[11941] in Perl-Users-Digest
Perl-Users Digest, Issue: 5540 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Sun May 2 08:37:13 1999
Date: Sun, 2 May 99 05:00:21 -0700
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Sun, 2 May 1999 Volume: 8 Number: 5540
Today's topics:
Re: Can I get multiple line or cut/paste input? gertyk@mail.cybernex ADD .net
Re: checking the existence of a directory (Bart Lateur)
Re: finding the right doc WAS Re: using perl to manage <ebohlman@netcom.com>
Re: finding the right doc WAS Re: using perl to manage (Larry Rosler)
Re: finding the right doc WAS Re: using perl to manage <tchrist@mox.perl.com>
FMTEYEWTK about open() but were afraid to ask <tchrist@mox.perl.com>
getting a remote URL (Cim)
Re: global var disappearing, reappearing <tchrist@mox.perl.com>
How to get POP3D hostname from a given domain name? <hkchan@mbox4.singnet.com.sg>
Special: Digest Administrivia (Last modified: 12 Dec 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Sun, 02 May 1999 11:44:02 GMT
From: gertyk@mail.cybernex ADD .net
Subject: Re: Can I get multiple line or cut/paste input?
Message-Id: <372c39d8.72157731@news.cybernex.net>
try this little script...
Type as many lines as you want, when done hit the 's' key three times
and hit <Enter>.
$/="sss"; #Change record seperator to three s'
$input = (<>);
print "\n\n>>>\n$input\n<<<\n";
On Sat, 01 May 1999 04:17:45 GMT, Mitch <portboy@home.com> wrote:
>Okay, when my script is running, I need to prompt a user for input that
>contains multiple lines. The user will probably only cut and paste into
>this portion. So here's what it
>should look like:
>
>prompt>
>input something aseodhasdlkfja;sldkjfa;ldksjf
>asdfaadsfSDfasdfasdfasdfasdfadsfasdfasdfasdfasdf
>sdfasdlfkjalsdkjfa;lsdkjf;laksdjf;laksdjf;laksdjf
>asdfasdlfkja;sdlkfjal;skdjfl;aksdjf;laksdjf;laksdjf
>asdfa;lksdjf;alskdjf;lasdkjf;laskdjf;alsdkjf
>...
>prompt>
>
>The user can either input or cut/paste in the stuff, and then on the
>final line hit three .'s and then the contents should be written to a
>file. Can this be done in PerL? If so, can somebody give me some
>direction on finding the solution to this question.
>
>Thanks,
>
>Mitch
------------------------------
Date: Sun, 02 May 1999 08:21:37 GMT
From: bart.lateur@skynet.be (Bart Lateur)
Subject: Re: checking the existence of a directory
Message-Id: <372d0066.976511@news.skynet.be>
Larry Rosler wrote:
>I've seen that advice before, but I don't think much of it. If the
>mkdir fails, you don't know whether the directory already existed or
>couldn't be created. So you have to either examine $! or do the -d test
>anyway. So why not just test first and avoid the unnecessary mkdir
>system call?
Like Tad McClennan wrote: there may be a file with tha same name.
If you do:
mkdir $directory,0755;
-d $directory
or die "Directory $directory still doesn't exist ($!)";
I think this covers all bases. Note that $! will be set through the
failure of mkdir().
Although, you have the habit of using a file extension for plain files,
and none for directories, you should never have a file><directory
conflict.
Bart.
------------------------------
Date: Sun, 2 May 1999 07:38:42 GMT
From: Eric Bohlman <ebohlman@netcom.com>
Subject: Re: finding the right doc WAS Re: using perl to manage passwords?
Message-Id: <ebohlmanFB3H8I.Mxy@netcom.com>
Dan Baker <dtbaker@bus-prod.com> wrote:
: perhaps now, with some experience, I could and might create a utility to
: do just that.... but you must admit it is not *trivial* and certainly
: not possible for a newbie who doesn't know perl well enough to even know
: which FAQ to read. Wouldn't it be nice and hugely useful for one of the
: more expert programmers out there to write a dandy little search engine
: and include it with the standard docs! Even nicer would be one with a
: simple little GUI front end written with TK maybe?!
There's already a Tk-based GUI POD reader, tkpod, available from CPAN.
Of course, you already need Tk installed to use it, so it's not for rank
beginners.
perldoc has quite a few nifty search facilities; about the only thing
missing is the ability to do a Boolean line-by-line (or
paragraph-by-paragraph) search of the standard documentation, and that
wouldn't be all that hard to add (I've thought about doing it myself).
