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+.\" -*- mode: troff; coding: utf-8 -*-
+.\" Automatically generated by Pod::Man 5.01 (Pod::Simple 3.43)
+.\"
+.\" Standard preamble:
+.\" ========================================================================
+.de Sp \" Vertical space (when we can't use .PP)
+.if t .sp .5v
+.if n .sp
+..
+.de Vb \" Begin verbatim text
+.ft CW
+.nf
+.ne \\$1
+..
+.de Ve \" End verbatim text
+.ft R
+.fi
+..
+.\" \*(C` and \*(C' are quotes in nroff, nothing in troff, for use with C<>.
+.ie n \{\
+. ds C` ""
+. ds C' ""
+'br\}
+.el\{\
+. ds C`
+. ds C'
+'br\}
+.\"
+.\" Escape single quotes in literal strings from groff's Unicode transform.
+.ie \n(.g .ds Aq \(aq
+.el .ds Aq '
+.\"
+.\" If the F register is >0, we'll generate index entries on stderr for
+.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
+.\" entries marked with X<> in POD. Of course, you'll have to process the
+.\" output yourself in some meaningful fashion.
+.\"
+.\" Avoid warning from groff about undefined register 'F'.
+.de IX
+..
+.nr rF 0
+.if \n(.g .if rF .nr rF 1
+.if (\n(rF:(\n(.g==0)) \{\
+. if \nF \{\
+. de IX
+. tm Index:\\$1\t\\n%\t"\\$2"
+..
+. if !\nF==2 \{\
+. nr % 0
+. nr F 2
+. \}
+. \}
+.\}
+.rr rF
+.\" ========================================================================
+.\"
+.IX Title "PERLOPENTUT 1"
+.TH PERLOPENTUT 1 2023-11-28 "perl v5.38.2" "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 \- simple recipes for opening files and pipes in Perl
+.SH DESCRIPTION
+.IX Header "DESCRIPTION"
+Whenever you do I/O on a file in Perl, you do so through what in Perl is
+called a \fBfilehandle\fR. A filehandle is an internal name for an external
+file. It is the job of the \f(CW\*(C`open\*(C'\fR function to make the association
+between the internal name and the external name, and it is the job
+of the \f(CW\*(C`close\*(C'\fR function to break that association.
+.PP
+For your convenience, Perl sets up a few special filehandles that are
+already open when you run. These include \f(CW\*(C`STDIN\*(C'\fR, \f(CW\*(C`STDOUT\*(C'\fR, \f(CW\*(C`STDERR\*(C'\fR,
+and \f(CW\*(C`ARGV\*(C'\fR. Since those are pre-opened, you can use them right away
+without having to go to the trouble of opening them yourself:
+.PP
+.Vb 1
+\& print STDERR "This is a debugging message.\en";
+\&
+\& print STDOUT "Please enter something: ";
+\& $response = <STDIN> // die "how come no input?";
+\& print STDOUT "Thank you!\en";
+\&
+\& while (<ARGV>) { ... }
+.Ve
+.PP
+As you see from those examples, \f(CW\*(C`STDOUT\*(C'\fR and \f(CW\*(C`STDERR\*(C'\fR are output
+handles, and \f(CW\*(C`STDIN\*(C'\fR and \f(CW\*(C`ARGV\*(C'\fR are input handles. They are
+in all capital letters because they are reserved to Perl, much
+like the \f(CW@ARGV\fR array and the \f(CW%ENV\fR hash are. Their external
+associations were set up by your shell.
+.PP
+You will need to open every other filehandle on your own. Although there
+are many variants, the most common way to call Perl's \fBopen()\fR function
+is with three arguments and one return value:
+.PP
+\&\f(CW\*(C` \fR\f(CIOK\fR\f(CW = open(\fR\f(CIHANDLE\fR\f(CW, \fR\f(CIMODE\fR\f(CW, \fR\f(CIPATHNAME\fR\f(CW)\*(C'\fR
+.PP
+Where:
+.IP \fIOK\fR 4
+.IX Item "OK"
+will be some defined value if the open succeeds, but
+\&\f(CW\*(C`undef\*(C'\fR if it fails;
+.IP \fIHANDLE\fR 4
+.IX Item "HANDLE"
+should be an undefined scalar variable to be filled in by the
+\&\f(CW\*(C`open\*(C'\fR function if it succeeds;
+.IP \fIMODE\fR 4
+.IX Item "MODE"
+is the access mode and the encoding format to open the file with;
+.IP \fIPATHNAME\fR 4
+.IX Item "PATHNAME"
+is the external name of the file you want opened.
