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2.2 Part or Whole?

 
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2.2 Part or Whole?

Are you in fact taking over a complete program or a module used by other programs (or both)? Let's see how we can find out.

2.2.1 Shebang-a-Lang-a-Ding-Dong

You can recognize a Perl program file by the fact that the first two characters are:


#!

and somewhere on the rest of that line the word "perl" appears.[1] Developers call this the shebang line. It is possible to create a Perl program without this line by requiring it to be run explicitly with a command something like

[1] A Perl program on Windows could get away without this line, because the .pl suffix is sufficient to identify it as a Perl program, but it is good practice to leave the line in on Windows anyway. (The path to Perl isn't important in that case.)


% perl program_file_name

although it would be strange to receive a main program in this state. Don't depend on the filename ending with an extension like .pl or .plx; this is not necessary on many systems. A .pl extension is commonplace on Windows systems, where the file extension is required to tell the operating system the type of the file; aside from that .pl extensions were often used as a convention for "Perl Library": files containing specialized subroutines. These mostly precede the introduction in Perl 5 of objects, which provide a better paradigm for code reuse.

One time when the extension of a file is guaranteed, however, is for a Perl module; if the filename ends in .pm, then it's a module, intended to be used by another file that's the actual program.

Caveat: Sometimes a .pm file may begin with a shebang; this almost certainly means that someone created a module that contains its own tests so that executing the module as a program also works. If you see a .pm like this, try running it to see what happens. A file that's not a .pm can't be a module, but could still be a dependency rather than the main program. If it begins with a shebang, it could be a library of subroutine or constant definitions that's been endowed with self-testing capabilities. It may not be possible to tell the difference between this type of file and the main program without careful inspection or actual execution if the developer did not comment the file clearly.

It is quite possible that you will have to change the shebang line to refer to a different perl. The previous owners of the program may have located their perl somewhere other than where the one you plan to use is. If the code consists of a lot of files containing that path, here's how you can change them all at once, assuming that /their/path/to/perl is on the original shebang line and /your/path/to/perl is the location of your perl:


% perl -pi.bak -e \

  's#/their/path/to/perl#/your/path/to/perl#g' *

This command puts the original version of each filebefore any changes were madein a file of the same name but with .bak appended to it. If you've been using a revision control system to store the files in, you don't need to make copies like that. (I told you that would turn out to be a good decision.) Leaving out the .bak:


% perl -pi -e 's#/their/path/to/perl#/your/path/to/perl#g' *

results in in-place editing; that is, the original files are overwritten with the new contents.

This command assumes that all the files to be changed are in the current directory. If they are contained in multiple subdirectories, you can combine this with the find command like this:


% find . -type f -print | xargs perl -pi -e \

's#/their/path/to/perl#/your/path/to/perl#g'

Of course, you can use this command for globally changing other strings besides the path to perl, and you might have frequent occasion to do so. Put the command in an alias, like so:


% alias gchange "find . -type f -print | xargs \

perl -pi.bak -e '\!1'"

the syntax of which may vary depending on which shell you are using. Then you can invoke it thusly:


% gchange s,/their/path/to/perl,/your/path/to/perl,g

Note that I changed the substitution delimiter from a # to a ,: A shell might take the # as a comment-introducing character. Because you might be using this alias to change pieces of code containing characters like $ and ! that can have special meaning to your shell, learn about how your shell does quoting and character escaping so you'll be prepared to handle those situations.

Note also that I put the .bak back. Because otherwise one day, you'll forget to check the files into your version control system first, because the alias isn't called something like gchange_with_no_backups.

If you want to develop this concept further, consider turning the alias into a script that checks each file in before altering it.

2.2.2 .ph Files

You may encounter a .ph file. This is a Perl version of a C header (.h) file, generated by the h2ph program that comes with perl. The odds are that you can eliminate the need for this file in rewriting the program. These .ph files have not been commonly used since Perl 4 because the dynamic module loading capability introduced in Perl 5 made it possible, and desirable, for modules to incorporate any header knowledge they required. A private .ph file is probably either a copy of what h2ph would have produced from a system header (but the author lacked the permission to install it in perl's library), or a modified version of the same. Read the .ph file to see what capability it is providing and then research modules that perform the same function.

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