Discover a few other useful special variables.
- [Voiceover] Perl uses special variables for a number of purposes, including default values, parameter passing, process control and configuration options. Here's a working copy of hello.pl from Chapter 7 of the exercise files and here I'm going to show you a few of the more commonly used special variables. Let's get started. I'm going to start out with the environment variable. This one actually looks like this. It's a special variable that's a hash that'll display the environment variables used for access from the system environment and it looks like this.
That percent ENV, that's the special variable and this'll show us the system environment variables. We're getting the keys here in for loop and this is actually a for each so I'm going to say for each, and we're getting the keys sorted in alphabetical order and then we're taking each key and we're using that to index into this ENV hash. When I run this, you'll see there's a lot of them here and your results will be very different from mine.
The results will be very different from computer to computer and from OS to OS, but this is the environment variables on this system at this time. That's the ENV variable and that can be really useful, especially if you're doing web applications and you want to get the parameters that are passed to a script from the browser. Here's another one, this one's really simple. Say $0 and when I run that, we get the full path name of this Perl script.
Again, this will be very different for you. It's specific to your OS and to your file system and to, obviously, where you have all these files stored. Another one is the operating system name and sometimes you'll want this. This is a caret character that's shift 6 on my keyboard and the letter O, that's not the number zero. When I run this, you'll see this says darwin which is the system name for the Apple OS 10 operating system.
This can be useful if you have a script out there and it runs on different operating systems. It might have different capabilities on different operating systems so you need to know I'm going to turn on this flag for this operating system and that switch for that operating system. Likewise, you might want to know the version of Perl that's being run so that's what the, again, the caret and the capital V as in Victor or as in Version. This says that this is 5.18.2. That's, again, for this computer and this operating system.
There are many, many more special variables in Perl and for a complete reference, see the Perl Var page in the standard Perl documentation. Here it is, it's a long list of these special characters and you might just find that this is a fun afternoon of reading and experimenting. I suggest that you go through this and you experiment with these and if you can, experiment with them on different operating systems and really learn about these because some of these are very, very useful in small, specific circumstances and you'll want to know that it's there when you say, "Hey, I could use "that value from the operating system" or "that capability from the operating system." You'll know that you can find that and you'll know where to find it.
Watch to learn the details of the Perl syntax, from variables, conditionals, loops, and data structures to regular expressions, functions, and references. A quick-start guide is included for experienced developers who want to get up and running with Perl 5 fast, and the entire course is recommended for both new and experienced programmers alike. Later chapters cover file handling and reusing code with Perl modules, plus Perl best coding practices.
- Understanding Perl's general syntax and the anatomy of a Perl script
- Writing statements and expressions
- Creating assignments
- Working with variables and strings
- Using data types effectively
- Defining logical flow with conditionals and loops
- Using special variables
- Using Perl operators
- Performing simple Perl programming tasks with expressions
- Matching data
- Defining and calling functions
- Using references
- Handling files in the file I/O
- Using built-in functions
- Reusing code with modules
- Coding with Perl best practices
Skill Level Intermediate
Programming Foundations: Refactoring Codewith Simon Allardice1h 44m Intermediate
1. Setting Up
About Perl3m 36s
2. Quick Start
3. Basic Syntax
4. Values and Variables
7. Special Variables
9. Regular Expressions
11. References and Structures
12. File I/O
13. Built-In Functions
15. Best Practices
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