Originally Posted by durden_tyler

It would be much more helpful if you actually cut and paste what you tried, and show the error message clearly.



Sure.

Code:

cat -n f7

"f7" is my test file. "cat -n" displays the contents of my test file with the line numbers printed on the left. I've created the test file in such a way that it's easy to figure out the line number by looking at the content. For example, the first line says "this is line 1", 8th line says "this is line 8" and so on. That way, after the perl script is run, you can easily figure out-

(a) if some text was added before or after a line
(b) if some text replaced a line

Code:

$ perl -lne 'BEGIN {$RM="BLAH"}

- "perl" invokes the Perl interpreter
- "l", "n" and "e" are the command-line options.
- "l" prints a newline after a "print" command so you do not have to add an explicit "\n" in your "print" command.
- "n" creates a "while (<>){...}" loop around the program i.e. it executes the perl program on each line of the input stream (file "f7" in my case)
- "e" specifies that a program follows in the single quotes after it

- "BEGIN {$RM="BLAH"}" this is where all initialization resides. Code inside BEGIN executes only once - at the beginning, before the file is processed. Over here, the variable $RM is set to "BLAH" only once. If I do not put it inside "BEGIN", it will be unnecessarily initialized for every line in the file f7.

Code:

             if ($. == 3) {print qq(RM="rm2")}

$. is a special Perl variable that stores the line number of the input stream that is being processed currently. In my case the input stream is the file "f7". It is similar to the "NR" variable of awk. Perl has an extensive list of such variables that are of the form "$" followed by (I guess) almost every special character on your keyboard.

"qq" is a quote-like operator that is used along with "print" to print double quoted interpolated strings.

Code:

             elsif ($. == 5) {print qq($RM)}  
             elsif ($. == 7) {print qq(rmdesc="Result process")}
             else {print}' f7

"elsif" and "else" is used because $. - the line number can only be one of 3, 5, 7, something else. Note that "if ... elsif ... else" is preferable to "if(){...} if(){...} if(){...}" over here for each line.

So what the conditional loop does is this -

(a) if current line number is 3 then print RM="rm2" to the console, which means the original line 3 in the file f7 is not displayed.

(b) otherwise if current line number is 5 then print value of $RM i.e. BLAH, which means that the original line 5 in the file f7 is not displayed.

(c) otherwise if current line number is 7 then print rmdesc="Result process", which means that the original line 7 in the file f7 is not displayed.

(d) for every other line, print the original content in the file.

Note that the script reads the file and shows something to the standard output. That "something" could be original file content or something you want to display. It does not replace or alter or modify the file contents in any way.



Yes, it's called a "one-liner". Not sure what makes you think it will not work. It may not work for your requirements, but it does work as seen in the output of my previous post.



Again, if there is some text on line 30, and if you change the if/elsif condition accordingly, then this one-liner will read line 30 and print something else to the standard output (console). That's all it does. The file will remain as it was earlier.

HTH,
tyler_durden