OPEN
open FILEHANDLE,EXPR
open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. If the filename begins with <" or nothing, the file is opened for input. If the filename begins with ">", the file is opened for output. If the filename begins with >>", the file is opened for appending. (You can put a '+' in front of the '>' or <' to indicate that you want both read and write access to the file.) If the filename begins with "|", the filename is interpreted as a command to which output is to be piped, and if the filename ends with a "|", the filename is interpreted as command which pipes input to us. (You may not have a command that pipes both in and out.) Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns non-zero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess. Examples:

$ARTICLE = 100; open ARTICLE or die "Can't find article $ARTICLE: $!\n"; while (<ARTICLE>) {... 
    open(LOG, >>/usr/spool/news/twitlog'); # (log is reserved)
    open(article, "caesar <$article |");    # decrypt article
    open(extract, "|sort >/tmp/Tmp$$");     # $$ is our process id
    # process argument list of files along with any includes
    foreach $file (@ARGV) {
        process($file, 'fh00');
    }
    sub process {
        local($filename, $input) = @_;
        $input++;               # this is a string increment
        unless (open($input, $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }
        while (<$input>) {              # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            ...         # whatever
        }
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) which is to be duped and opened. You may use & after >,>>,<, +>, >> and <. The mode you specify should match the mode of the original filehandle. Here is a script that saves, redirects, and restores STDOUT and STDERR:

#!/usr/bin/perl open(SAVEOUT, ">&STDOUT"); open(SAVEERR, ">&STDERR"); open(STDOUT, ">foo.out") || die "Can't redirect stdout"; open(STDERR, ">&STDOUT") || die "Can't dup stdout"; select(STDERR); $| = 1; # make unbuffered select(STDOUT); $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too close(STDOUT); close(STDERR); open(STDOUT, ">&SAVEOUT"); open(STDERR, ">&SAVEERR"); print STDOUT "stdout 2\n"; print STDERR "stderr 2\n";

If you specify <&=N", where N is a number, then Perl will do an equivalent of C's fdopen() of that file descriptor. For example:

open(FILEHANDLE, "<&=$fd")

If you open a pipe on the command "-", i.e. either "|-" or "-|", then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are more or less equivalent:

open(FOO, "|tr '[a-z]' '[A-Z]'"); open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, "cat -n '$file'|"); open(FOO, "-|") || exec 'cat', '-n', $file;

Explicitly closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $?. Note: on any operation which may do a fork, unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output.

The filename that is passed to open will have leading and trailing whitespace deleted. In order to open a file with arbitrary weird characters in it, it's necessary to protect any leading and trailing whitespace thusly:

$file =~ s#^(\s)#./$1#; open(FOO, "< $file\0");

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