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



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 shell way also has 2- and 3-argument forms, which have different semantics for
       handling the filename.  The choice is yours.

Open a 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$
           $ 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 "<" is optional.  If omitted, Perl assumes that you want
       to open the file for reading.

       Note also that the first example uses the "||" logical operator, and the second uses "or", which has
       lower precedence.  Using "||" 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 "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 "Dispelling the Dweomer".

       There is also a 3-argument version of "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 $datafile, so you don't have to worry
       about $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.

   Indirect Filehandles
       "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 "INFILE" will clash.  With two functions opening indirect
       filehandles like "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 there are no
       more references to it:

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

       Indirect filehandles also make it easy to pass filehandles to and return filehandles from
       subroutines:

           for my $file ( qw(this.conf that.conf) ) {
               my $fin = open_or_throw('<', $file);
               process_conf( $fin );
               # no close() needed
           }

           use Carp;
           sub open_or_throw {
               my ($mode, $filename) = @_;
               open my $h, $mode, $filename
                   or croak "Could not open '$filename': $!";
               return $h;
           }

   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, "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 $! as usual.  But if the command contains special shell characters, such
       as ">" or "*", 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 "open" call will only indicate failure if Perl can't even run
       the shell.  See "How can I capture STDERR from an external command?" in perlfaq8 to see how to cope
       with this.  There's also an explanation in perlipc.

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

       perl-5.6.x introduced a version of piped open that executes a process based on its command line
       arguments without relying on the shell. (Similar to the "system(@LIST)" notation.) This is safer and
       faster than executing a single argument pipe-command, but does not allow special shell constructs.
       (It is also not supported on Microsoft Windows, Mac OS Classic or RISC OS.)

       Here's an example of "open '-|'", which prints a random Unix fortune cookie as uppercase:

           my $collection = shift(@ARGV);
           open my $fortune, '-|', 'fortune', $collection
               or die "Could not find fortune - $!";
           while (<$fortune>)
           {
               print uc($_);
           }
           close($fortune);

       And this "open '|-'" pipes into lpr:

           open my $printer, '|-', 'lpr', '-Plp1'
               or die "can't run lpr: $!";
           print {$printer} "stuff\n";
           close($printer)
               or die "can't close lpr: $!";

   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 currently running!
       This is actually a stealth "fork" hidden inside an "open" call.  See "Safe Pipe Opens" in perlipc 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, "+> 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 "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 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 perlfaq5 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 files named "-" (and so on) in your directory, 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 a 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 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 whitespace.
       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.
       (Note: starting from Perl release 5.6 the "O_LARGEFILE" flag, if available, is automatically added to
       the sysopen() flags because large files are the default.)

       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 whitespace, 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'll
       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 a 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 file's 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 of 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 "Classic" 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> line 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, rather than the handle?

       If you aren't running with "strict refs", or if you've turned 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> line 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 accessed 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 if $pid = open(STDOUT, "|-");       # return if parent
               die "cannot fork: $!" unless defined $pid;
               while (<STDIN>) {
                   last if --$lines < 0;
                   print;
               }
               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 text files.  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 and 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";
               }
           }

   Opening Named Pipes
       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 "Named Pipes" in
       perlipc.  Unix-domain sockets are rather different beasts as well; they're described in "Unix-Domain
       TCP Clients and Servers" in perlipc.

       When it comes to opening devices, it can be easy and it can be 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 "How do
       I read and write the serial port?" in perlfaq8 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 sockets, you can set them to be
       non-blocking using "fcntl":

           use Fcntl;
           my $old_flags = fcntl($handle, F_GETFL, 0)
               or die "can't get flags: $!";
           fcntl($handle, F_SETFL, $old_flags | 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 POSIX, 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.

   Opening Sockets
       What else can you open?  To open a connection using sockets, you won't use one of Perl's two open
       functions.  See "Sockets: Client/Server Communication" in perlipc 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.cpan.org/');

   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" 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 non-standard system 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, Plan 9,
       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 you 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.  It's possible for another
       process to create a file in the slice of time between your existence check and your attempt to create
       the file.  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 Windows.  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 an 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: $!";

   IO Layers
       In Perl 5.8.0 a new I/O framework called "PerlIO" was introduced.  This is a new "plumbing" for all
       the I/O happening in Perl; for the most part everything will work just as it did, but PerlIO also
       brought in some new features such as the ability to think of I/O as "layers".  One I/O layer may in
       addition to just moving the data also do transformations on the data.  Such transformations may
       include compression and decompression, encryption and decryption, and transforming between various
       character encodings.

       Full discussion about the features of PerlIO is out of scope for this tutorial, but here is how to
       recognize the layers being used:

       •   The three-(or more)-argument form of "open" is being used and the second argument contains
           something else in addition to the usual '<', '>', '>>', '|' and their variants, for example:

               open(my $fh, "<:crlf", $fn);

       •   The two-argument form of "binmode" is being used, for example

               binmode($fh, ":encoding(utf16)");

       For more detailed discussion about PerlIO see PerlIO; for more detailed discussion about Unicode and
       I/O see perluniintro.

SEE ALSO
       The "open" and "sysopen" functions in perlfunc(1); the system open(2), dup(2), fopen(3), and
       fdopen(3) manpages; the POSIX documentation.

AUTHOR and COPYRIGHT
       Copyright 1998 Tom Christiansen.

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

       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



perl v5.16.2                                     2012-10-11                                   PERLOPENTUT(1)