I learned a fun new perl one-liner today:

perl -00pe0 foo >bar

What does it do? Well, it’s nearly the same as cp foo bar, except that it squeezes multiple blank lines into foo into a single blank line in bar.

How does it do that? Well, let’s break it down piece by piece. Perl’s -0 flag lets you change perl’s “input record separator”. This lets you fiddle with perl’s idea of what a “line” is. Normally perl reads in text a line at a time, where a “line” is a string of text terminated by a newline character. But when you set the -0 flag to 0, perl will keep reading until it hits a string of 2 or more newlines in a row. In other words, perl will read in text a paragraph at a time, and stop when it hits a blank line. If there are multiple blank lines, perl will treat them as if there was just a single blank line and throw away the extras. (You can also do the same thing by setting perl’s special $/ variable to “”.)

The next flag is -p. That tells perl to read in text a “line” at a time, execute some code against each line, and then print the line to standard output. Of course, because of the -00 right before it, each “line” is actually a paragraph.

The final flag is -e. This tells perl that the following parameter is a program that it should execute after it reads in each line. Normally this is given in between single quotes, but here the entire program is simply “0”. This is a perl idiom for a no-op. This is because we don’t need any additional code — the combination of the -00 and -p flags are all we need.

Incidentally, blank lines can be squeezed much simply, and with far less obfuscation, by running cat(1) with the -s flag:

cat -s foo bar