The main design issue would be determining what should get searched (since
a full-body search on the POD of every installed module would be too
time-consuming to do as the default).
------------------------------
Date: Sun, 2 May 1999 00:46:27 -0700
From: lr@hpl.hp.com (Larry Rosler)
Subject: Re: finding the right doc WAS Re: using perl to manage passwords?
Message-Id: <MPG.1195a2ac9b2971ff989990@nntp.hpl.hp.com>
In article <372BCC3F.43A26571@bus-prod.com> on Sat, 01 May 1999 22:53:35
-0500, Dan Baker <dtbaker@bus-prod.com> says...
> Matthew Bafford wrote:
...
> > Or, you could even *gasp* resort to the program that comes with Windows!
> > The Find program (start menu) will search files.
> -------
> file names perhaps, but it certainly won't search the contents of files
> for keywords...
It certainly will. Look again. They call that capability 'Advanced'.
But it is not advanced enough to use regexes.
--
(Just Another Larry) Rosler
Hewlett-Packard Company
http://www.hpl.hp.com/personal/Larry_Rosler/
lr@hpl.hp.com
------------------------------
Date: 2 May 1999 03:43:10 -0700
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: Re: finding the right doc WAS Re: using perl to manage passwords?
Message-Id: <372c1e2e@cs.colorado.edu>
[courtesy cc of this posting sent to cited author via email]
In comp.lang.perl.misc, Eric Bohlman <ebohlman@netcom.com> writes:
:perldoc has quite a few nifty search facilities;
I think not. Its days are numbered.
--tom
--
I eschew embedded capital letters in names; to my prose-oriented eyes,
they are too awkward to read comfortably. They jangle like bad typography.
--Rob Pike
------------------------------
Date: 2 May 1999 05:44:00 -0700
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: FMTEYEWTK about open() but were afraid to ask
Message-Id: <372c3a80@cs.colorado.edu>
NAME
perlopentut - tutorial on opening things in Perl
DESCRIPTION
Perl has two simple, built-in ways to open files: the shell way for
convenience, and the C way for precision. The choice is yours.
Open ` la shell
Perl's `open' function was designed to mimic the way command-line
redirection in the shell works. Here are some basic examples from
the shell:
$ myprogram file1 file2 file3
$ myprogram < inputfile
$ myprogram > outputfile
$ myprogram >> outputfile
$ myprogram | otherprogram
$ otherprogram | myprogram
And here are some more advanced examples:
$ otherprogram | myprogram f1 - f2
$ otherprogram 2>&1 | myprogram -
$ myprogram <&3
$ myprogram >&4
Programmers accustomed to constructs like those above can take
comfort in learning that Perl directly supports these familiar
constructs using virtually the same syntax as the shell.
Simple Opens
The `open' function takes two arguments: the first is a filehandle,
and the second is a single string comprising both what to open and
how to open it. `open' returns true when it works, and when it
fails, returns a false value and sets the special variable $! to
reflect the system error. If the filehandle was previously opened,
it will be implicitly closed first.
For example:
open(INFO, "datafile") || die("can't open datafile: $!");
open(INFO, "< datafile") || die("can't open datafile: $!");
open(RESULTS,"> runstats") || die("can't open runstats: $!");
open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write that
this way:
open INFO, "< datafile" or die "can't open datafile: $!";
open RESULTS,"> runstats" or die "can't open runstats: $!";
open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading less-than is optional.
If omitted, Perl assumes that you want to open the file for
reading.
The other important thing to notice is that, just as in the shell,
any white space before or after the filename is ignored. This is
good, because you wouldn't want these to do different things:
open INFO, "<datafile"
open INFO, "< datafile"
open INFO, "< datafile"
Ignoring surround whitespace also helps for when you read a
filename in from a different file, and forget to trim it before
opening:
$filename = <INFO>; # oops, \n still there
open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because `open' mimics the shell
in its style of using redirection arrows to specify how to open the
file, it also does so with respect to extra white space around the
filename itself as well. For accessing files with naughty names,
see the section on "Dispelling the Dweomer".
Pipe Opens
In C, when you want to open a file using the standard I/O library,
you use the `fopen' function, but when opening a pipe, you use the
`popen' function. But in the shell, you just use a different
redirection character. That's also the case for Perl. The `open'
call remains the same--just its argument differs.
If the leading character is a pipe symbol, C<open) starts up a new
command and open a write-only filehandle leading into that command.
