perlopentut.1

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.\" ========================================================================
.\"
.IX Title "PERLOPENTUT 1"
.TH PERLOPENTUT 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlopentut \- tutorial on opening things in Perl
.SH "DESCRIPTION"
.IX Header "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.
.SH "Open a\*` la shell"
.IX Header "Open a` la shell"
Perl's \f(CW\*(C`open\*(C'\fR function was designed to mimic the way command-line
redirection in the shell works.  Here are some basic examples
from the shell:
.PP
.Vb 6
\&    $ myprogram file1 file2 file3
\&    $ myprogram    <  inputfile
\&    $ myprogram    >  outputfile
\&    $ myprogram    >> outputfile
\&    $ myprogram    |  otherprogram 
\&    $ otherprogram |  myprogram
.Ve
.PP
And here are some more advanced examples:
.PP
.Vb 4
\&    $ otherprogram      | myprogram f1 \- f2
\&    $ otherprogram 2>&1 | myprogram \-
\&    $ myprogram     <&3
\&    $ myprogram     >&4
.Ve
.PP
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.
.Sh "Simple Opens"
.IX Subsection "Simple Opens"
The \f(CW\*(C`open\*(C'\fR 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.  \f(CW\*(C`open\*(C'\fR returns true when it works, and when it fails,
returns a false value and sets the special variable \f(CW$!\fR to reflect
the system error.  If the filehandle was previously opened, it will
be implicitly closed first.
.PP
For example:
.PP
.Vb 4
\&    open(INFO,      "datafile") || die("can\*(Aqt open datafile: $!");
\&    open(INFO,   "<  datafile") || die("can\*(Aqt open datafile: $!");
\&    open(RESULTS,">  runstats") || die("can\*(Aqt open runstats: $!");
\&    open(LOG,    ">> logfile ") || die("can\*(Aqt open logfile:  $!");
.Ve
.PP
If you prefer the low-punctuation version, you could write that this way:
.PP
.Vb 3
\&    open INFO,   "<  datafile"  or die "can\*(Aqt open datafile: $!";
\&    open RESULTS,">  runstats"  or die "can\*(Aqt open runstats: $!";
\&    open LOG,    ">> logfile "  or die "can\*(Aqt open logfile:  $!";
.Ve
.PP
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.
.PP
Note also that the first example uses the \f(CW\*(C`||\*(C'\fR logical operator, and the
second uses \f(CW\*(C`or\*(C'\fR, which has lower precedence.  Using \f(CW\*(C`||\*(C'\fR in the latter
examples would effectively mean
.PP
.Vb 1
\&    open INFO, ( "<  datafile"  || die "can\*(Aqt open datafile: $!" );
.Ve
.PP
which is definitely not what you want.
.PP
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:
.PP
.Vb 3
\&    open INFO,   "<datafile"   
\&    open INFO,   "< datafile" 
\&    open INFO,   "<  datafile"
.Ve
.PP
Ignoring surrounding whitespace also helps for when you read a filename
in from a different file, and forget to trim it before opening:
.PP
.Vb 2
\&    $filename = <INFO>;         # oops, \en still there
\&    open(EXTRA, "< $filename") || die "can\*(Aqt open $filename: $!";
.Ve
.PP
This is not a bug, but a feature.  Because \f(CW\*(C`open\*(C'\fR 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\*(R".
.PP
There is also a 3\-argument version of \f(CW\*(C`open\*(C'\fR, which lets you put the
special redirection characters into their own argument:
.PP
.Vb 1
\&    open( INFO, ">", $datafile ) || die "Can\*(Aqt create $datafile: $!";
.Ve
.PP
In this case, the filename to open is the actual string in \f(CW$datafile\fR,
so you don't have to worry about \f(CW$datafile\fR 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.
.Sh "Indirect Filehandles"
.IX Subsection "Indirect Filehandles"
\&\f(CW\*(C`open\*(C'\fR'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:
.PP
.Vb 5
\&    open( my $in, $infile )   or die "Couldn\*(Aqt read $infile: $!";
\&    while ( <$in> ) {
\&        # do something with $_
\&    }
\&    close $in;
.Ve
.PP
Indirect filehandles make namespace management easier.  Since filehandles
are global to the current package, two subroutines trying to open
\&\f(CW\*(C`INFILE\*(C'\fR will clash.  With two functions opening indirect filehandles
like \f(CW\*(C`my $infile\*(C'\fR, there's no clash and no need to worry about future
conflicts.
.PP
Another convenient behavior is that an indirect filehandle automatically
closes when it goes out of scope or when you undefine it:
.PP
.Vb 4
\&    sub firstline {
\&        open( my $in, shift ) && return scalar <$in>;
\&        # no close() required
\&    }
.Ve
.Sh "Pipe Opens"
.IX Subsection "Pipe Opens"
In C, when you want to open a file using the standard I/O library,
you use the \f(CW\*(C`fopen\*(C'\fR function, but when opening a pipe, you use the
\&\f(CW\*(C`popen\*(C'\fR function.  But in the shell, you just use a different redirection
character.  That's also the case for Perl.  The \f(CW\*(C`open\*(C'\fR call 
remains the same\*(--just its argument differs.
.PP
If the leading character is a pipe symbol, \f(CW\*(C`open\*(C'\fR 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:
.PP
.Vb 3
\&    open(PRINTER, "| lpr \-Plp1")    || die "can\*(Aqt run lpr: $!";
\&    print PRINTER "stuff\en";
\&    close(PRINTER)                  || die "can\*(Aqt close lpr: $!";
.Ve
.PP
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