print blank lines

[Tdlearner]

Hi Guys,

I am trying to print blank lines on using perl command line.

cat blank.dat
Hello
### Blank line commented for visibility ####
Second Blank Line
### Blank line commented for visibility ####

Command 1 :

perl -npe 'if ($_ == m/^$/) {print "This is a blank line \n"}' blank.dat

Output :
This is a blank line
Hello
### Blank line commented for visibility ####
This is a blank line
Second Blank Line
### Blank line commented for visibility ####


Command 2 :

perl -npe 'if (/^$/) {print "This is a blank line \n"}' blank.dat

Output :
Hello
This is a blank line
### Blank line commented for visibility ####
Second Blank Line
This is a blank line


I don't understand the order of print statements using command 1 .

Command 2 prints everything in the file , plus the print statements .


Shouldn't both the above should print just print statements ?

If I write a small perl program using while loop , it just works fine. But commandline behavior is different .

I am sure I am missing some important subtle points being a beginner in perl


thanks,

 

[feherke]

Hi

Wrong operator.

perl -pe 'if ($_ =[red][b]~[/b][/red] m/^$/) {print "This is a blank line \n"}'  blank.dat

And if you use -p there is no need for -n.

Feherke.

http://rootshell.be/~feherke/

 

[MillerH]

Because you're explicitly print out information, you don't need or want the -p option to perl. Also don't use == for string equality test, use eq instead. But the easiest solution is to just use a regular expression.

perl -ne 'print "blank at line # $.\n" if /^$/' blank.dat

You might have to change your cmd prompt quoting style if you're on a windows system. And don't forget to remove the comment from your blank line. Made that mistake temporarily when using your test data.

- Miller

 

[KevinADC]

perl -pe 'if (/^\s*$/) {print "Line number $. is a blank line\n"}'  blank.dat

------------------------------------------
- Kevin, perl coder unexceptional!

 

[Tdlearner]

Thanks guys,

Miller, I tried perl -n and it does the job but

BUt why perl -p is also printing non-matching lines ?


The following works but it also prints non-matching lines

perl -pe 'if ($_ =~ /^$/) {$_ = "This is a blank line \n" }' blank.dat

Output :
Hello
This is a blank line
Second Blank Line
This is a blank line


While -n option only prints matching line , -p option prints everything.

Is this default behaviour, to my undestanding , -p and -n options are same except the print part.

thanks

 

[MillerH]

Yes, this is just documented behavior of the perl command line. -n simply loops over ARGV, while -p loops but also prints each line.

Given you're explicitly printing output, you probably want -n like in my example.

>perl --help

Usage: perl.exe [switches] [--] [programfile] [arguments]
  -e program      one line of program (several -e's allowed, omit programfile)
  -n              assume "while (<>) { ... }" loop around program
  -p              assume loop like -n but print line also, like sed

- Miller

 

[Tdlearner]

Thanks Miller .