Question

I have the follwoing script

#!/usr/bin/perl 
open IN, "/tmp/file"; 
s/(.*)=/$k{$1}++;"$1$k{$1}="/e and print while <IN>; 

how to print the output of the script to file_out in place to print to standard output?

lidia

Answer 1

#!/usr/bin/perl 
open IN, "/tmp/file"; 
open OUT, ">file_out.txt";
s/(.*)=/$k{$1}++;"$1$k{$1}="/e and print OUT while <IN>; 

Explanation:

• `open IN, “/tmp/file”
  • open command to open file
  • IN filehandle name
  • /tmp/file name of file and specifier that it is for reading
    • if there is no modifier, it means reading
    • if there is a <, i.e. "</tmp/file" it also means reading
• `open OUT, “>file_out.txt”
  • open command to open file
  • OUT filehandle name
  • >file_out.txt name of file and specifier that it is for reading
    • there must be a >, i.e. ">file_out.txt" to write
• s/.../.../e your substitution (I assume you know what it does)
• and is a boolean operator that short-circuits, meaning it only does the thing afterwards if the thing beforehand is true. In this case, it will only print if the substitution actually matched something.
• print OUT print to the filehandle OUT
• while <IN> for each line from the file behind filehandle IN

Note:

Used this way, it makes extensive use of the magical default variable $_. Do a search for $_ on the perlintro site. In short:

• If you don’t tell a s/// substitution what string to work on, it uses $_
• If you don’t tell a print what to print, it prints $_
• If you don’t tell a while loop going through a filehandle’s data where to put each line, it gets put into $_

Your program could have been rewritten:

#!/usr/bin/perl 
open IN, "/tmp/file"; 
open OUT, ">file_out.txt";
while( defined( $line = <IN> ) )
{
  $line =~ s/(.*)=/$k{$1}++;"$1$k{$1}="/e or next;
  print OUT $line;
}