open - open a file, pipe, or descriptor
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 Using open() for
IPC 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 Bidirectional Communication with Another Process 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 Using open() for
IPC 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 Bidirectional Communication 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 binmode in the perlfunc manpage 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 (<ARTICLE>) {...
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, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/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 Safe Pipe Opens 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 $^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: ", <HANDLE>;
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 seek in the perlfunc manpage for some details about mixing reading and writing.