This lets you write into that handle and have what you write show
up on that command's standard input. For example:
open(PRINTER, "| lpr -Plp1") || die "cannot fork: $!";
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
If the trailing character is a pipe, you start up a new command and
open a read-only filehandle leading out of that command. This lets
whatever that command writes to its standard output show up on your
handle for reading. For example:
open(NET, "netstat -i -n |") || die "cannot fork: $!";
while (<NET>) { } # do something with input
close(NET) || die "can't close netstat: $!";
What happens if you try to open a pipe to or from a non-existent
command? In most systems, such an `open' will not return an error.
That's because in the traditional `fork'/`exec' model, running the
other program happens only in the forked child process, which means
that the failed `exec' can't be reflected in the return value of
`open'. Only a failed `fork' shows up there. See the section on
"Why doesn't open() return an error when a pipe open fails?" in the
perlfaq8 manpage to see how to cope with this. There's also an
explanation in the perlipc manpage.
If you would like to open a bidirectional pipe, the IPC::Open2
library will handle this for you. Check out the section on
"Bidirectional Communication with Another Process" in the perlipc
manpage
The Minus File
Again following the lead of the standard shell utilities, Perl's
`open' function treats a file whose name is a single minus, "-", in
a special way. If you open minus for reading, it really means to
access the standard input. If you open minus for writing, it really
means to access the standard output.
If minus can be used as the default input or default output? What
happens if you open a pipe into or out of minus? What's the default
command it would run? The same script as you're current running!
This is actually a stealth `fork' hidden inside an `open' call. See
the section on "Safe Pipe Opens" in the perlipc manpage for
details.
Mixing Reads and Writes
It is possible to specify both read and write access. All you do is
add a "+" symbol in front of the redirection. But as in the shell,
using a less-than on a file never creates a new file; it only opens
an existing one. On the other hand, using a greater-than always
clobbers (truncates to zero length) an existing file, or creates a
brand-new one if there isn't an old one. Adding a "+" for read-
write doesn't affect whether it only works on existing files or
always clobbers existing ones.
open(WTMP, "+< /usr/adm/wtmp")
|| die "can't open /usr/adm/wtmp: $!";
open(SCREEN, "+> /tmp/lkscreen")
|| die "can't open /tmp/lkscreen: $!";
open(LOGFILE, "+>> /tmp/applog"
|| die "can't open /tmp/applog: $!";
The first one won't create a new file, and the second one will
always clobber an old one. The third one will create a new file if
necessary and not clobber an old one, and it will allow you to read
at any point in the file, but all writes will always go to the end.
In short, the first case is substantially more common than the
second and third cases, which are almost always wrong. (If you know
C, the plus in Perl's `open' is historically derived from the one
in C's fopen(3S), which it ultimately calls.)
In fact, when it comes to updating a file, unless you're working on
a binary file as in the WTMP case above, you probably don't want to
use this approach for updating. Instead, Perl's -i flag comes to
the rescue. The following command takes all the C, C++, or yacc
source or header files and changes all their foo's to bar's,
leaving the old version in the original file name with a ".orig"
tacked on the end:
$ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
This is a short cut for some renaming games that are really the
best way to update textfiles. See the second question in the
perlfaq5 manpage for more details.
Filters
One of the most common uses for `open' is one you never even
notice. When you process the ARGV filehandle using `<ARGV>', Perl
actually does an implicit open on each file in @ARGV. Thus a
program called like this:
$ myprogram file1 file2 file3
Can have all its files opened and processed one at a time using a
construct no more complex than:
while (<>) {
# do something with $_
}
If @ARGV is empty when the loop first begins, Perl pretends you've
opened up minus, that is, the standard input. In fact, $ARGV, the
currently open file during `<ARGV>' processing, is even set to "-"
in these circumstances.
You are welcome to pre-process your @ARGV before starting the loop
to make sure it's to your liking. One reason to do this might be to
remove command options beginning with a minus. While you can always
roll the simple ones by hand, the Getopts modules are good for
this.
use Getopt::Std;
# -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
getopts("vDo:");
# -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
getopts("vDo:", \%args);
Or the standard Getopt::Long module to permit named arguments:
use Getopt::Long;
GetOptions( "verbose" => \$verbose, # --verbose
"Debug" => \$debug, # --Debug
"output=s" => \$output );
# --output=somestring or --output somestring
Another reason for preprocessing arguments is to make an empty
argument list default to all files:
@ARGV = glob("*") unless @ARGV;
You could even filter out all but plain, text files. This is a bit
silent, of course, and you might prefer to mention them on the way.
