使用Perl更改文本文件中的行

Question

I read other places on how to do this but they were confusing for me.

I want to read lines from a text file and when I come across a certain line I want to append something to it.

My code is:

 open my $p, "$username_filename" or die "can not open $username_filename: $!";

 foreach $line (<$p>){
     if ($line =~ /^listen/){
         `echo "whatever" >> $username_file`;
     }
 }

However when I run this I get this error

sh: -c: line 0: syntax error near unexpected token `newline' sh: -c: line 0: `echo "current_user" >> '

Is this way correct to edit the file and why am I getting this error?

Answer 1

Working with files is not like editing in a word processor. Lines are an illusion, a file is just a big string of characters. You can't change a line in the middle of a file for the same reason you can't change a line in the middle of a book, the words can't be moved around to make room.

Instead, like a book, if you want to change something you need to rewrite the whole thing.

The basic algorithm is to...

1. Open the file for reading.
2. Open a temporary file for writing.
3. Read a line, alter the line, write the line.
4. Repeat 3 until done reading.
5. Overwrite the file with the temp file.

Some other notes...

print writes to STDOUT by default, but you can give it a filehandle to write to instead.

foreach my $line (<$fh>) is unfortunately not optimized to read files. It will read the possibly enormous file into memory. while(my $line = <$fh>) reads one line at a time.

I've turned on strict . This forces you to declare your variables. It protects you from typos like the one you made of $username_file vs $username_filename .

You could use something like "$filename.tmp" but File::Temp provides temp files that are guaranteed to be temporary, unique and cleaned up when the program exits.

use strict;
use warnings;
use autodie;     # because writing 'or die' gets old fast
use File::Temp;  # provides safe temp files    

my $filename = ...; # set it somehow

open my $read, "<", $filename;
my $temp = File::Temp->new;

while(my $line = <$read>) {
    if( $line =~ /^listen/ ) {
        chomp $line;             # remove the newline
        $line .= " whatever\n";  # add our content and put a newline back
    }

    # Write the line to the temp file
    print $temp $line;
}

# Overwrite our file with the rewritten temp file
rename $temp->filename, $filename;

That's inside a program. If you just want to do it quickly, you can do it on the command line with -i and -p .

perl -i.bak -pe 'if( /^listen/ ) { chomp; $_ .= "whatever" }' filename

-p says to run the code on each line of the file. The line will be put into $_ and whatever is in $_ will be printed. -i says to edit the file in place. -i.bak makes a backup of the original file just in case you make a mistake.

Answer 2

There are a few problems with your attempt. The big one is that using echo >> file will append to the file, not insert at some arbitrary place inside the file.

Another problem is that you're trying to append to a file called $username_file , and you haven't declared or defined that variable.

I don't think perl lets you insert into the middle of a file. I think your best bet would be to read the file a line at a time, and on the correct line(s), append the text you want. Write each line to a new file, then swap the files around at the end.

For example:

#!/usr/bin/perl
my $in_filename = "in.txt";
my $out_filename = "out.txt";

open (my $in, "<", $in_filename) or die;
open (my $out, ">", $out_filename) or die;

while (my $lline = <$in>)
{
    chomp $lline;
    if ( $lline =~ /listen/ )
    {
        print "$lline whatever\n";
    }
    else
    {
       print "$lline\n";
    }
 } 

close $in;
close $out;

rename $in_filename, "$in_filename.original";
rename $out_filename, $in_filename;

I use chomp to remove line endings, because <$in> gives us a line including its line endings, wish otherwise messes up the append.

As always there are many ways to achieve this. I think using sed is probably a better option for this, but you specifically asked how to do it in perl, so perl it is.