perlopentut.pod

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=head1 NAME

perlopentut - tutorial on opening things in Perl

=head1 DESCRIPTION

Perl has two simple, built-in ways to open files: the shell way for
convenience, and the C way for precision.  The shell way also has 2- and
3-argument forms, which have different semantics for handling the filename.
The choice is yours.

=head1 Open E<agrave> la shell

Perl's C<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.

=head2 Simple Opens

The C<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.  C<open> returns true when it works, and when it fails,
returns a false value and sets the special variable C<$!> 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.

Note also that the first example uses the C<||> logical operator, and the
second uses C<or>, which has lower precedence.  Using C<||> in the latter
examples would effectively mean

    open INFO, ( "<  datafile"  || die "can't open datafile: $!" );

which is definitely not what you want.

The other important thing to notice is that, just as in the shell,
any whitespace 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 surrounding 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 C<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 whitespace around the filename itself
as well.  For accessing files with naughty names, see 
L<"Dispelling the Dweomer">.

There is also a 3-argument version of C<open>, which lets you put the
special redirection characters into their own argument:

    open( INFO, ">", $datafile ) || die "Can't create $datafile: $!";

In this case, the filename to open is the actual string in C<$datafile>,
so you don't have to worry about C<$datafile> containing characters
that might influence the open mode, or whitespace at the beginning of
the filename that would be absorbed in the 2-argument version.  Also,
any reduction of unnecessary string interpolation is a good thing.

=head2 Indirect Filehandles

C<open>'s first argument can be a reference to a filehandle.  As of
perl 5.6.0, if the argument is uninitialized, Perl will automatically
create a filehandle and put a reference to it in the first argument,
like so:

    open( my $in, $infile )   or die "Couldn't read $infile: $!";
    while ( <$in> ) {
	# do something with $_
    }
    close $in;

Indirect filehandles make namespace management easier.  Since filehandles
are global to the current package, two subroutines trying to open
C<INFILE> will clash.  With two functions opening indirect filehandles
like C<my $infile>, there's no clash and no need to worry about future
conflicts.

Another convenient behavior is that an indirect filehandle automatically
closes when it goes out of scope or when you undefine it:

    sub firstline {
	open( my $in, shift ) && return scalar <$in>;
	# no close() required
    }

=head2 Pipe Opens

In C, when you want to open a file using the standard I/O library,
you use the C<fopen> function, but when opening a pipe, you use the
C<popen> function.  But in the shell, you just use a different redirection
character.  That's also the case for Perl.  The C<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 opens 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 "can't run lpr: $!";
    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 "can't fork netstat: $!";
    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?  If possible, Perl will detect the failure and set C<$!> as
usual.  But if the command contains special shell characters, such as
C<E<gt>> or C<*>, called 'metacharacters', Perl does not execute the
command directly.  Instead, Perl runs the shell, which then tries to
run the command.  This means that it's the shell that gets the error
indication.  In such a case, the C<open> call will only indicate
failure if Perl can't even run the shell.  See L<perlfaq8/"How can I
capture STDERR from an external command?"> to see how to cope with
this.  There's also an explanation in L<perlipc>.

If you would like to open a bidirectional pipe, the IPC::Open2
library will handle this for you.  Check out 
L<perlipc/"Bidirectional Communication with Another Process">

=head2 The Minus File

Again following the lead of the standard shell utilities, Perl's
C<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 currently running!  This is actually
a stealth C<fork> hidden inside an C<open> call.  See 
L<perlipc/"Safe Pipe Opens"> for details.

=head2 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, "+> lkscreen")
        || die "can't open lkscreen: $!";

    open(LOGFILE, "+>> /var/log/applog")
        || die "can't open /var/log/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 C<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 B<-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 filename 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 
L<perlfaq5> for more details.

=head2 Filters 

One of the most common uses for C<open> is one you never
even notice.  When you process the ARGV filehandle using
C<< <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 C<< <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 B<-n> or B<-p> command-line options, you
should put changes to @ARGV in a C<BEGIN{}> block.

Remember that a normal C<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
C<< <ARGV> >> processing employs the normal, shell-style Perl C<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 F<f1>, the process F<cmd1>, standard
input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command,
and finally the F<f3> file.

Yes, this also means that if you have files named "-" (and so on) in
your directory, they won't be processed as literal files by C<open>.
You'll need to pass them as "./-", much as you would for the I<rm> program,
or you could use C<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 I<gzip>:

    @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_  } @ARGV;

Or, if you have the I<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 C<< <ARGV> >>.
Pretty nifty, eh?