@ARGV = grep { -f && -T } @ARGV;
If you're using the -n or -p command-line options, you should put
changes to @ARGV in a `BEGIN{}' block.
Remember that a normal `open' has special properties, in that it
might call fopen(3S) or it might called popen(3S), depending on
what its argument looks like; that's why it's sometimes called
"magic open". Here's an example:
$pwdinfo = `domainname` =~ /^(\(none\))?$/
? '< /etc/passwd'
: 'ypcat passwd |';
open(PWD, $pwdinfo)
or die "can't open $pwdinfo: $!";
This sort of thing also comes into play in filter processing.
Because `<ARGV>' processing employs the normal, shell-style Perl
`open', it respects all the special things we've already seen:
$ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
That program will read from the file f1, the process cmd1, standard
input (tmpfile in this case), the f2 file, the cmd2 command, and
finally the f3 file.
Yes, this also means that if you have a file named "-" (and so on)
in your directory, that they won't be processed as literal files by
`open'. You'll need to pass them as "./-" much as you would for the
*rm* program. Or you could use `sysopen' as described below.
One of the more interesting applications is to change files of a
certain name into pipes. For example, to autoprocess gzipped or
compressed files by decompressing them with *gzip*:
@ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
Or, if you have the *GET* program installed from LWP, you can fetch
URLs before processing them:
@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
It's not for nothing that this is called magic `<ARGV>'. Pretty
nifty, eh?
Open ` la C
If you want the convenience of the shell, then Perl's `open' is
definitely the way to go. On the other hand, if you want finer
precision than C's simplistic fopen(3S) provides, then you should
look to Perl's `sysopen', which is a direct hook into the open(2)
system call. That does mean it's a bit more involved, but that's
the price of precision.
`sysopen' takes 3 (or 4) arguments.
sysopen HANDLE, PATH, FLAGS, [MASK]
The HANDLE argument is a filehandle just as with `open'. The PATH
is a literal path, one that doesn't pay attention to any greater-
thans or less-thans or pipes or minuses, nor ignore white space. If
it's there, it's part of the path. The FLAGS argument contains one
or more values derived from the Fcntl module that have been or'd
together using the bitwise "|" operator. The final argument, the
MASK, is optional; if present, it is combined with the user's
current umask for the creation mode of the file. You should usually
omit this.
Although the traditional values of read-only, write-only, and read-
write are 0, 1, and 2 respectively, this is known not to hold true
on some systems. Instead, it's best to load in the appropriate
constants first from the Fcntl module, which supplies the following
standard flags:
O_RDONLY Read only
O_WRONLY Write only
O_RDWR Read and write
O_CREAT Create the file if it doesn't exist
O_EXCL Fail if the file already exists
O_APPEND Append to the file
O_TRUNC Truncate the file
O_NONBLOCK Non-blocking access
Less common flags that are sometimes available on some operating
systems include `O_BINARY', `O_TEXT', `O_SHLOCK', `O_EXLOCK',
`O_DEFER', `O_SYNC', `O_ASYNC', `O_DSYNC', `O_RSYNC', `O_NOCTTY',
`O_NDELAY' and `O_LARGEFILE'. Consult your open(2) manpage or its
local equivalent for details.
Here's how to use `sysopen' to emulate the simple `open' calls we
had before. We'll omit the `|| die $!' checks for clarity, but make
sure you always check the return values in real code. These aren't
quite the same, since `open' will trim leading and trailing white
space, but you'll get the idea:
To open a file for reading:
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
To open a file for writing, creating a new file if needed or else
truncating an old file:
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
To open a file for appending, creating one if necessary:
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
To open a file for update, where the file must already exist:
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
And here are things you can do with `sysopen' that you cannot do
with a regular `open'. As you see, it's just a matter of
controlling the flags in the third argument.
To open a file for writing, creating a new file which must not
previously exist:
sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
To open a file for appending, where that file must already exist:
sysopen(FH, $path, O_WRONLY | O_APPEND);
To open a file for update, creating a new file if necessary:
sysopen(FH, $path, O_RDWR | O_CREAT);
To open a file for update, where that file must not already exist:
sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
To open a file without blocking, creating one if necessary:
sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
Permissions ` la mode
If you omit the MASK argument to `sysopen', Perl uses the octal
value 0666. The normal MASK to use for executables and directories
should be 0777, and for anything else, 0666.
Why so permissive? Well, it isn't really. The MASK will be modified
by your process's current `umask'. A umask is a number representing
*disabled* permissions bits; that is, bits that will not be turned
on in the created files' permissions field.
For example, if your `umask' were 027, then the 020 part would
disable the group from writing, and the 007 part would disable
others from reading, writing, or executing. Under these conditions,
passing `sysopen' 0666 would create a file with mode 0640, since
`0666 &~ 027' is 0640.
You should seldom use the MASK argument to `sysopen()'. That takes
away the user's freedom to choose what permission new files will
have. Denying choice is almost always a bad thing. One exception
would be for cases where sensitive or private data is being stored,
such as with mail folders, cookie files, and internal temporary
files.
Obscure Open Tricks
Re-Opening Files (dups)
Sometimes you already have a filehandle open, and want to make
another handle that's a duplicate of the first one. In the shell,
we place an ampersand in front of a file descriptor number when
doing redirections. For example, `2>&1' makes descriptor 2 (that's
STDERR in Perl) be redirected into descriptor 1 (which is usually
Perl's STDOUT). The same is essentially true in Perl: a filename
that begins with an ampersand is treated instead as a file
descriptor if a number, or as a filehandle if a string.
open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!";
That means that if a function is expecting a filename, but you
don't want to give it a filename because you already have the file
open, you can just pass the filehandle with a leading ampersand.
It's best to use a fully qualified handle though, just in case the
function happens to be in a different package:
somefunction("&main::LOGFILE");
This way if somefunction() is planning on opening its argument, it
can just use the already opened handle. This differs from passing a
handle, because with a handle, you don't open the file. Here you
have something you can pass to open.
If you have one of those tricky, newfangled I/O objects that the
C++ folks are raving about, then this doesn't work because those
aren't a proper filehandle in the native Perl sense. You'll have to
use fileno() to pull out the proper descriptor number, assuming you
can:
use IO::Socket;
$handle = IO::Socket::INET->new("www.perl.com:80");
$fd = $handle->fileno;
somefunction("&$fd"); # not an indirect function call
It can be easier (and certainly will be faster) just to use real
filehandles though:
use IO::Socket;
local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
die "can't connect" unless defined(fileno(REMOTE));
somefunction("&main::REMOTE");
If the filehandle or descriptor number is preceded not just with a
simple "&" but rather with a "&=" combination, then Perl will not
create a completely new descriptor opened to the same place using
the dup(2) system call. Instead, it will just make something of an
alias to the existing one using the fdopen(3S) library call This is
slightly more parsimonious of systems resources, although this is
less a concern these days. Here's an example of that:
$fd = $ENV{"MHCONTEXTFD"};
open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!";
If you're using magic `<ARGV>', you could even pass in as a command
line argument in @ARGV something like `"<&=$MHCONTEXTFD"', but
we've never seen anyone actually do this.
Dispelling the Dweomer
Perl is more of a DWIMmer language than something like Java--where
DWIM is an acronym for "do what I mean". But this principle
sometimes leads to more hidden magic than one knows what to do
with. In this way, Perl is also filled with *dweomer*, an obscure
word meaning an enchantment. Sometimes, Perl's DWIMmer is just too
much like dweomer for comfort.
If magic `open' is a bit too magical for you, you don't have to
turn to `sysopen'. To open a file with arbitrary weird characters
in it, it's necessary to protect any leading and trailing
whitespace. Leading whitespace is protected by inserting a `"./"'
in front of a filename that starts with whitespace. Trailing
whitespace is protected by appending an ASCII NUL byte (`"\0"') at
the end off the string.
$file =~ s#^(\s)#./$1#;
open(FH, "< $file\0") || die "can't open $file: $!";
This assumes, of course, that your system considers dot the current
working directory, slash the directory separator, and disallows
ASCII NULs within a valid filename. Most systems follow these
conventions, including all POSIX systems as well as proprietary
Microsoft systems. The only vaguely popular system that doesn't
work this way is the proprietary Macintosh system, which uses a
colon where the rest of us use a slash. Maybe `sysopen' isn't such
a bad idea after all.
If you want to use `<ARGV>' processing in a totally boring and non-
magical way, you could do this first:
# "Sam sat on the ground and put his head in his hands.
# 'I wish I had never come here, and I don't want to see
# no more magic,' he said, and fell silent."
for (@ARGV) {
s#^([^./])#./$1#;
$_ .= "\0";
}
while (<>) {
# now process $_
}
But be warned that users will not appreciate being unable to use "-
" to mean standard input, per the standard convention.
Paths as Opens
You've probably noticed how Perl's `warn' and `die' functions can
produce messages like:
Some warning at scriptname line 29, <FH> chunk 7.
That's because you opened a filehandle FH, and had read in seven
records from it. But what was the name of the file, not the handle?
If you aren't running with `strict refs', or if you've turn them
off temporarily, then all you have to do is this:
open($path, "< $path") || die "can't open $path: $!";
while (<$path>) {
# whatever
}
Since you're using the pathname of the file as its handle, you'll
get warnings more like
Some warning at scriptname line 29, </etc/motd> chunk 7.
Single Argument Open
Remember how we said that Perl's open took two arguments? That was
a passive prevarication. You see, it can also take just one
argument. If and only if the variable is a global variable, not a
lexical, you can pass `open' just one argument, the filehandle, and
it will get the path from the global scalar variable of the same
name.
$FILE = "/etc/motd";
open FILE or die "can't open $FILE: $!";
while (<FILE>) {
# whatever
}
Why is this here? Someone has to cater to the hysterical porpoises.
It's something that's been in Perl since the very beginning, if not
before.
Playing with STDIN and STDOUT
One clever move with STDOUT is to explicitly close it when you're
done with the program.
END { close(STDOUT) || die "can't close stdout: $!" }
If you don't do this, and your program fills up the disk partition
due to a command line redirection, it won't report the error exit
with a failure status.
You don't have to accept the STDIN and STDOUT you were given. You
are welcome to reopen them if you'd like.
open(STDIN, "< datafile")
|| die "can't open datafile: $!";
open(STDOUT, "> output")
|| die "can't open output: $!";
And then these can be read directly or passed on to subprocesses.
This makes it look as though the program were initially invoked
with those redirections from the command line.
It's probably more interesting to connect these to pipes. For
example:
$pager = $ENV{PAGER} || "(less || more)";
open(STDOUT, "| $pager")
|| die "can't fork a pager: $!";
This makes it appear as though your program were called with its
stdout already piped into your pager. You can also use this kind of
thing in conjunction with an implicit fork to yourself. You might
do this if you would rather handle the post processing in your own
program, just in a different process:
head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return unless $pid = open(STDOUT, "|-");
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
print;
last if --$lines < 0;
}
exit;
}
This technique can be applied to repeatedly push as many filters on
your output stream as you wish.
Other I/O Issues
These topics aren't really arguments related to `open' or
`sysopen', but they do affect what you do with your open files.
Opening Non-File Files
When is a file not a file? Well, you could say when it exists but
isn't a plain file. We'll check whether it's a symbolic link first,
just in case.
if (-l $file || ! -f _) {
print "$file is not a plain file\n";
}
What other kinds of files are there than, well, files? Directories,
symbolic links, named pipes, Unix-domain sockets, and block and
character devices. Those are all files, too--just not *plain*
files. This isn't the same issue as being a text file. Not all text
files are plain files. Not all plain files are textfiles. That's
why there are separate `-f' and `-T' file tests.
To open a directory, you should use the `opendir' function, then
process it with `readdir', carefully restoring the directory name
if necessary:
opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
while (defined($file = readdir(DIR))) {
# do something with "$dirname/$file"
}
closedir(DIR);
If you want to process directories recursively, it's better to use
the File::Find module. For example, this prints out all files
recursively, add adds a slash to their names if the file is a
directory.
@ARGV = qw(.) unless @ARGV;
use File::Find;
find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;
This finds all bogus symbolic links beneath a particular directory:
find sub { print "$File::Find::name\n" if -l && !-e }, $dir;
As you see, with symbolic links, you can just pretend that it is
what it points to. Or, if you want to know *what* it points to,
then `readlink' is called for:
if (-l $file) {
if (defined($whither = readlink($file))) {
print "$file points to $whither\n";
} else {
print "$file points nowhere: $!\n";
}
}
Named pipes are a different matter. You pretend they're regular
files, but their opens will normally block until there is both a
reader and a writer. You can read more about them in the section on
"Named Pipes" in the perlipc manpage. Unix-domain sockets are
rather different beasts as well; they're described in the section
on "Unix-Domain TCP Clients and Servers" in the perlipc manpage.
When it comes to opening devices, it can be easy and it can tricky.
We'll assume that if you're opening up a block device, you know
what you're doing. The character devices are more interesting.