+.PP
+Most of the complexity of the \f(CW\*(C`open\*(C'\fR function lies in the many
+possible values that the \fIMODE\fR parameter can take on.
+.PP
+One last thing before we show you how to open files: opening
+files does not (usually) automatically lock them in Perl. See
+perlfaq5 for how to lock.
+.SH "Opening Text Files"
+.IX Header "Opening Text Files"
+.SS "Opening Text Files for Reading"
+.IX Subsection "Opening Text Files for Reading"
+If you want to read from a text file, first open it in
+read-only mode like this:
+.PP
+.Vb 3
+\& my $filename = "/some/path/to/a/textfile/goes/here";
+\& my $encoding = ":encoding(UTF\-8)";
+\& my $handle = undef; # this will be filled in on success
+\&
+\& open($handle, "< $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename for reading: $!";
+.Ve
+.PP
+As with the shell, in Perl the \f(CW"<"\fR is used to open the file in
+read-only mode. If it succeeds, Perl allocates a brand new filehandle for
+you and fills in your previously undefined \f(CW$handle\fR argument with a
+reference to that handle.
+.PP
+Now you may use functions like \f(CW\*(C`readline\*(C'\fR, \f(CW\*(C`read\*(C'\fR, \f(CW\*(C`getc\*(C'\fR, and
+\&\f(CW\*(C`sysread\*(C'\fR on that handle. Probably the most common input function
+is the one that looks like an operator:
+.PP
+.Vb 2
+\& $line = readline($handle);
+\& $line = <$handle>; # same thing
+.Ve
+.PP
+Because the \f(CW\*(C`readline\*(C'\fR function returns \f(CW\*(C`undef\*(C'\fR at end of file or
+upon error, you will sometimes see it used this way:
+.PP
+.Vb 7
+\& $line = <$handle>;
+\& if (defined $line) {
+\& # do something with $line
+\& }
+\& else {
+\& # $line is not valid, so skip it
+\& }
+.Ve
+.PP
+You can also just quickly \f(CW\*(C`die\*(C'\fR on an undefined value this way:
+.PP
+.Vb 1
+\& $line = <$handle> // die "no input found";
+.Ve
+.PP
+However, if hitting EOF is an expected and normal event, you do not want to
+exit simply because you have run out of input. Instead, you probably just want
+to exit an input loop. You can then test to see if an actual error has caused
+the loop to terminate, and act accordingly:
+.PP
+.Vb 6
+\& while (<$handle>) {
+\& # do something with data in $_
+\& }
+\& if ($!) {
+\& die "unexpected error while reading from $filename: $!";
+\& }
+.Ve
+.PP
+\&\fBA Note on Encodings\fR: Having to specify the text encoding every time
+might seem a bit of a bother. To set up a default encoding for \f(CW\*(C`open\*(C'\fR so
+that you don't have to supply it each time, you can use the \f(CW\*(C`open\*(C'\fR pragma:
+.PP
+.Vb 1
+\& use open qw< :encoding(UTF\-8) >;
+.Ve
+.PP
+Once you've done that, you can safely omit the encoding part of the
+open mode:
+.PP
+.Vb 2
+\& open($handle, "<", $filename)
+\& || die "$0: can\*(Aqt open $filename for reading: $!";
+.Ve
+.PP
+But never use the bare \f(CW"<"\fR without having set up a default encoding
+first. Otherwise, Perl cannot know which of the many, many, many possible
+flavors of text file you have, and Perl will have no idea how to correctly
+map the data in your file into actual characters it can work with. Other
+common encoding formats including \f(CW"ASCII"\fR, \f(CW"ISO\-8859\-1"\fR,
+\&\f(CW"ISO\-8859\-15"\fR, \f(CW"Windows\-1252"\fR, \f(CW"MacRoman"\fR, and even \f(CW"UTF\-16LE"\fR.
+See perlunitut for more about encodings.
+.SS "Opening Text Files for Writing"
+.IX Subsection "Opening Text Files for Writing"
+When you want to write to a file, you first have to decide what to do about
+any existing contents of that file. You have two basic choices here: to
+preserve or to clobber.
