Problem

You want to read or write to a filename from Perl.

Solution

Use open for convenience, sysopen for precision, or the IO::File module to get an anonymous filehandle.

The open function takes two arguments: the filehandle to open and one string containing the filename and special characters indicating how to open it (the mode):

open(SOURCE, "< $path")
    or die "Couldn't open $path for reading: $!\n";

open(SINK, "> $path")
    or die "Couldn't open $path for writing: $!\n";

The sysopen function takes three or four arguments: filehandle, filename, mode, and an optional permissions value. The mode is a number constructed from constants provided by the Fcntl module:

use Fcntl;

sysopen(SOURCE, $path, O_RDONLY)
    or die "Couldn't open $path for reading: $!\n";

sysopen(SINK, $path, O_WRONLY)
    or die "Couldn't open $path for writing: $!\n";

The IO::File module's new method accepts both open and sysopen style arguments and returns an anonymous filehandle. The new method also accepts a mode in the style of fopen (3):

use IO::File;

# like Perl's open
$fh = IO::File->new("> $filename")
    or die "Couldn't open $filename for writing: $!\n";

# like Perl's sysopen
$fh = IO::File->new($filename, O_WRONLY|O_CREAT)
    or die "Couldn't open $filename for writing: $!\n";

# like stdio's fopen(3)
$fh = IO::File->new($filename, "r+")
    or die "Couldn't open $filename for read and write: $!\n";

Discussion

All input and output goes through filehandles, whether filehandles are mentioned or not. Filehandles aren't exclusively connected to files  - they're also used to communicate with other programs (see Chapter 16, Process Management and Communication) and for network communication (see Chapter 17, Sockets). The open function can also be used to manipulate file descriptors, discussed in Recipe 7.19.

The open function quickly and conveniently solves the problem of associating a filehandle with a file. It permits a shorthand for common modes (reading, writing, reading and writing, appending) passed in with the filename. It doesn't let you control the permission that files are created with or even whether files are created. For this level of control, you need sysopen, which uses constants provided by the Fcntl module to control individual settings like read, write, create, and truncate.

Most programmers meet open long before they meet sysopen. The following table shows how open modes (the Filename column) correspond to sysopen constants (O_ flags) and to the fopen (3) strings that IO::File->new can take (Char). Read and Write indicate that the filehandle may be read from or written to. Append means no matter where you are in the file, output goes to the end of the file (on most systems). Create indicates whether the open statement creates a file if one having the given name doesn't already exist. Trunc indicates open will clobber any existing data if the file already exists.

Here's a tip: you almost never want to use +> or +>>. The first clobbers your file before you can read it, and the second one is confusing because your read pointer can be anywhere, but on many systems, the writer always jumps to the end of the file.

The sysopen function takes three or four arguments:

sysopen(FILEHANDLE, $name, $flags)         or die "Can't open $name : $!";
sysopen(FILEHANDLE, $name, $flags, $perms) or die "Can't open $name : $!";

$name is the name of the file, without any < or + funny business. $flags is a number, formed by ORing together separate mode values for O_CREAT, O_WRONLY, O_TRUNC, etc. The exact availability of O_* constants depends on your operating system, so consult the online documentation for this (usually open (2), but not always), or look in /usr/include/fcntl.h. Common ones are:

Less common O_* flags sometimes available include O_SHLOCK, O_EXLOCK, O_BINARY, O_NOCTTY, and O_SYNC. Consult your open (2) manpage or its local equivalent for details.

If you omit the $perms argument to sysopen, Perl uses the octal value 0666. These permissions values need to be in octal and are modified by your process's current umask. A umask value is a number representing disabled permissions bits  - if your umask were 027 (group can't write; others can't read, write, or execute), then passing sysopen 0666 would create a file with mode 0640 (mathematically: 0666 &~ 027 is 0640).

If umask seems confusing, here's some advice: supply a creation mode of 0666 for regular files and one of 0777 for directories and executable files. This gives users a choice: if they want protected files, they can choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. One exception is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on. In short, seldom if ever use 0644 as argument to sysopen because that takes away the user's option to have a more permissive umask.

Here are examples of open and sysopen in action.

To open file for reading:

open(FH, "< $path")                                 or die $!;
sysopen(FH, $path, O_RDONLY)                        or die $!;

To open file for writing, create new file if needed, or else truncate old file:

open(FH, "> $path")                                 or die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        or die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0600)  or die $!;

To open file for writing, create new file, file must not exist:

sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         or die $!;
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0600)   or die $!;

To open file for appending, create if necessary:

open(FH, ">> $path")                                or die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       or die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0600) or die $!;

To open file for appending, file must exist:

sysopen(FH, $path, O_WRONLY|O_APPEND)               or die $!;

To open file for update, file must exist:

open(FH, "+< $path")                                or die $!;
sysopen(FH, $path, O_RDWR)                          or die $!;

To open file for update, create file if necessary:

sysopen(FH, $path, O_RDWR|O_CREAT)                  or die $!;
sysopen(FH, $path, O_RDWR|O_CREAT, 0600)            or die $!;

To open file for update, file must not exist:

sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           or die $!;
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0600)     or die $!;

We demonstrate using a creation mask of 0600 here only to show how to create a private file. The argument is normally omitted.

See Also

The open, sysopen, and umask functions in perlfunc (1) and Chapter 3 of Programming Perl; the documentation for the standard IO::File and Fcntl modules (also in Chapter 7 of Programming Perl); your system's open (2), fopen (3), and umask (2) manpages; Recipe 7.2