Google Answers Logo
View Question
Q: simple Perl script to print from two files ( Answered 5 out of 5 stars,   0 Comments )
Subject: simple Perl script to print from two files
Category: Computers > Programming
Asked by: spangler-ga
List Price: $20.00
Posted: 11 Sep 2006 12:25 PDT
Expires: 11 Oct 2006 12:25 PDT
Question ID: 764213
I have two tab-delimited text files (File1, File2) with the same
number of lines ... I want to create a new file with the same number
of lines, but has the information on each line of File2 after the
information on the corresponding line in File1 (with a tab separating
the File2 content
from the File1 content)
Subject: Re: simple Perl script to print from two files
Answered By: maniac-ga on 11 Sep 2006 18:49 PDT
Rated:5 out of 5 stars
Hello Spangler,

The program listed at the end will combine the two files as you
specified and generate output to "standard output" which can be
directed to a new file. I kept the program simple to make it easier to
explain but I am also describing several "improvements" that may be
helpful as well.

The first line
tells a Unix (or Linux) system that you want to perl to process the
following commands. This assumes that perl is installed in a system
directory (/usr/bin) - the typical location. On systems that do not
support this, it should be processed as a comment and ignored.

The next few lines are comments to help explain what the program does.

The line starting with "die" is executed only if there are less than
two parameters (the file names) on the command line. The $0 is
replaced by the name of your script. The @ARGV refers to the last
index in the $ARGV array. If die is executed, it puts out an
informative message to help the user run the perl script and the
script quits. If you want a third parameter (say the output file) -
change the 2 to a 3.

The line starting with $tab creates a string variable and assigns the
"tab" character to it. This is used only to aid in readability in the
print statement later.

The two open statements, create file handles for FILE1 and FILE2 using
the first two parameters of the command line. As-is these do not do
any error checking - if that is desired rewrite to something like this
  open(FILE1, $ARGV[0]) || die "Cannot open $ARGV[0]: $!\n";
which will execute "die" only if the open fails and explains the error
message (usually something like "file not found"). Note that the array
starts with zero (0), so the references are to $ARGV[0] for the first
argument and $ARGV[1] for the second argument.

If you need to generate output into a new file, add a statement like
  open(FILE3, "> " . $ARGV[2]);
[or with an added "die"] to create a file using the third argument.
The open function is defined such that
  "filename" - open for reading
  "> filename" - open for writing
  ">> filename" - open for append
and so on.

The while loop repeats, reading a line at a time from the filehandle
FILE1 until that first file is exhausted. The next statement strips
off the newline from the end of the line - otherwise the output would
look something like

and so on. The last statement concatenates the line from FILE1, a tab,
and a line from FILE2 and prints the output to the standard output. If
you added a third open statement to create an output file, change
STDOUT to FILE3 [that file will be closed automatically when the
script exits - or use "close" to close it].

As a side comment, the program should also run to completion if FILE1
has more lines than FILE2. In this case, the output will still look
OK. If FILE2 is longer than FILE1, and it is important that all the
lines get output - add something like this to the end
  while ($line = <FILE2>) {
    print (FILE3 $tab . <FILE2>);
which will walk through the rest of FILE2 and preceed the output of
each line with a tab.

Let me also provide you with some good text and on line references as well.

"Programmin Perl" by Larry Wall and Randal L Schwartz has an excellent
explanation of perl. I used a first edition of that book extensively
in preparing this answer. For example:
 - use of the die statement in the chapter titled "Real Perl Programs"
 - the chapters on "The Gory Details" and "Functions" for operator and function use
 - the chapter on "An Overview of Perl" for a refresher on the
language to put it together

On line, see
or  (this link did not work, but the next one did)
for a couple good Perl tutorials or search with phrases such as
  perl tutorial
  perl sample scripts
or check out the learning resource at at

If any part of the answer is incomplete or unclear, please make a
clarification request.
Good luck with your work.



# A program to concatenate two files.
# For files FILE1 & FILE2, the output is
# FILE1 <tab> FILE2
# on each line.

die "Usage: $0 file1 file2\nwhere file1 and file2 are files with equal
number of lines.\n" if @ARGV < 2;

$tab = "\t";

open(FILE1, $ARGV[0]);
open(FILE2, $ARGV[1]);

while ($line = <FILE1>) {
    $line =~ s/\n//;
    print STDOUT $line . $tab . <FILE2>
spangler-ga rated this answer:5 out of 5 stars
thank you maniac for the helpful #comment lines that make clear the
actions invoked by the perl script ... the program ran first time out
of the box and i am going to replace a larger inelegant script with
yours ...

There are no comments at this time.

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy