3 ways to iterate over lines of a file in Linux

Frequently I need to run a process for each item in a list, stored in a text file one item per line: usernames, filenames, e-mail addresses, etc. Obviously there are more than 3 ways to do this, but here are 3 I have found useful:

sh prog1.sh list.txt

Source: prog1.sh

while read line
    echo $line
done < $1

4 lines. Not bad.

perl prog2.pl list.txt

Source: prog2.pl

while(<>) {
    print `echo $_`;

3 lines. Pretty good.

Perl -n
perl -n prog3.pl list.txt

Source: prog3.pl

print `echo $_`;

1 line! The -n switch basically wraps your Perl code in a loop that processes each line of the input file. I just discovered this while flipping through my 17-year-old copy of Programming Perl (link is to a newer edition).

I really like this method because you can write a script that processes a single input that could easily be reused by another script, but can also easily be used to process an entire list by adding just the -n switch. (There’s also a similar -p switch that does the same thing, but additionally prints out each line.)

I should note that in the examples above, I am using echo as a substitute for any command external to the script itself. In the Perl examples, there would be no need to call echo to merely print the contents of the line, but it’s a convenient stand-in for a generic command.

As suggested by a comment on a previous post, I have made these examples available in a git repository: iterate over lines.

Perl error when running W3C checklink

I’m using ActiveState Perl 5.14.2 on a 64-bit Windows 7 machine. I downloaded and installed the W3C checklink package via the Perl Package Manager.

When I attempted to run checklink on the command line like this:
C:\Users\chris\>checklinks http://osric.com
I got the following error message:
"-T" is on the #! line, it must also be used on the command line.

To get this to work, I had to run the command as follows:
C:\Users\chris\>perl -T C:\perl64\site\bin\checklink http://osric.com

The -T switch is to enable taint mode, which helps protect the program from malicious input.

Using Perl and PDF::API2 to Update PDF Properties and Metadata

What do you do when you have 600 PDF documents with titles in all caps, when you need the titles to be title-cased? I dreaded the thought of asking anyone to open each document and edit the titles by hand, not to mention fearing the typos that process might introduce.

For better or worse, here was my solution:

Sounds fast and easy, right? Well, there were a few hitches:
Continue reading Using Perl and PDF::API2 to Update PDF Properties and Metadata