Synopsis

open filehandle, filename

open filehandle, mode, filename

open filehandle, mode, expr, list (new in 5.8)

open filehandle, mode, reference (new in 5.8)
                  

Opens the file given by filename and associates it with filehandle. If filehandle is omitted, the scalar variable of the same name as the filehandle must contain the filename. (And you must also be careful to use or die after the statement rather than || die, because the precedence of || is higher than list operators such as open.)

If filename is preceded by either < or nothing, the file is opened for input (read-only). If filename is preceded by >, the file is opened for output. If the file doesn’t exist, it will be created; if the file exists, it will be overwritten with output using >. Preceding the filename with >> opens an output file for appending. For both read and write access, use a + before < or >.

If you choose to use the three-or-more-arguments form of open, you can use separate mode and filename arguments, such as open($ fh , $ mode , $ filename ), in which $ moderepresents an open mode or pipe. For example:

my $mode = '+<';
my $filename = 'whatever.txt';
open(FH, $mode, $filename)
    or die("can't open $filename: $!");

As covered in Chapter 4, you can build Perl 5.8 and newer with PerlIO support, which offers additional features for your system’s I/O (STDIO). This allows you to do neat things, such as specify utf-8 as your default encoding for all of your I/O, or set your default line endings with 'crlf'. In addition, you can select piping to or extracting information from an external program with '|-' and '-|', respectively. For example:

my $prog = "webster overworked";
open($fh, '-|', $prog)
    or die("can't open pipe to '$prog': $!");

or, similarly:

my @prog_info = qw(/usr/contrib/bin/webster overworked);
open($fh, '-|', @prog_info)
    or die(...);

A filehandle may also be attached to a process by using a piped command. If the filename begins with |, the filename is interpreted as a command to which output is to be piped. If the filename ends with a |, the filename is interpreted as a command that pipes input to you. You may not have an open command that pipes both in and out.

Any pipe command containing shell metacharacters is passed to the shell for execution; otherwise, it is executed directly by Perl. The filename - refers to STDIN, and > refers to STDOUT. open returns nonzero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the process ID of the subprocess.