+.PP
+If you want to preserve any existing contents, then you want to open the file
+in append mode. As in the shell, in Perl you use \f(CW">>"\fR to open an
+existing file in append mode. \f(CW">>"\fR creates the file if it does not
+already exist.
+.PP
+.Vb 3
+\& my $handle = undef;
+\& my $filename = "/some/path/to/a/textfile/goes/here";
+\& my $encoding = ":encoding(UTF\-8)";
+\&
+\& open($handle, ">> $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename for appending: $!";
+.Ve
+.PP
+Now you can write to that filehandle using any of \f(CW\*(C`print\*(C'\fR, \f(CW\*(C`printf\*(C'\fR,
+\&\f(CW\*(C`say\*(C'\fR, \f(CW\*(C`write\*(C'\fR, or \f(CW\*(C`syswrite\*(C'\fR.
+.PP
+As noted above, if the file does not already exist, then the append-mode open
+will create it for you. But if the file does already exist, its contents are
+safe from harm because you will be adding your new text past the end of the
+old text.
+.PP
+On the other hand, sometimes you want to clobber whatever might already be
+there. To empty out a file before you start writing to it, you can open it
+in write-only mode:
+.PP
+.Vb 3
+\& my $handle = undef;
+\& my $filename = "/some/path/to/a/textfile/goes/here";
+\& my $encoding = ":encoding(UTF\-8)";
+\&
+\& open($handle, "> $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename in write\-open mode: $!";
+.Ve
+.PP
+Here again Perl works just like the shell in that the \f(CW">"\fR clobbers
+an existing file.
+.PP
+As with the append mode, when you open a file in write-only mode,
+you can now write to that filehandle using any of \f(CW\*(C`print\*(C'\fR, \f(CW\*(C`printf\*(C'\fR,
+\&\f(CW\*(C`say\*(C'\fR, \f(CW\*(C`write\*(C'\fR, or \f(CW\*(C`syswrite\*(C'\fR.
+.PP
+What about read-write mode? You should probably pretend it doesn't exist,
+because opening text files in read-write mode is unlikely to do what you
+would like. See perlfaq5 for details.
+.SH "Opening Binary Files"
+.IX Header "Opening Binary Files"
+If the file to be opened contains binary data instead of text characters,
+then the \f(CW\*(C`MODE\*(C'\fR argument to \f(CW\*(C`open\*(C'\fR is a little different. Instead of
+specifying the encoding, you tell Perl that your data are in raw bytes.
+.PP
+.Vb 3
+\& my $filename = "/some/path/to/a/binary/file/goes/here";
+\& my $encoding = ":raw :bytes"
+\& my $handle = undef; # this will be filled in on success
+.Ve
+.PP
+And then open as before, choosing \f(CW"<"\fR, \f(CW">>"\fR, or
+\&\f(CW">"\fR as needed:
+.PP
+.Vb 2
+\& open($handle, "< $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename for reading: $!";
+\&
+\& open($handle, ">> $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename for appending: $!";
+\&
+\& open($handle, "> $encoding", $filename)
+\& || die "$0: can\*(Aqt open $filename in write\-open mode: $!";
+.Ve
+.PP
+Alternately, you can change to binary mode on an existing handle this way:
+.PP
+.Vb 1
+\& binmode($handle) || die "cannot binmode handle";
+.Ve
+.PP
+This is especially handy for the handles that Perl has already opened for you.
+.PP
+.Vb 2
+\& binmode(STDIN) || die "cannot binmode STDIN";
+\& binmode(STDOUT) || die "cannot binmode STDOUT";
+.Ve
+.PP
+You can also pass \f(CW\*(C`binmode\*(C'\fR an explicit encoding to change it on the fly.
+This isn't exactly "binary" mode, but we still use \f(CW\*(C`binmode\*(C'\fR to do it:
+.PP
+.Vb 2
+\& binmode(STDIN, ":encoding(MacRoman)") || die "cannot binmode STDIN";
+\& binmode(STDOUT, ":encoding(UTF\-8)") || die "cannot binmode STDOUT";
+.Ve
+.PP
+Once you have your binary file properly opened in the right mode, you can
+use all the same Perl I/O functions as you used on text files. However,
+you may wish to use the fixed-size \f(CW\*(C`read\*(C'\fR instead of the variable-sized
+\&\f(CW\*(C`readline\*(C'\fR for your input.
