man > perlopentut(1)

PERLOPENTUT(1)                    Perl Programmers Reference Guide                    PERLOPENTUT(1)

NAME
       perlopentut - simple recipes for opening files and pipes in Perl

DESCRIPTION
       Whenever you do I/O on a file in Perl, you do so through what in Perl is called a filehandle.
       A filehandle is an internal name for an external file.  It is the job of the "open" function
       to make the association between the internal name and the external name, and it is the job of
       the "close" function to break that association.

       For your convenience, Perl sets up a few special filehandles that are already open when you
       run.  These include "STDIN", "STDOUT", "STDERR", and "ARGV".  Since those are pre-opened, you
       can use them right away without having to go to the trouble of opening them yourself:

           print STDERR "This is a debugging message.\n";

           print STDOUT "Please enter something: ";
           $response = <STDIN> // die "how come no input?";
           print STDOUT "Thank you!\n";

           while (<ARGV>) { ... }

       As you see from those examples, "STDOUT" and "STDERR" are output handles, and "STDIN" and
       "ARGV" are input handles.  They are in all capital letters because they are reserved to Perl,
       much like the @ARGV array and the %ENV hash are.  Their external associations were set up by
       your shell.

       You will need to open every other filehandle on your own. Although there are many variants,
       the most common way to call Perl's open() function is with three arguments and one return
       value:

       "    OK = open(HANDLE, MODE, PATHNAME)"

       Where:

       OK  will be some defined value if the open succeeds, but "undef" if it fails;

       HANDLE
           should  be  an  undefined  scalar  variable  to be filled in by the "open" function if it
           succeeds;

       MODE
           is the access mode and the encoding format to open the file with;

       PATHNAME
           is the external name of the file you want opened.

       Most of the complexity of the "open" function lies in the many possible values that the  MODE
       parameter can take on.

       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.

Opening Text Files
   Opening Text Files for Reading
       If you want to read from a text file, first open it in read-only mode like this:

           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't open $filename for reading: $!";

       As with the shell, in Perl the "<" 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 $handle argument with a reference to that handle.

       Now you may use functions like "readline", "read", "getc",  and  "sysread"  on  that  handle.
       Probably the most common input function is the one that looks like an operator:

           $line = readline($handle);
           $line = <$handle>;          # same thing

       Because  the  "readline"  function  returns  "undef"  at  end of file or upon error, you will
       sometimes see it used this way:

           $line = <$handle>;
           if (defined $line) {
               # do something with $line
           }
           else {
               # $line is not valid, so skip it
           }

       You can also just quickly "die" on an undefined value this way:

           $line = <$handle> // die "no input found";

       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:

           while (<$handle>) {
               # do something with data in $_
           }
           if ($!) {
               die "unexpected error while reading from $filename: $!";
           }

       A  Note  on  Encodings:  Having to specify the text encoding every time might seem a bit of a
       bother.  To set up a default encoding for "open" so that you don't have  to  supply  it  each
       time, you can use the "open" pragma:

           use open qw< :encoding(UTF-8) >;

       Once you've done that, you can safely omit the encoding part of the open mode:

           open($handle, "<", $filename)
               || die "$0: can't open $filename for reading: $!";

       But  never  use the bare "<" 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  "ASCII",  "ISO-8859-1",  "ISO-8859-15",
       "Windows-1252", "MacRoman", and even "UTF-16LE".  See perlunitut for more about encodings.

   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.

       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 ">>" to open an existing file in append mode.  ">>"  creates
       the file if it does not already exist.

           my $handle   = undef;
           my $filename = "/some/path/to/a/textfile/goes/here";
           my $encoding = ":encoding(UTF-8)";

           open($handle, ">> $encoding", $filename)
               || die "$0: can't open $filename for appending: $!";

       Now  you  can  write  to  that  filehandle using any of "print", "printf", "say", "write", or
       "syswrite".

       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.

       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:

           my $handle   = undef;
           my $filename = "/some/path/to/a/textfile/goes/here";
           my $encoding = ":encoding(UTF-8)";

           open($handle, "> $encoding", $filename)
               || die "$0: can't open $filename in write-open mode: $!";

       Here again Perl works just like the shell in that the ">" clobbers an existing file.

       As  with  the append mode, when you open a file in write-only mode, you can now write to that
       filehandle using any of "print", "printf", "say", "write", or "syswrite".

       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.

