PERLFAQ5

NAME

DESCRIPTION

How do I flush/unbuffer an output filehandle? Why must I do this?

The C standard I/O library (stdio) normally buffers characters sent to devices so that there isn't a system call for each byte. In most stdio implementations, the type of output buffering and the size of the buffer varies according to the type of device. Perl's "print()" and "write()" functions normally buffer output, while "syswrite()" bypasses buffering all together.

If you want your output to be sent immediately when you execute "print()" or "write()" (for instance, for some network protocols), you must set the handle's autoflush flag. This flag is the Perl variable $| and when it is set to a true value, Perl will flush the handle's buffer after each "print()" or "write()". Setting $| affects buffering only for the currently selected default filehandle. You choose this handle with the one argument "select()" call (see ``$|'' in perlvar and ``select'' in perlfunc).

Use "select()" to choose the desired handle, then set its per-filehandle variables.

        $old_fh = select(OUTPUT_HANDLE);
        $| = 1;
        select($old_fh);

Some modules offer object-oriented access to handles and their variables, although they may be overkill if this is the only thing you do with them. You can use "IO::Handle":

        use IO::Handle;
        open my( $printer ), ">", "/dev/printer");   # but is this?
        $printer->autoflush(1);

or "IO::Socket" (which inherits from "IO::Handle"):

        use IO::Socket;           # this one is kinda a pipe?
        my $sock = IO::Socket::INET->new( 'www.example.com:80' );

        $sock->autoflush();

You can also flush an "IO::Handle" object without setting "autoflush". Call the "flush" method to flush the buffer yourself:

        use IO::Handle;
        open my( $printer ), ">", "/dev/printer"); 
        $printer->flush; # one time flush

How do I change, delete, or insert a line in a file, or append to the beginning of a file?

The basic idea of inserting, changing, or deleting a line from a text file involves reading and printing the file to the point you want to make the change, making the change, then reading and printing the rest of the file. Perl doesn't provide random access to lines (especially since the record input separator, $/, is mutable), although modules such as "Tie::File" can fake it.

A Perl program to do these tasks takes the basic form of opening a file, printing its lines, then closing the file:

        open my $in,  '<',  $file      or die "Can't read old file: $!";
        open my $out, '>', "$file.new" or die "Can't write new file: $!";

        while( <$in> )
                {
                print $out $_;
                }

   close $out;

Within that basic form, add the parts that you need to insert, change, or delete lines.

To prepend lines to the beginning, print those lines before you enter the loop that prints the existing lines.

        open my $in,  '<',  $file      or die "Can't read old file: $!";
        open my $out, '>', "$file.new" or die "Can't write new file: $!";

        print "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

        while( <$in> )
                {
                print $out $_;
                }

   close $out;

To change existing lines, insert the code to modify the lines inside the "while" loop. In this case, the code finds all lowercased versions of ``perl'' and uppercases them. The happens for every line, so be sure that you're supposed to do that on every line!

        open my $in,  '<',  $file      or die "Can't read old file: $!";
        open my $out, '>', "$file.new" or die "Can't write new file: $!";

        print "# Add this line to the top\n";

        while( <$in> )
                {
                s/\b(perl)\b/Perl/g;
                print $out $_;
                }

   close $out;

To change only a particular line, the input line number, $., is useful. First read and print the lines up to the one you want to change. Next, read the single line you want to change, change it, and print it. After that, read the rest of the lines and print those:

        while( <$in> )   # print the lines before the change
                {
                print $out $_;
                last if $. == 4; # line number before change
                }

        my $line = <$in>;
        $line =~ s/\b(perl)\b/Perl/g;
        print $out $line;

        while( <$in> )   # print the rest of the lines
                {
                print $out $_;
                }

To skip lines, use the looping controls. The "next" in this example skips comment lines, and the "last" stops all processing once it encounters either "__END__" or "__DATA__".

        while( <$in> )
                {
                next if /^\s+#/;             # skip comment lines
                last if /^__(END|DATA)__$/;  # stop at end of code marker
                print $out $_;
                }

Do the same sort of thing to delete a particular line by using "next" to skip the lines you don't want to show up in the output. This example skips every fifth line:

        while( <$in> )
                {
                next unless $. % 5;
                print $out $_;
                }

If, for some odd reason, you really want to see the whole file at once rather than processing line by line, you can slurp it in (as long as you can fit the whole thing in memory!):

        open my $in,  '<',  $file      or die "Can't read old file: $!"
        open my $out, '>', "$file.new" or die "Can't write new file: $!";

        my @lines = do { local $/; <$in> }; # slurp!

