(This excerpt is from the perlfunc manpage, part of the standard set of
documentation included with every valid Perl distribution--like the one on
your own system. See also http://www.*-*-*.com/
or http://www.*-*-*.com/
if your negligent system adminstrator has been remiss in his duties.)
NAME
open - open a file, pipe, or descriptor
SYNOPSIS
open FILEHANDLE,EXPR
open FILEHANDLE
DESCRIPTION
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. (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 the filename begins with `'<'' or nothing, the file is opened
for input. If the filename begins with `'>'', the file is truncated
and opened for output, being created if necessary. If the filename
begins with `'>>'', 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.
The prefix and the filename may be separated with spaces. These
various prefixes correspond to the fopen(3) modes of `'r'', `'r+'',
`'w'', `'w+'', `'a'', and `'a+''.
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 section on "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 the section on
"Bidirectional Communication" in the perlipc manpage for
alternatives.)
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
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: $!";
open(ARTICLE, "caesar <$article |") # decrypt article
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
process($file, 'fh00');
}
sub process {
$input++; # this is a string increment
unless (open($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.) 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")
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', $file;
See the section on "Safe Pipe Opens" in the perlipc manpage for
more examples of this.
NOTE: On any operation that may do a fork, any unflushed buffers
remain unflushed in both processes, which means you may need to set
`$|' to avoid duplicate output. 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 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: $!";
However, to open a file with arbitrary weird characters in it, it's
necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
If you want a "real" C `open' (see the open(2) manpage on your
system), then you should use the `sysopen' function, which involves
no such magic. 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 the "seek" entry in the perlfunc manpage for some details about
mixing reading and writing.
--
To err is human; to really{*filter*}things up requires the root password