Opening Binary Files
       If the file to be opened contains binary data instead of text  characters,  then  the  "MODE"
       argument  to "open" is a little different.  Instead of specifying the encoding, you tell Perl
       that your data are in raw bytes.

           my $filename = "/some/path/to/a/binary/file/goes/here";
           my $encoding = ":raw :bytes"
           my $handle   = undef;     # this will be filled in on success

       And then open as before, choosing "<", ">>", or ">" as needed:

           open($handle, "< $encoding", $filename)
               || die "$0: can't open $filename for reading: $!";

           open($handle, ">> $encoding", $filename)
               || die "$0: can't open $filename for appending: $!";

           open($handle, "> $encoding", $filename)
               || die "$0: can't open $filename in write-open mode: $!";

       Alternately, you can change to binary mode on an existing handle this way:

           binmode($handle)    || die "cannot binmode handle";

       This is especially handy for the handles that Perl has already opened for you.

           binmode(STDIN)      || die "cannot binmode STDIN";
           binmode(STDOUT)     || die "cannot binmode STDOUT";

       You can also pass "binmode" an explicit encoding to change it on the fly.  This isn't exactly
       "binary" mode, but we still use "binmode" to do it:

         binmode(STDIN,  ":encoding(MacRoman)") || die "cannot binmode STDIN";
         binmode(STDOUT, ":encoding(UTF-8)")    || die "cannot binmode STDOUT";

       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
       "read" instead of the variable-sized "readline" for your input.

       Here's an example of how to copy a binary file:

           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't write to $name_out: $!";
               }
           }

           close($in_fh)       || die "couldn't close $name_in: $!";
           close($out_fh)      || die "couldn't close $name_out: $!";

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.

       Filehandles  into  commands are also known as pipes, 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.

       As such, you open a pipe using the same "open" call that you use for opening  files,  setting
       the second ("MODE") argument to special characters that indicate either an input or an output
       pipe.  Use  "-|"  for a filehandle that will let your Perl program read data from an external
       program, and "|-" for a filehandle that will send data to that program instead.

   Opening a pipe for reading
       Let's say you'd like your Perl program to process data stored in a  nearby  directory  called
       "unsorted",  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.

       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. Or, you could offload
       all that merging and sorting into your operating system's own "sort"  command  by  opening  a
       pipe directly into its output, and get to work that much faster.

       Here's how that might look:

           open(my $sort_fh, '-|', 'sort -u unsorted/*.txt')
               or die "Couldn't 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 ...
               #
           }

       The  second  argument  to  "open", "-|", makes it a read-pipe into a separate program, rather
       than an ordinary filehandle into a file.

       Note that the third argument to "open" is a string containing the program name ("sort")  plus
       all  its arguments: in this case, "-u" to specify unqiue sort, and then a fileglob specifying
       the files to sort.  The resulting filehandle $sort_fh  works  just  like  a  read-only  ("<")
       filehandle,  and your program can subsequently read data from it as if it were opened onto an
       ordinary, single file.

   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 @processed. You want to print these lines to a file
       called "numbered.txt" with a neatly formatted column of line-numbers.

       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, "cat", running with its own "-n"  option  to  activate
       line numbering, should do the trick:

           open(my $cat_fh, '|-', 'cat -n > numbered.txt')
               or die "Couldn't open a pipe into cat: $!";

           for my $line (@processed) {
               print $cat_fh $line;
           }

       Here,  we  use  a  second "open" argument of "|-", signifying that the filehandle assigned to
       $cat_fh should be a write-pipe. We can then use it just as we  would  a  write-only  ordinary
       filehandle, including the basic function of "print"-ing data to it.

       Note  that  the third argument, specifying the command that we wish to pipe to, sets up "cat"
       to redirect its output via that ">" symbol into the file "numbered.txt". 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 "open"!  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.

   Expressing the command as a list
       For opening pipes, Perl offers the option to call "open" 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  "open"  call
       in the first example like this:

           open(my $sort_fh, '-|', 'sort', '-u', glob('unsorted/*.txt'))
               or die "Couldn't open a pipe into sort: $!";

       When  you call "open" 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 "open" calls, useful in cases such as passing in variables as arguments, or  even
       just referring to filenames with spaces in them.

       However,  when you do want to pass a meaningful metacharacter to the shell, such with the "*"
       inside that final "unsorted/*.txt" argument here, you can't use  this  alternate  syntax.  In
       this  case,  we  have  worked  around  it  via  Perl's  handy "glob" built-in function, which
       evaluates its argument into a list of filenames — and we can safely pass that resulting  list
       right into "open", as shown above.

       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 "fork" function  (e.g.
       macOS or Linux), as well as on Windows when running Perl 5.22 or later.

SEE ALSO
       The  full documentation for "open" provides a thorough reference to this function, beyond the
       best-practice basics covered here.

AUTHOR and COPYRIGHT
       Copyright 2013 Tom Christiansen; now maintained by Perl5 Porters

       This documentation is free; you can redistribute it and/or modify it under the same terms  as
       Perl itself.

perl v5.38.2                                 2026-06-12                               PERLOPENTUT(1)