Monday, May 12, 2008

String Variables In Bash, Perl, C and Awk on Linux or Unix - Porting

Hey There,

Once again, we're back to porting. In this series of posts that's run the gamut from a somewhat brief explanation of the shebang line (The rightful starting place) to shell, Perl and C porting examples of a fully functional useradd program, we're finally coming back around to the beginning and getting back to basics. Trust me; this will eventually all fall together. When it does, I'll be sure to put up a road map so no one has to try the hit-and-miss blog-search method of information nesting ;)

Today, we're going to look at the simple variable (also referred to as scalar, string, etc); defining, assigning value to and extracting value from it on both Unix and Linux based systems. Our approach is going to be concept-based. That is to say that, for each post on cross-language porting, we'll be hitting on a single concept (such as the simple variable) and showing how each can be applied to our chosen four languages: bash (or shell), Perl, C and Awk (Some folks think Awk isn't a programming language for some reason, but we'll demonstrate, over time, that it must be, since it contains all the constructs that generally define a language as a "programming" language; as opposed to a "markup" language like HTML). I'm going to try and keep this linear, so we're starting out with the very basics and will, eventually, work toward more complex programming constructs.

Here we go:

The simple variable is relatively accurately described in its name. This is one of the simplest forms of variables, as it can be realistically thought of as having only two parts: Part 1 is the variable itself, and part 2 is that variables definition or value. If we say x=y, then the variable x (itself) equals y (its definition/value). Simple enough.

1.Defining, Initializing or Declaring the simple variable. This part is going to be simple for every kind of variable (simple and otherwise), because (except in C), no explicit declaration of most variables is necessary. For the simple variable, Bash, Perl and Awk allow you to define the variable when you assign it value. C requires that you define your variable before you use it. In Bash, Perl and Awk, you have the option to define your variable before use if you wish. Examples below (Note that all beginning and ending double quotes are for emphasis only and not actual code):

Ex: Defining a variable called MySimpleVariable.

In Bash: Just type "declare MySimpleVariable" (you can also use "typeset," and both have options to specify what type of variable you want your simple variable to be. For instance, you could type "declare -i MySimpleVariable" if you wanted your variable to be limited to only being an integer. For now, we're not imposing any restrictions.

In Perl: Just type "$MySimpleVariable;"

In Awk: Just type "declare MySimpleVariable"

In C: You "need" to declare/initialize your variables before you can use them. For this post, we'll stick to numbers and strings for the simple variable (even though, technically, a char string is an array in C). Pretty much everything else isn't simple ;)

For a simple integer variable, just type: "int MySimpleVariable;"
For a simple string variable, just type: "char *MySimpleVariable;" (This generally needs to be followed by a declaration of the size/memory-allocation-requirement of the string, like "MySimpleVariable = (char *)malloc(8*sizeof(char));" for an 8 character string)

2. Assigning values to the simple variable. This is very straightforward in all of our four languages:

Ex: We want to assign the value "MySimpleValue" to the simple variable named MySimpleVariable (Note that any values that contain spaces should be quoted).

In Bash: Just type "MySimpleVariable=MySimpleValue" - Spaces between the variable, "=" sign and value are not permitted.

In Perl: Just type "$MySimpleVariable = MySimpleValue;" - Spaces between the variable, "=" sign and value are optional.

In Awk: Just type "MySimpleVariable = "MySimpleValue"" - Spaces between the variable, "=" sign and value are not, technically, necessary, but recommended. Also, note that "MySimpleValue" is placed within double quotes in the assignment. This is sometimes necessary for string variables, but not for numeric variables (e.g. sometimes "a = b" doesn't work, but "a = 1" does. In this case "a = "b"" ( double quoted value) is required for the string variable assignment, but not the integer).

In C: Just type: "MySimpleVariable = MySimpleValue;" for an integer assignment. For a character, or string, assignment you must surround the value with double quotes (e.g. "MySimpleVariable = "MySimpleValue";").

3. Extracting the value from your simple variable. Finally, it's all going to pay off :)

Ex: We want to print the value of the MySimpleVariable variable. This is also fairly simple in all four languages (Okay, C is always going to be a bit more of a pain ;)

In Bash: Just type "echo $MySimpleVariable" - Note that the $ character needs to precede the variable name when you want to get the value.

host # echo $MySimpleVariable

In Perl: Just type "print "$MySimpleVariable\n";" - Note that the $ character needs to precede the variable name when you want to get the value - The \n, indicating a carriage-return, line-feed or new-line isn't necessary, but is nice if you don't want your output on the same line as your next command prompt:

host # perl -e '$MySimpleVariable = MySimpleValue;print "$MySimpleVariable\n";'

In Awk: Just Type "print MySimpleVariable" - Note that the $ character "must not" precede the variable name when you want to get the value.

host # echo|awk '{MySimpleVariable="MySimpleValue";print MySimpleVariable}'

In C: Just type "printf("%d\n", MySimpleVariable);" for an integer assignment. For a character, or string assignment, you would type: "printf("%s\n", MySimpleVariable);" -- The %s in printf indicates a "string" value and the %d indicates a simple decimal (or integer) value. Note that, for this post, we're going to skip the whole compile part of getting your C program to get you output. You can just take the examples from the preceding steps as guidance. There is a bit more to making a standalone C program than there is to making a standalone program with our other three languages.

host # ./c_program

And that's all there is to the simple variable (for the most part ;)

Enjoy, and Cheers,

, Mike