+.PP
+Here's an example of how to copy a binary file:
+.PP
+.Vb 3
+\& my $BUFSIZ = 64 * (2 ** 10);
+\& my $name_in = "/some/input/file";
+\& my $name_out = "/some/output/flie";
+\&
+\& my($in_fh, $out_fh, $buffer);
+\&
+\& open($in_fh, "<", $name_in)
+\& || die "$0: cannot open $name_in for reading: $!";
+\& open($out_fh, ">", $name_out)
+\& || die "$0: cannot open $name_out for writing: $!";
+\&
+\& for my $fh ($in_fh, $out_fh) {
+\& binmode($fh) || die "binmode failed";
+\& }
+\&
+\& while (read($in_fh, $buffer, $BUFSIZ)) {
+\& unless (print $out_fh $buffer) {
+\& die "couldn\*(Aqt write to $name_out: $!";
+\& }
+\& }
+\&
+\& close($in_fh) || die "couldn\*(Aqt close $name_in: $!";
+\& close($out_fh) || die "couldn\*(Aqt close $name_out: $!";
+.Ve
+.SH "Opening Pipes"
+.IX Header "Opening Pipes"
+Perl also lets you open a filehandle into an external program or shell
+command rather than into a file. You can do this in order to pass data
+from your Perl program to an external command for further processing, or
+to receive data from another program for your own Perl program to
+process.
+.PP
+Filehandles into commands are also known as \fIpipes\fR, since they work on
+similar inter-process communication principles as Unix pipelines. Such a
+filehandle has an active program instead of a static file on its
+external end, but in every other sense it works just like a more typical
+file-based filehandle, with all the techniques discussed earlier in this
+article just as applicable.
+.PP
+As such, you open a pipe using the same \f(CW\*(C`open\*(C'\fR call that you use for
+opening files, setting the second (\f(CW\*(C`MODE\*(C'\fR) argument to special
+characters that indicate either an input or an output pipe. Use \f(CW"\-|"\fR for a
+filehandle that will let your Perl program read data from an external
+program, and \f(CW"|\-"\fR for a filehandle that will send data to that
+program instead.
+.SS "Opening a pipe for reading"
+.IX Subsection "Opening a pipe for reading"
+Let's say you'd like your Perl program to process data stored in a nearby
+directory called \f(CW\*(C`unsorted\*(C'\fR, which contains a number of textfiles.
+You'd also like your program to sort all the contents from these files
+into a single, alphabetically sorted list of unique lines before it
+starts processing them.
+.PP
+You could do this through opening an ordinary filehandle into each of
+those files, gradually building up an in-memory array of all the file
+contents you load this way, and finally sorting and filtering that array
+when you've run out of files to load. \fIOr\fR, you could offload all that
+merging and sorting into your operating system's own \f(CW\*(C`sort\*(C'\fR command by
+opening a pipe directly into its output, and get to work that much
+faster.
+.PP
+Here's how that might look:
+.PP
+.Vb 2
+\& open(my $sort_fh, \*(Aq\-|\*(Aq, \*(Aqsort \-u unsorted/*.txt\*(Aq)
+\& or die "Couldn\*(Aqt open a pipe into sort: $!";
+\&
+\& # And right away, we can start reading sorted lines:
+\& while (my $line = <$sort_fh>) {
+\& #
+\& # ... Do something interesting with each $line here ...
+\& #
+\& }
+.Ve
+.PP
+The second argument to \f(CW\*(C`open\*(C'\fR, \f(CW"\-|"\fR, makes it a read-pipe into a
+separate program, rather than an ordinary filehandle into a file.
+.PP
+Note that the third argument to \f(CW\*(C`open\*(C'\fR is a string containing the
+program name (\f(CW\*(C`sort\*(C'\fR) plus all its arguments: in this case, \f(CW\*(C`\-u\*(C'\fR to
+specify unqiue sort, and then a fileglob specifying the files to sort.
+The resulting filehandle \f(CW$sort_fh\fR works just like a read-only (\f(CW"<"\fR) filehandle, and your program can subsequently read data
+from it as if it were opened onto an ordinary, single file.