These are typically used for modems, mice, and some kinds of
printers. This is described in the section on "How do I read and
write the serial port?" in the perlfaq8 manpage It's often enough
to open them carefully:
sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
# (O_NOCTTY no longer needed on POSIX systems)
or die "can't open /dev/ttyS1: $!";
open(TTYOUT, "+>&TTYIN")
or die "can't dup TTYIN: $!";
$ofh = select(TTYOUT); $| = 1; select($ofh);
print TTYOUT "+++at\015";
$answer = <TTYIN>;
With descriptors that you haven't opened using `sysopen', such as a
socket, you can set them to be non-blocking using `fcntl':
use Fcntl;
fcntl(Connection, F_SETFL, O_NONBLOCK)
or die "can't set non blocking: $!";
Rather than losing yourself in a morass of twisting, turning
`ioctl's, all dissimilar, if you're going to manipulate ttys, it's
best to make calls out to the stty(1) program if you have it, or
else use the portable POSIX interface. To figure this all out,
you'll need to read the termios(3) manpage, which describes the
POSIX interface to tty devices, and then the POSIX manpage, which
describes Perl's interface to POSIX. There are also some high-level
modules on CPAN that can help you with these games. Check out
Term::ReadKey and Term::ReadLine.
What else can you open? To open a connection using sockets, you
won't use one of Perl's two open functions. See the section on
"Sockets: Client/Server Communication" in the perlipc manpage for
that. Here's an example. Once you have it, you can use FH as a
bidirectional filehandle.
use IO::Socket;
local *FH = IO::Socket::INET->new("www.perl.com:80");
For opening up a URL, the LWP modules from CPAN are just what the
doctor ordered. There's no filehandle interface, but it's still
easy to get the contents of a document:
use LWP::Simple;
$doc = get('http://www.sn.no/libwww-perl/');
Binary Files
On certain legacy systems with what could charitably be called
terminally convoluted (some would say broken) I/O models, a file
isn't a file--at least, not with respect to the C standard I/O
library. On these old systems whose libraries (but not kernels)
distinguish between text and binary streams, to get files to behave
properly you'll have to bend over backwards to avoid nasty
problems. On such infelicitous systems, sockets and pipes are
already opened in binary mode, and there is currently no way to
turn that off. With files, you have more options.
Another option is to use the `binmode' function on the appropriate
handles before doing regular I/O on them:
binmode(STDIN);
binmode(STDOUT);
while (<STDIN>) { print }
Passing `sysopen' a non-standard flag option will also open the
file in binary mode on those systems that support it. This is the
equivalent of opening the file normally, then calling `binmode'ing
on the handle.
sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
|| die "can't open records.data: $!";
Now you can use `read' and `print' on that handle without worrying
about the system non-standard I/O library breaking your data. It's
not a pretty picture, but then, legacy systems seldom are. CP/M
will be with us until the end of days, and after.
On systems with exotic I/O systems, it turns out that,
astonishingly enough, even unbuffered I/O using `sysread' and
`syswrite' might do sneaky data mutilation behind your back.
while (sysread(WHENCE, $buf, 1024)) {
syswrite(WHITHER, $buf, length($buf));
}
Depending on the vicissitudes of your runtime system, even these
calls may need `binmode' or `O_BINARY' first. Systems known to be
free of such difficulties include Unix, the Mac OS, Plan9, and
Inferno.
File Locking
In a multitasking environment, you may need to be careful not to
collide with other processes who want to do I/O on the same files
as others are working on. You'll often need shared or exclusive
locks on files for reading and writing respectively. You might just
pretend that only exclusive locks exist.
Never use the existence of a file `-e $file' as a locking
indication, because there is a race condition between the test for
the existence of the file and its creation. Atomicity is critical.
Perl's most portable locking interface is via the `flock' function,
whose simplicity is emulated on systems that don't directly support
it, such as SysV or WindowsNT. The underlying semantics may affect
how it all works, so you should learn how `flock' is implemented on
your system's port of Perl.
File locking *does not* lock out another process that would like to
do I/O. A file lock only locks out others trying to get a lock, not
processes trying to do I/O. Because locks are advisory, if one
process uses locking and another doesn't, all bets are off.
By default, the `flock' call will block until a lock is granted. A
request for a shared lock will be granted as soon as there is no
exclusive locker. A request for a exclusive lock will be granted as
soon as there is no locker of any kind. Locks are on file
descriptors, not file names. You can't lock a file until you open
it, and you can't hold on to a lock once the file has been closed.