                # do your magic here

        print $out @lines;

Modules such as "File::Slurp" and "Tie::File" can help with that too. If you can, however, avoid reading the entire file at once. Perl won't give that memory back to the operating system until the process finishes.

You can also use Perl one-liners to modify a file in-place. The following changes all 'Fred' to 'Barney' in inFile.txt, overwriting the file with the new contents. With the "-p" switch, Perl wraps a "while" loop around the code you specify with "-e", and "-i" turns on in-place editing. The current line is in $_. With "-p", Perl automatically prints the value of $_ at the end of the loop. See perlrun for more details.

        perl -pi -e 's/Fred/Barney/' inFile.txt

To make a backup of "inFile.txt", give "-i" a file extension to add:

        perl -pi.bak -e 's/Fred/Barney/' inFile.txt

To change only the fifth line, you can add a test checking $., the input line number, then only perform the operation when the test passes:

        perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

To add lines before a certain line, you can add a line (or lines!) before Perl prints $_:

        perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

You can even add a line to the beginning of a file, since the current line prints at the end of the loop:

        perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

To insert a line after one already in the file, use the "-n" switch. It's just like "-p" except that it doesn't print $_ at the end of the loop, so you have to do that yourself. In this case, print $_ first, then print the line that you want to add.

        perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt

To delete lines, only print the ones that you want.

        perl -ni -e 'print unless /d/' inFile.txt

                ... or ...

        perl -pi -e 'next unless /d/' inFile.txt

How do I count the number of lines in a file?

        $lines = 0;
        open(FILE, $filename) or die "Can't open `$filename': $!";
        while (sysread FILE, $buffer, 4096) {
                $lines += ($buffer =~ tr/\n//);
                }
        close FILE;

This assumes no funny games with newline translations.  

How can I use Perl's -i option from within a program?

        # ...
        {
        local($^I, @ARGV) = ('.orig', glob("*.c"));
        while (<>) {
                if ($. == 1) {
                        print "This line should appear at the top of each file\n";
                }
                s/\b(p)earl\b/${1}erl/i;        # Correct typos, preserving case
                print;
                close ARGV if eof;              # Reset $.
                }
        }
        # $^I and @ARGV return to their old values here

This block modifies all the ".c" files in the current directory, leaving a backup of the original data from each file in a new ".c.orig" file.  

How can I copy a file?

Use the File::Copy module. It comes with Perl and can do a true copy across file systems, and it does its magic in a portable fashion.

        use File::Copy;

        copy( $original, $new_copy ) or die "Copy failed: $!";

If you can't use File::Copy, you'll have to do the work yourself: open the original file, open the destination file, then print to the destination file as you read the original.  

How do I make a temporary file name?

        open my $tmp, '+>', undef or die $!;

Otherwise, you can use the File::Temp module.

        use File::Temp qw/ tempfile tempdir /;

        $dir = tempdir( CLEANUP => 1 );
        ($fh, $filename) = tempfile( DIR => $dir );

        # or if you don't need to know the filename

        $fh = tempfile( DIR => $dir );

The File::Temp has been a standard module since Perl 5.6.1. If you don't have a modern enough Perl installed, use the "new_tmpfile" class method from the IO::File module to get a filehandle opened for reading and writing. Use it if you don't need to know the file's name:

        use IO::File;
        $fh = IO::File->new_tmpfile()
        or die "Unable to make new temporary file: $!";

If you're committed to creating a temporary file by hand, use the process ID and/or the current time-value. If you need to have many temporary files in one process, use a counter:

        BEGIN {
        use Fcntl;
        my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
        my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;

        sub temp_file {
                local *FH;
                my $count = 0;
                until( defined(fileno(FH)) || $count++ > 100 ) {
                        $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                        # O_EXCL is required for security reasons.
                        sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT;
                        }

                if( defined fileno(FH) ) {
                        return (*FH, $base_name);
                        }
                else {
                        return ();
                        }
                }
                
        }

How can I manipulate fixed-record-length files?

Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case from the output of a normal, Berkeley-style ps:

        # sample input line:
        #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
        my $PS_T = 'A6 A4 A7 A5 A*';
        open my $ps, '-|', 'ps';
        print scalar <$ps>;
        my @fields = qw( pid tt stat time command );
        while (<$ps>) {
                my %process;
                @process{@fields} = unpack($PS_T, $_);
        for my $field ( @fields ) {
                print "$field: <$process{$field}>\n";
        }
        print 'line=', pack($PS_T, @process{@fields} ), "\n";
        }

We've used a hash slice in order to easily handle the fields of each row. Storing the keys in an array means it's easy to operate on them as a group or loop over them with for. It also avoids polluting the program with global variables and using symbolic references.  

How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?

        open my    $fh, $file_name;

        open local $fh, $file_name;

        print $fh "Hello World!\n";

        process_file( $fh );

If you like, you can store these filehandles in an array or a hash. If you access them directly, they aren't simple scalars and you need to give "print" a little help by placing the filehandle reference in braces. Perl can only figure it out on its own when the filehandle reference is a simple scalar.

        my @fhs = ( $fh1, $fh2, $fh3 );

        for( $i = 0; $i <= $#fhs; $i++ ) {
                print {$fhs[$i]} "just another Perl answer, \n";
                }

Before perl5.6, you had to deal with various typeglob idioms which you may see in older code.

        open FILE, "> $filename";
        process_typeglob(   *FILE );
        process_reference( \*FILE );

        sub process_typeglob  { local *FH = shift; print FH  "Typeglob!" }
        sub process_reference { local $fh = shift; print $fh "Reference!" }

If you want to create many anonymous handles, you should check out the Symbol or IO::Handle modules.  

How can I use a filehandle indirectly?

        $fh =   SOME_FH;       # bareword is strict-subs hostile
        $fh =  "SOME_FH";      # strict-refs hostile; same package only
        $fh =  *SOME_FH;       # typeglob
        $fh = \*SOME_FH;       # ref to typeglob (bless-able)
        $fh =  *SOME_FH{IO};   # blessed IO::Handle from *SOME_FH typeglob

Or, you can use the "new" method from one of the IO::* modules to create an anonymous filehandle, store that in a scalar variable, and use it as though it were a normal filehandle.

        use IO::Handle;                     # 5.004 or higher
        $fh = IO::Handle->new();

Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar variable that contains a filehandle. Functions like "print", "open", "seek", or the "<FH>" diamond operator will accept either a named filehandle or a scalar variable containing one:

        ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
        print $ofh "Type it: ";
        $got = <$ifh>
        print $efh "What was that: $got";

If you're passing a filehandle to a function, you can write the function in two ways:

        sub accept_fh {
                my $fh = shift;
                print $fh "Sending to indirect filehandle\n";
        }

Or it can localize a typeglob and use the filehandle directly:

        sub accept_fh {
                local *FH = shift;
                print  FH "Sending to localized filehandle\n";
        }

Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.)

        accept_fh(*STDOUT);
        accept_fh($handle);

In the examples above, we assigned the filehandle to a scalar variable before using it. That is because only simple scalar variables, not expressions or subscripts of hashes or arrays, can be used with built-ins like "print", "printf", or the diamond operator. Using something other than a simple scalar variable as a filehandle is illegal and won't even compile:

        @fd = (*STDIN, *STDOUT, *STDERR);
        print $fd[1] "Type it: ";                           # WRONG
        $got = <$fd[0]>                                     # WRONG
        print $fd[2] "What was that: $got";                 # WRONG

With "print" and "printf", you get around this by using a block and an expression where you would place the filehandle:

        print  { $fd[1] } "funny stuff\n";
        printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
        # Pity the poor deadbeef.

That block is a proper block like any other, so you can put more complicated code there. This sends the message out to one of two places:

        $ok = -x "/bin/cat";
        print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
        print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";

This approach of treating "print" and "printf" like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you can use the built-in function named "readline" to read a record just as "<>" does. Given the initialization shown above for @fd, this would work, but only because readline() requires a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet.

        $got = readline($fd[0]);

Let it be noted that the flakiness of indirect filehandles is not related to whether they're strings, typeglobs, objects, or anything else. It's the syntax of the fundamental operators. Playing the object game doesn't help you at all here.  

How can I set up a footer format to be used with write()?

How can I write() into a string?

How can I open a filehandle to a string? , , ,

Since Perl 5.8.0, you can pass a reference to a scalar instead of the filename to create a file handle which you can used to read from or write to a string:

        open(my $fh, '>', \$string) or die "Could not open string for writing";
        print $fh "foo\n";
        print $fh "bar\n";      # $string now contains "foo\nbar\n"

        open(my $fh, '<', \$string) or die "Could not open string for reading";
        my $x = <$fh>;  # $x now contains "foo\n"

With older versions of Perl, the "IO::String" module provides similar functionality.  

