PERLOPENTUT
NAME DESCRIPTION Open la shell A Open A la C Obscure Open Tricks Other I/O Issues SEE ALSO AUTHOR and COPYRIGHT HISTORY
perlopentut - tutorial on opening things in Perl
Perl has two simple, built-in ways to open files: the shell way for convenience, and the C way for precision. The choice is yours.
Perl's open function was designed to mimic the way command-line redirection in the shell works. Here are some
$ myprogram
$ otherprogram 2 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.
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.
The other important thing to notice is that, just as in the shell, any white space before or after the filename is ignored. This is good, because you wouldn't want these to do
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 white space around the filename itself as well. For accessing files with naughty names, see ``Dispelling the Dweomer''.
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 open 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
What happens if you try to open a pipe to or from a non-existent command? In most systems, such an open will not return an error. That's because in the traditional fork/exec model, running the other program happens only in the forked child process, which means that the failed exec can't be reflected in the return value of open. Only a failed fork shows up there. See ``Why doesn't open() return an error when a pipe open fails?'' 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
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, open(SCREEN, open(LOGFILE,
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
changes all their foo's to bar's, leaving the old version in the original file name with a ``.orig'' tacked on the
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 , Perl actually does an implicit open on each file in @ARGV. Thus a
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 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(
getopts(
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
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
? '
open(PWD, $pwdinfo)
or die
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 a file named ``- (and so on) in your directory, that 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
It's not for nothing that this is called magic . Pretty nifty, eh?
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, then 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 white space. 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,
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 white space, but you'll get the idea:
And here are things you can do with sysopen that you cannot do with a regular open. As you 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
Permissionsla modeA
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 files' 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 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.
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 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,
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
$handle = IO::Socket::INET-
local *REMOTE = IO::Socket::INET-
If you're using magic , you could even pass in as a command line argument in @ARGV something like , 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 () at the end off the string.
$file = s#^(s)#./$1#;
open(FH, 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 proprietary 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 processing in a totally boring and non-magical way, you could do this
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
That's because you opened a filehandle FH , and had read in seven records from it. But what was the name of the file, not the handle?
If you aren't running with strict refs, or if you've turn them off temporarily, then all you have to do is
Since you're using the pathname of the file as its handle, you'll get warnings more like
Some warning at scriptname line 29,
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 =
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
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, open(STDOUT,
And then these can be read 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.
while (
sub head {
my $lines = shift 20; return unless $pid = open(STDOUT, This technique can be applied to repeatedly push as many filters on your output stream as you wish.
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 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 textfiles. 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
If you want to process directories recursively, it's better to use the File::Find module. For example, this prints out all files recursively, add adds a slash to their names if the file is a directory.
@ARGV = qw(.) unless @ARGV;
use File::Find; find sub { print $File::Find::name, -d
if (defined($whither = readlink($file))) { print 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 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
fcntl(Connection, F_SETFL, O_NONBLOCK) or die Rather than losing yourself in a morass of twisting, turning ioctls, 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?.
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-
$doc = get('
http://www.linpro.no/lwp/');
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
binmode(STDOUT); while ( 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 binmodeing on the handle.
sysopen(BINDAT,
Now you can use read and print on that handle without worrying about the system non-standard 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 , Plan9, 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 others 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. 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 WindowsNT. 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 a 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,
use Fcntl qw(:DEFAULT :flock); open(FH, You can get a non-blocking lock by using LOCK_NB.
flock(FH, LOCK_SH LOCK_NB)
or die
use Fcntl qw(:DEFAULT :flock); open(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,
or die
The open and sysopen function in perlfunc(1); the standard open(2), dup(2), fopen(3), and fdopen(3) manpages; the POSIX documentation.
Copyright 1998 Tom Christiansen.
When included as part of the Standard Version of Perl, or as part of its complete documentation whether printed or otherwise, this work may be distributed only under the terms of Perl's Artistic License. Any distribution of this file or derivatives thereof outside of that package require that special arrangements be made with copyright holder.
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.
First release: Sat Jan 9 08:09:11 MST 1999
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