Here's how to get a blocking shared lock on a file, typically used
for reading:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename: $!";
flock(FH, LOCK_SH) or die "can't lock filename: $!";
# now read from FH
You can get a non-blocking lock by using `LOCK_NB'.
flock(FH, LOCK_SH | LOCK_NB)
or die "can't lock filename: $!";
This can be useful for producing more user-friendly behaviour by
warning if you're going to be blocking:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename: $!";
unless (flock(FH, LOCK_SH | LOCK_NB)) {
$| = 1;
print "Waiting for lock...";
flock(FH, LOCK_SH) or die "can't lock filename: $!";
print "got it.\n"
}
# now read from FH
To get an exclusive lock, typically used for writing, you have to
be careful. We `sysopen' the file so it can be locked before it
gets emptied. You can get a nonblocking version using `LOCK_EX |
LOCK_NB'.
use 5.004;
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "filename", O_WRONLY | O_CREAT)
or die "can't open filename: $!";
flock(FH, LOCK_EX)
or die "can't lock filename: $!";
truncate(FH, 0)
or die "can't truncate filename: $!";
# now write to FH
Finally, due to the uncounted millions who cannot be dissuaded from
wasting cycles on useless vanity devices called hit counters,
here's how to increment a number in a file safely:
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR | O_CREAT)
or die "can't open numfile: $!";
# autoflush FH
$ofh = select(FH); $| = 1; select ($ofh);
flock(FH, LOCK_EX)
or die "can't write-lock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0)
or die "can't rewind numfile : $!";
print FH $num+1, "\n"
or die "can't write numfile: $!";
truncate(FH, tell(FH))
or die "can't truncate numfile: $!";
close(FH)
or die "can't close numfile: $!";
SEE ALSO
The `open' and `sysopen' function in perlfunc(1); the standard
open(2), dup(2), fopen(3), and fdopen(3) manpages; the POSIX
documentation.
AUTHOR and COPYRIGHT
Copyright 1998 Tom Christiansen.
When included as part of the Standard Version of Perl, or as part
of its complete documentation whether printed or otherwise, this
work may be distributed only under the terms of Perl's Artistic
License. Any distribution of this file or derivatives thereof
outside of that package require that special arrangements be made
with copyright holder.
Irrespective of its distribution, all code examples in these files
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun or for
profit as you see fit. A simple comment in the code giving credit
would be courteous but is not required.
HISTORY
First release: Sat Jan 9 08:09:11 MST 1999
--
"I'd explain it to you, but your head would blow up."
- Steven Wright
------------------------------
Date: Sun, 02 May 1999 10:16:59 GMT
From: cim@online.ee (Cim)
Subject: getting a remote URL
Message-Id: <372c23ea.4609999@news.online.ee>
How could I access a remote URL via perl?
http://someplace.com/cgi-bin/abc.cgi?img=123&xx=xxxxx ........
This URL could return a HTML with img links inside or just an image.
Then I want to print it.
What will the <img src> be? I want to hide the original location. So
that it would look as if the output would come from my server.
help. cim out.
------------------------------
Date: 2 May 1999 03:51:32 -0700
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: Re: global var disappearing, reappearing
Message-Id: <372c2024@cs.colorado.edu>
[courtesy cc of this posting sent to cited author via email]
In comp.lang.perl.misc,
Otis Gospodnetic <otis@my-dejanews.com> writes:
:What is the correct way to declare a global variable that is to be visible
:from all subroutines in the program?
:
:I thought:
:my $foo = 'bar'; # correct
:$foo = 'bar'; # bad
my is not global.
--tom
--
I'm TRYING to be a back end! - --Andrew Hume
------------------------------
Date: Sun, 02 May 1999 17:02:06 +0800
From: hkchan <hkchan@mbox4.singnet.com.sg>
Subject: How to get POP3D hostname from a given domain name?
Message-Id: <372C148D.F931E34D@mbox4.singnet.com.sg>
Hello,
I am wondering how to write a PERL script to get POP3D server hostname,
for example, a user just type one's email address without specifying the
POP3D server but only the domain name, username@domain.com, and the
script would be intelligence enough to return the value such as
username@pop3d_server_name.domain.com.
Thanks in advance,
Gerald
------------------------------
Date: 12 Dec 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Dec 98)
Message-Id: <null>
Administrivia:
Well, after 6 months, here's the answer to the quiz: what do we do about
comp.lang.perl.moderated. Answer: nothing.
]From: Russ Allbery <rra@stanford.edu>
]Date: 21 Sep 1998 19:53:43 -0700
]Subject: comp.lang.perl.moderated available via e-mail
]
]It is possible to subscribe to comp.lang.perl.moderated as a mailing list.
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The Perl-Users Digest is a retransmission of the USENET newsgroup
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------------------------------
End of Perl-Users Digest V8 Issue 5540
**************************************