+.SS "Opening a pipe for writing"
+.IX Subsection "Opening a pipe for writing"
+Continuing the previous example, let's say that your program has
+completed its processing, and the results sit in an array called
+\&\f(CW@processed\fR. You want to print these lines to a file called
+\&\f(CW\*(C`numbered.txt\*(C'\fR with a neatly formatted column of line-numbers.
+.PP
+Certainly you could write your own code to do this — or, once again,
+you could kick that work over to another program. In this case, \f(CW\*(C`cat\*(C'\fR,
+running with its own \f(CW\*(C`\-n\*(C'\fR option to activate line numbering, should do
+the trick:
+.PP
+.Vb 2
+\& open(my $cat_fh, \*(Aq|\-\*(Aq, \*(Aqcat \-n > numbered.txt\*(Aq)
+\& or die "Couldn\*(Aqt open a pipe into cat: $!";
+\&
+\& for my $line (@processed) {
+\& print $cat_fh $line;
+\& }
+.Ve
+.PP
+Here, we use a second \f(CW\*(C`open\*(C'\fR argument of \f(CW"|\-"\fR, signifying that the
+filehandle assigned to \f(CW$cat_fh\fR should be a write-pipe. We can then
+use it just as we would a write-only ordinary filehandle, including the
+basic function of \f(CW\*(C`print\*(C'\fR\-ing data to it.
+.PP
+Note that the third argument, specifying the command that we wish to
+pipe to, sets up \f(CW\*(C`cat\*(C'\fR to redirect its output via that \f(CW">"\fR
+symbol into the file \f(CW\*(C`numbered.txt\*(C'\fR. This can start to look a little
+tricky, because that same symbol would have meant something
+entirely different had it showed it in the second argument to \f(CW\*(C`open\*(C'\fR!
+But here in the third argument, it's simply part of the shell command that
+Perl will open the pipe into, and Perl itself doesn't invest any special
+meaning to it.
+.SS "Expressing the command as a list"
+.IX Subsection "Expressing the command as a list"
+For opening pipes, Perl offers the option to call \f(CW\*(C`open\*(C'\fR with a list
+comprising the desired command and all its own arguments as separate
+elements, rather than combining them into a single string as in the
+examples above. For instance, we could have phrased the \f(CW\*(C`open\*(C'\fR call in
+the first example like this:
+.PP
+.Vb 2
+\& open(my $sort_fh, \*(Aq\-|\*(Aq, \*(Aqsort\*(Aq, \*(Aq\-u\*(Aq, glob(\*(Aqunsorted/*.txt\*(Aq))
+\& or die "Couldn\*(Aqt open a pipe into sort: $!";
+.Ve
+.PP
+When you call \f(CW\*(C`open\*(C'\fR this way, Perl invokes the given command directly,
+bypassing the shell. As such, the shell won't try to interpret any
+special characters within the command's argument list, which might
+overwise have unwanted effects. This can make for safer, less
+error-prone \f(CW\*(C`open\*(C'\fR calls, useful in cases such as passing in variables
+as arguments, or even just referring to filenames with spaces in them.
+.PP
+However, when you \fIdo\fR want to pass a meaningful metacharacter to the
+shell, such with the \f(CW"*"\fR inside that final \f(CW\*(C`unsorted/*.txt\*(C'\fR argument
+here, you can't use this alternate syntax. In this case, we have worked
+around it via Perl's handy \f(CW\*(C`glob\*(C'\fR built-in function, which evaluates
+its argument into a list of filenames — and we can safely pass that
+resulting list right into \f(CW\*(C`open\*(C'\fR, as shown above.
+.PP
+Note also that representing piped-command arguments in list form like
+this doesn't work on every platform. It will work on any Unix-based OS
+that provides a real \f(CW\*(C`fork\*(C'\fR function (e.g. macOS or Linux), as well as
+on Windows when running Perl 5.22 or later.
+.SH "SEE ALSO"
+.IX Header "SEE ALSO"
+The full documentation for \f(CW\*(C`open\*(C'\fR
+provides a thorough reference to this function, beyond the best-practice
+basics covered here.
+.SH "AUTHOR and COPYRIGHT"
+.IX Header "AUTHOR and COPYRIGHT"
+Copyright 2013 Tom Christiansen; now maintained by Perl5 Porters
+.PP
+This documentation is free; you can redistribute it and/or modify it under
+the same terms as Perl itself.
generated by cgit v1.2.3 (git 2.39.1) at 2025-02-14 07:39:18 +0000