What's the use of <> in Perl?

Question

What's the use of <> in Perl. How to use it ? If we simply write

<>; 

and

while(<>) 

what is that the program doing in both cases?

Answer 1

The answers above are all correct, but it might come across more plainly if you understand general UNIX command line usage. It is very common to want a command to work on multiple files. E.g.

ls -l *.c 

The command line shell (bash et al) turns this into:

ls -l a.c b.c c.c ... 

in other words, ls never see '*.c' unless the pattern doesn't match. Try this at a command prompt (not perl):

echo * 

you'll notice that you do not get an *.

So, if the shell is handing you a bunch of file names, and you'd like to go through each one's data in turn, perl's <> operator gives you a nice way of doing that...it puts the next line of the next file (or stdin if no files are named) into $_ (the default scalar).

Here is a poor man's grep:

while(<>) {    print if m/pattern/; } 

Running this script:

./t.pl * 

would print out all of the lines of all of the files that match the given pattern.

cat /etc/passwd | ./t.pl 

would use cat to generate some lines of text that would then be checked for the pattern by the loop in perl.

So you see, while(<>) gets you a very standard UNIX command line behavior...process all of the files I give you, or process the thing I piped to you.