/O functions for reading, writing, appending data to/from disk storage. A file handle is needed for opening a new file, an existing file, etc. A number of functions for file manipulation is listed below:

 

print      

# Output to standard output (stdout) device

 

open(FILEANDLE, "file.html");

 

# open a file for read, write, or append

 

open(FILEHANDLE, ">file.html");

# use > to overwrite if file.html exists

 

open(FILEHANDLE, ">>file.html");

# use >> to append if file.html content exists

 

open(FILEHANDLE, "| \perl5/sbin\sendmail"); 

# open a process

 

close(FILEHANDLE);     

# close a file or process

 

opendir(DIR, "directory");

# specify a file folder of directory

 

readdir(DIR);

# allow the access of every file in the file folder that

# specified by DIR file handle

 

 

closedir(DIR);   

      # close a directory file handle


 

Example 4: Reading an external Email text file.

 

 

#!/usr/bin/perl

# reademail.pl

# Description:

# This program open an email file and print it.

# - EMAIL is a file handler as the open() function call.

# - prop.txt is a saved email file.

# - print function will display text on the STDOUT

#   standard output screen (monitor)

# - close(EMAIL) to return file handler to OS

#

open (EMAIL, "prop.txt");

while(<EMAIL>)

 {

  print;

 }

close (EMAIL);

 

OUTPUT: Partial output from monitor screen

 

...

The four parts are:

1)  Setting up the Microsoft Advanced Server  platform running IIS to

enable JAVA Servlets to be run.

2)  Creating an HTML page that will call the JAVA servlet

3)  Creating a JAVA servlet

4)  Establishing database connectivity with the servlet to Microsoft

Access database

...

 

 


Example 5: Reading a saved Email text file through a browser. This Perl script is saved in the cgi-bin directory. Remote access through a web browser is required to access "reademail_html.pl" program.

 

 

#!/usr/bin/perl

# reademail_html.pl

# Description:

# This program open an email file and print it.

# - EMAIL is a file handler as the open() function call.

# - prop.txt is a saved email file.

# - print function will display text on the STDOUT

#   standard output screen (monitor)

# - close(EMAIL) to return file handler to OS

#

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

print "<HTML>\n";

print "<HEAD>\n";

print "<TITLE> Read Email </TITLE>\n";

print "</HEAD>\n";

print "<BODY BGCOLOR =\"#FFFFFF\">\n\n";

      open (EMAIL, "prop.txt");

      while(<EMAIL>)

       {

        print;

       }

      close (EMAIL);

print "</BODY>\n";

print "</HTML>\n";

 

 

 

 

 

 

 


 

6.5 Advanced File Manipulation Examples

 

Example 6: Locking and Unclocking Files

 

flock(FILEHANDLE, operation);

 

Where operation can be one of the following:

Name        Operation         Description

lock_sh     1                 Create a shared lock

lock_ex     2                 Create an exclusive lock

lock_nb     4                 Create a non-blocking lock

lock_un     8                 Unlock an existing lock

 

 

$lock_sh = 1;     #Create a shared lock

$lock_ex = 2;     #Create an exclusive lock so other can't write to it

$lock_nb = 4;     #Create a non-blocking lock

$lock_un = 8;     #Unlock an existing lock

open(FILE1, ">>file.txt");

flock(FILE1, "$lock_ex");     # lock the file

print "FILE1, "write some other data to the file that you locked";

flock(FILE1, "$lock_un");     # unlock the file

 

Example 7: print out every file and directory in the current directory (.)

 

opendir(DIR, ".");  # open current directory

foreach $name (readdir(DIR))

      {

      print "$name\n\n";

      }

closedir(DIR);