How can I output my numbers with commas added?

You can use Number::Format to separate places in a number. It handles locale information for those of you who want to insert full stops instead (or anything else that they want to use, really).

This subroutine will add commas to your number:

        sub commify {
                local $_  = shift;
                1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
                return $_;
                }

This regex from Benjamin Goldberg will add commas to numbers:

        s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;

It is easier to see with comments:

        s/(
                ^[-+]?             # beginning of number.
                \d+?               # first digits before first comma
                (?=                # followed by, (but not included in the match) :
                        (?>(?:\d{3})+) # some positive multiple of three digits.
                        (?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
                )
                |                  # or:
                \G\d{3}            # after the last group, get three digits
                (?=\d)             # but they have to have more digits after them.
        )/$1,/xg;

How can I translate tildes (~) in a filename?

Within Perl, you may use this directly:

        $filename =~ s{
          ^ ~             # find a leading tilde
          (               # save this in $1
              [^/]        # a non-slash character
                    *     # repeated 0 or more times (0 means me)
          )
        }{
          $1
              ? (getpwnam($1))[7]
              : ( $ENV{HOME} || $ENV{LOGDIR} )
        }ex;

How come when I open a file read-write it wipes it out?

        open(FH, "+> /path/name");              # WRONG (almost always)

Whoops. You should instead use this, which will fail if the file doesn't exist.

        open(FH, "+< /path/name");      # open for update

Using ``>'' always clobbers or creates. Using ``<'' never does either. The ``+'' doesn't change this.

Here are examples of many kinds of file opens. Those using sysopen() all assume

        use Fcntl;

To open file for reading:

        open(FH, "< $path")                                 || die $!;
        sysopen(FH, $path, O_RDONLY)                        || die $!;

To open file for writing, create new file if needed or else truncate old file:

        open(FH, "> $path") || die $!;
        sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        || die $!;
        sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666)  || die $!;

To open file for writing, create new file, file must not exist:

        sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         || die $!;
        sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666)   || die $!;

To open file for appending, create if necessary:

        open(FH, ">> $path") || die $!;
        sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       || die $!;
        sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;

To open file for appending, file must exist:

        sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

To open file for update, file must exist:

        open(FH, "+< $path")                                || die $!;
        sysopen(FH, $path, O_RDWR)                          || die $!;

To open file for update, create file if necessary:

        sysopen(FH, $path, O_RDWR|O_CREAT)                  || die $!;
        sysopen(FH, $path, O_RDWR|O_CREAT, 0666)            || die $!;

To open file for update, file must not exist:

        sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           || die $!;
        sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666)     || die $!;

To open a file without blocking, creating if necessary:

        sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
            or die "can't open /foo/somefile: $!":

Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS. That is, two processes might both successfully create or unlink the same file! Therefore O_EXCL isn't as exclusive as you might wish.

See also the new perlopentut if you have it (new for 5.6).  

Why do I sometimes get an Argument list too long when I use <*>?

To get around this, either upgrade to Perl v5.6.0 or later, do the glob yourself with readdir() and patterns, or use a module like File::KGlob, one that doesn't use the shell to do globbing.  

Is there a leak/bug in glob()?

How can I open a file with a leading > or trailing blanks?

The special two argument form of Perl's open() function ignores trailing blanks in filenames and infers the mode from certain leading characters (or a trailing ``|''). In older versions of Perl this was the only version of open() and so it is prevalent in old code and books.

Unless you have a particular reason to use the two argument form you should use the three argument form of open() which does not treat any characters in the filename as special.

        open FILE, "<", "  file  ";  # filename is "   file   "
        open FILE, ">", ">file";     # filename is ">file"

How can I reliably rename a file?

        rename($old, $new) or system("mv", $old, $new);

It may be more portable to use the File::Copy module instead. You just copy to the new file to the new name (checking return values), then delete the old one. This isn't really the same semantically as a rename(), which preserves meta-information like permissions, timestamps, inode info, etc.

Newer versions of File::Copy export a move() function.  

How can I lock a file?

1.

Produces a fatal error if none of the three system calls (or their close equivalent) exists.

2.

lockf(3) does not provide shared locking, and requires that the filehandle be open for writing (or appending, or read/writing).

3.

