Recipe 8.2 Counting Lines (or Paragraphs or Records) in a File

8.2.1 Problem

You need to compute the number of lines in a file.

8.2.2 Solution

Many systems have a wc program to count lines in a file:

$count = `wc -l < $file`;
die "wc failed: $?" if $?;
chomp($count);

You could also open the file and read line-by-line until the end, counting lines as you go:

open(FILE, "<", $file) or die "can't open $file: $!";
$count++ while <FILE>;
# $count now holds the number of lines read

Here's the fastest solution, assuming your line terminator really is "\n":

$count += tr/\n/\n/ while sysread(FILE, $_, 2 ** 20);

8.2.3 Discussion

Although you can use -s $file to determine the file size in bytes, you generally cannot use it to derive a line count. See the Introduction in Chapter 9 for more on -s.

If you can't or don't want to call another program to do your dirty work, you can emulate wc by opening up and reading the file yourself:

open(FILE, "<", $file) or die "can't open $file: $!";
$count++ while <FILE>;
# $count now holds the number of lines read

Another way of writing this is:

open(FILE, "<", $file) or die "can't open $file: $!";
for ($count=0; <FILE>; $count++) { }

If you're not reading from any other files, you don't need the $count variable in this case. The special variable $. holds the number of lines read since a filehandle was last explicitly closed:

1 while <FILE>;
$count = $.;

This reads in all records in the file, then discards them.

To count paragraphs, set the global input record separator variable $/ to the empty string ("") before reading to make the input operator (<FH>) read a paragraph at a time.

$/ = "";            # enable paragraph mode for all reads
open(FILE, "<", $file) or die "can't open $file: $!";
1 while <FILE>;
$para_count = $.;

The sysread solution reads the file a megabyte at a time. Once end-of-file is reached, sysread returns 0. This ends the loop, as does undef, which would indicate an error. The tr operation doesn't really substitute \n for \n in the string; it's an old idiom for counting occurrences of a character in a string.

8.2.4 See Also

The tr operator in perlop(1) and Chapter 5 of Programming Perl; your system's wc(1) manpage; the $/ entry in perlvar(1), and in the "Special Variables in Alphabetical Order" section of Chapter 28 of Programming Perl; the Introduction to Chapter 9