Oily Rag 1: CD Part 3: Difference between revisions

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'''Uniform Server running from a CD - Part 3'''

Before looking at any scripts I cover the Perl commands used. In many ways and because of its origin Perl is nothing more than a glorified command processor, admittedly a very powerful one. I find interactive input-output always poses a problem hence cover this first. The following four commands are more than adequate for most scripts.

!Instrustion

!Perl Input and Output

system( 'pause' )

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'''''system()'':''' is a function that will execute any command you would normally type at a command prompt. Enclose the command between quotes.

''''pause':''' This halts a script and prompts a user with the message '''Press any key to continue....'''

|Prints whatever is between the two quotes, \n start on a new line.

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'''my''' is not required for scripts run directly however when initiated from a batch file '''my''' forces the variable to be local to the script. Hence '''my''' is required to capture a variable, otherwise the script swans off.

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'''chomp''' $input;

|In most situations you do not want that enter key character (mentioned above) hence chomp removes it giving you a clean string.

'''chomp $some_name;'''

'''''Note'':''' For portability I use only core Perl commands.  

!Instrustion

!Perl Input and Output

'''close(INFILE);'''  

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'''@some_name''' @ creates an array who's name is some_name you can use whatever you like for some_name so long as it is unique.

'''close(INFILE)''' Just closes the file associated with handle INFILE and '''cleans up'''.

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'''close(OUTFILE);'''

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'''$file''' is a variable containing the complete path to the file including its name.

'''foreach $save(@some_name)''' foreach @some_name scans the array @some_name line by line after reaching the last line continues on to the next command. Every time a line is scanned it is read from the array and placed into the variable $save overwriting its previous content.

'''print OUTFILE $save''' This line is placed within the foreach loop. It prints (copies) the content of variable $save to the file associated with handle OUTFILE. Hence every line in the array is written to the file.

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'''}'''  

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In this example the variable $new_drive is checked to see it it is '''eq'''ual to the string in the quotes. If it is true then a drive letter has not been set to give it a value the $temp_drive is assigned to it.

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'''@mpg=();'''

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The quickest way to clear an array is to assign nothing to it. No content in the brackets.

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There are several ways to assign data to an array. Push is a function that takes two parameters, the first is the array to work on @mpg followed by the data to store. Data is stored in the next available location, arrays automatically expand to accommodate the data.

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'''$str1 = pop(@mpg);'''

'''''Note'':''' The data popped is the last one that was pushed into the array. Last in first out.

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'''@mpg=reverse(@mpg);'''

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The reverse function when applied to an array reveres the order of its content. It makes a temporary copy hence the reason to assigning it to itself. If you apply this command before popping data you now get what was first pushed into the array.

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'''sub update_files {'''

These values get passed as an array named '''@_''' you access these values using '''$_[0]''' etc these variable contain the following values:

$_[2]=30

You can pass arrays for example fred(10,20,@mpg) these get passed to the array @_ what I find a pain, arrays gets flattened. To access values within your sub, now looks like this:

$_[4]=$mpg[2]

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