Run
that and "a" will be displayed. That was easy enough. Now the command
line way:
If you called that t.pl, run ./t.pl a b c and you'll see each of your
files go by. Notice that I didn't bother to close F. There are cases
where being sloppy like that could get you in trouble, but in this
simple situation, Perl happily closes the previous file for you when
it sees you re-using F.
Now the magic angle bracket way:
Try that both by itself (type lines to it), and as ./t.pl a b c, and
finally as cat a b c | ./t.pl. You'll see that whatever you do, it
reads the lines or the files. When you give it file names, they act
like they came from STDIN. If you don't, it reads STDIN. If you want
to read BOTH the files and STDIN, do ./t.pl a b c -.
If you want, you can do things when each file ends:
And of course you can accumulate all the lines instead:
Now we're going to get much trickier. We're not going to read unless
there is something ready to read. This can be useful if you need to
do other things while waiting for input, or if you don't want lack
of input to hold you up forever. The code will use Perl's "select"
command, which just passes to the system level select() call. Perl's
"select" is one of those overloaded functions that does very different
things depending on how many arguments you give it, so don't get confused
here. This is the usage that tells us whether or not filehandles are
ready for io. We also use the "fileno" command to get the actual file
number of STDIN. The "vec" is used to turn "$rin" into a bit structure
that is what the system level select() is looking for; if STDIN is
file number 0 as it would be if not redirected, then the first bit
of $rin get sets to 1 before we call the Perl select (you don't have
to understand that to use it). The structure gets changed by select(),
so we protect it with the "$rout=$rin"; $rout will get changed, not
$rin. It all sounds very complicated, but it really isn't bad in usage:
(I'm making this code artificially simple. This works, but isn't the
way you would probably do it in reality.)
Try that first with just ./t.pl. You'll see it print 0 every two seconds.
If you type something, it only gets read when you press enter (buffered
input). Use CTRL-C or your INTR character to quit this.
0
0
0
hello
1
0 hello
0
0
Now try it like this: ./t.pl a. My "a" contains "This is A":
0
0
0
hello
1
1 This is A
1
0 hello
0
Nothing happened until we gave it a line on STDIN. Then, and only
then, did Perl rearrange things so that the file "a" came through
STDIN before our line. That makes sense, because until you try to
read STDIN, Perl has no idea what your intentions toward the files
on the command line are. You might want to process them specifically
as we did in the second little script.
Single character input is a little trickier, because you first need
to convince your OS that it should be feeding you that way. Exactly
how you do that depends on the heritage of your system; it's going
to be a "stty" command of some sort. This works on my Mac OS X box:
The angle brackets have one more trick up their sleeves:
That opens every file that begins with "a", "b' or "c". Use any wild
card characters you need just like you would at the command line.
This should give you some feel for the flexibility of Perl input.
About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com
Read this newsletter at:
http://www.perlpronews.com/2004/0129.html |
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Hi everyone,
I recently got a deal to display a banner on a website, but it can only be show to US visitors. Is there a script of some sort available that would let me do that? Or is it easy to implement myself?
Thanks! ...
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By
A.P. Lawrence
