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       open FILEHANDLE,MODE,LIST

       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.  (This is considered a
               symbolic reference, so `use strict 'refs'' should
               not be in effect.)

               If EXPR is omitted, the scalar variable of the
               same name as the FILEHANDLE contains the filename.
               (Note that lexical variables--those declared with
               `my'--will not work for this purpose; so if you're
               using `my', specify EXPR in your call to open.)
               See the perlopentut manpage for a kinder, gentler
               explanation of opening files.

               If MODE is `'<'' or nothing, the file is opened
               for input.  If MODE is `'>'', the file is
               truncated and opened for output, being created if
               necessary.  If MODE is `'>>'', the file is opened
               for appending, again being created if necessary.
               You can put a `'+'' in front of the `'>'' or `'<''
               to indicate that you want both read and write
               access to the file; thus `'+<'' is almost always
               preferred for read/write updates--the `'+>'' mode
               would clobber the file first.  You can't usually
               use either read-write mode for updating textfiles,
               since they have variable length records.  See the
               -i switch in the perlrun manpage for a better
               approach.  The file is created with permissions of
               `0666' modified by the process' `umask' value.

               These various prefixes correspond to the fopen(3)
               modes of `'r'', `'r+'', `'w'', `'w+'', `'a'', and
               `'a+''.

               In the 2-arguments (and 1-argument) form of the
               call the mode and filename should be concatenated
               (in this order), possibly separated by spaces.  It
               is possible to omit the mode if the mode is `'<''.

               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 a command which pipes
               output to us.  See the Using open() for IPC entry
               in the perlipc manpage for more examples of this.
               (You are not allowed to `open' to a command that
               pipes both in and out, but see the IPC::Open2
               manpage, the IPC::Open3 manpage, and the
               Bidirectional Communication with Another Process
               entry in the perlipc manpage for alternatives.)

               If MODE is `'|-'', the filename is interpreted as
               a command to which output is to be piped, and if
               MODE is `'-|'', the filename is interpreted as a
               command which pipes output to us.  In the
               2-arguments (and 1-argument) form one should
               replace dash (`'-'') with the command.  See the
               Using open() for IPC entry in the perlipc manpage
               for more examples of this.  (You are not allowed
               to `open' to a command that pipes both in and out,
               but see the IPC::Open2 manpage, the IPC::Open3
               manpage, and the Bidirectional Communication entry
               in the perlipc manpage for alternatives.)

               In the 2-arguments (and 1-argument) form opening
               `'-'' opens STDIN and opening `'>-'' opens STDOUT.

               Open returns nonzero upon success, the undefined
               value otherwise.  If the `open' involved a pipe,
               the return value happens to be the pid of the
               subprocess.

               If you're unfortunate enough to be running Perl on
               a system that distinguishes between text files and
               binary files (modern operating systems don't
               care), then you should check out the binmode entry
               elsewhere in this document for tips for dealing
               with this.  The key distinction between systems
               that need `binmode' and those that don't is their
               text file formats.  Systems like Unix, MacOS, and
               Plan9, which delimit lines with a single
               character, and which encode that character in C as
               `"\n"', do not need `binmode'.  The rest need it.

               When opening a file, it's usually a bad idea to
               continue normal execution if the request failed,
               so `open' is frequently used in connection with
               `die'.  Even if `die' won't do what you want (say,
               in a CGI script, where you want to make a nicely
               formatted error message (but there are modules
               that can help with that problem)) you should
               always check the return value from opening a file.
               The infrequent exception is when working with an
               unopened filehandle is actually what you want to
               do.

               Examples:

                   $ARTICLE = 100;
                   open ARTICLE or die "Can't find article $ARTICLE: $!\n";
                   while (
) {... open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) # if the open fails, output is discarded open(DBASE, '+<', 'dbase.mine') # open for update or die "Can't open 'dbase.mine' for update: $!"; open(DBASE, '+/tmp/Tmp$$") # $$ is our process id or die "Can't start sort: $!"; # process argument list of files along with any includes foreach $file (@ARGV) { process($file, 'fh00'); } sub process { my($filename, $input) = @_; $input++; # this is a string increment unless (open($input, $filename)) { print STDERR "Can't open $filename: $!\n"; return; } local $_; while (<$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) to be duped and opened. You may use `&' after `>', `>>', `<', `+>', `+>>', and `+<'. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of stdio buffers.) Duping file handles is not yet supported for 3-argument open(). Here is a script that saves, redirects, and restores STDOUT and STDERR: #!/usr/bin/perl open(OLDOUT, ">&STDOUT"); open(OLDERR, ">&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); $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too close(STDOUT); close(STDERR); open(STDOUT, ">&OLDOUT"); open(STDERR, ">&OLDERR"); print STDOUT "stdout 2\n"; print STDERR "stderr 2\n"; If you specify `'<&=N'', where `N' is a number, then Perl will do an equivalent of C's `fdopen' of that file descriptor; this is more parsimonious of file descriptors. For example: open(FILEHANDLE, "<&=$fd") Note that this feature depends on the fdopen() C library function. On many UNIX systems, fdopen() is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider rebuilding Perl to use the `sfio' library. If you open a pipe on the command `'-'', i.e., either `'|-'' or `'-|'' with 2-arguments (or 1-argument) form of open(), 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($pid)' to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following triples are more or less equivalent: open(FOO, "|tr '[a-z]' '[A-Z]'"); open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, "cat -n '$file'|"); open(FOO, '-|', "cat -n '$file'"); open(FOO, '-|') || exec 'cat', '-n', $file; See the Safe Pipe Opens entry in the perlipc manpage for more examples of this. Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see the perlport manpage). To be safe, you may need to set `$|' ($AUTOFLUSH in English) or call the `autoflush()' method of `IO::Handle' on any open handles. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See the section on "$^F" in the perlvar manpage. Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in `$?'. The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of "rsh cat file |", or you could change certain filenames as needed: $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; open(FH, $filename) or die "Can't open $filename: $!"; Use 3-argument form to open a file with arbitrary weird characters in it, open(FOO, '<', $file); otherwise it's necessary to protect any leading and trailing whitespace: $file =~ s#^(\s)#./$1#; open(FOO, "< $file\0"); (this may not work on some bizzare filesystems). One should conscientiously choose between the the magic and 3-arguments form of open(): open IN, $ARGV[0]; will allow the user to specify an argument of the form `"rsh cat file |"', but will not work on a filename which happens to have a trailing space, while open IN, '<', $ARGV[0]; will have exactly the opposite restrictions. If you want a "real" C `open' (see open(2) on your system), then you should use the `sysopen' function, which involves no such magic (but may use subtly different filemodes than Perl open(), which is mapped to C fopen()). This is another way to protect your filenames from interpretation. For example: use IO::Handle; sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) or die "sysopen $path: $!"; $oldfh = select(HANDLE); $| = 1; select($oldfh); print HANDLE "stuff $$\n"); seek(HANDLE, 0, 0); print "File contains: ", ; Using the constructor from the `IO::Handle' package (or one of its subclasses, such as `IO::File' or `IO::Socket'), you can generate anonymous filehandles that have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope: use IO::File; #... sub read_myfile_munged { my $ALL = shift; my $handle = new IO::File; open($handle, "myfile") or die "myfile: $!"; $first = <$handle> or return (); # Automatically closed here. mung $first or die "mung failed"; # Or here. return $first, <$handle> if $ALL; # Or here. $first; # Or here. } See the seek entry elsewhere in this document for some details about mixing reading and writing.


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