Some versions of flock() can't lock files over a network (e.g. on NFS file systems), so you'd need to force the use of fcntl(2) when you build Perl. But even this is dubious at best. See the flock entry of perlfunc and the INSTALL file in the source distribution for information on building Perl to do this.

Two potentially non-obvious but traditional flock semantics are that it waits indefinitely until the lock is granted, and that its locks are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with flock() may be modified by programs that do not also use flock(). Cars that stop for red lights get on well with each other, but not with cars that don't stop for red lights. See the perlport manpage, your port's specific documentation, or your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (If you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called ``features''). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)

For more information on file locking, see also ``File Locking'' in perlopentut if you have it (new for 5.6).

Why can't I just open(FH, >file.lock)?

        sleep(3) while -e "file.lock";  # PLEASE DO NOT USE
        open(LCK, "> file.lock");               # THIS BROKEN CODE

This is a classic race condition: you take two steps to do something which must be done in one. That's why computer hardware provides an atomic test-and-set instruction. In theory, this ``ought'' to work:

        sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
                or die "can't open  file.lock: $!";

except that lamentably, file creation (and deletion) is not atomic over NFS, so this won't work (at least, not every time) over the net. Various schemes involving link() have been suggested, but these tend to involve busy-wait, which is also less than desirable.  

I still don't get locking. I just want to increment the number in the file. How can I do this?

Anyway, this is what you can do if you can't help yourself.

        use Fcntl qw(:DEFAULT :flock);
        sysopen(FH, "numfile", O_RDWR|O_CREAT)   or die "can't open numfile: $!";
        flock(FH, LOCK_EX)                               or die "can't flock numfile: $!";
        $num = <FH> || 0;
        seek(FH, 0, 0)                           or die "can't rewind numfile: $!";
        truncate(FH, 0)                                  or die "can't truncate numfile: $!";
        (print FH $num+1, "\n")                  or die "can't write numfile: $!";
        close FH                                         or die "can't close numfile: $!";

Here's a much better web-page hit counter:

        $hits = int( (time() - 850_000_000) / rand(1_000) );

If the count doesn't impress your friends, then the code might. :-)  

All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?

If you know you are only going to use a system that does correctly implement appending (i.e. not Win32) then you can omit the seek() from the code in the previous answer.

If you know you are only writing code to run on an OS and filesystem that does implement append mode correctly (a local filesystem on a modern Unix for example), and you keep the file in block-buffered mode and you write less than one buffer-full of output between each manual flushing of the buffer then each bufferload is almost guaranteed to be written to the end of the file in one chunk without getting intermingled with anyone else's output. You can also use the syswrite() function which is simply a wrapper around your systems write(2) system call.

There is still a small theoretical chance that a signal will interrupt the system level write() operation before completion. There is also a possibility that some STDIO implementations may call multiple system level write()s even if the buffer was empty to start. There may be some systems where this probability is reduced to zero.  

How do I randomly update a binary file?

        perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

However, if you have fixed sized records, then you might do something more like this:

        $RECSIZE = 220; # size of record, in bytes
        $recno   = 37;  # which record to update
        open(FH, "+<somewhere") || die "can't update somewhere: $!";
        seek(FH, $recno * $RECSIZE, 0);
        read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
        # munge the record
        seek(FH, -$RECSIZE, 1);
        print FH $record;
        close FH;

Locking and error checking are left as an exercise for the reader. Don't forget them or you'll be quite sorry.  

How do I get a file's timestamp in perl?

Here's an example:

        $write_secs = (stat($file))[9];
        printf "file %s updated at %s\n", $file,
        scalar localtime($write_secs);

If you prefer something more legible, use the File::stat module (part of the standard distribution in version 5.004 and later):

        # error checking left as an exercise for reader.
        use File::stat;
        use Time::localtime;
        $date_string = ctime(stat($file)->mtime);
        print "file $file updated at $date_string\n";

The POSIX::strftime() approach has the benefit of being, in theory, independent of the current locale. See perllocale for details.  

How do I set a file's timestamp in perl?

        if (@ARGV < 2) {
                die "usage: cptimes timestamp_file other_files ...\n";
                }
        $timestamp = shift;
        ($atime, $mtime) = (stat($timestamp))[8,9];
        utime $atime, $mtime, @ARGV;

Error checking is, as usual, left as an exercise for the reader.

The perldoc for utime also has an example that has the same effect as touch(1) on files that already exist.

Certain file systems have a limited ability to store the times on a file at the expected level of precision. For example, the FAT and HPFS filesystem are unable to create dates on files with a finer granularity than two seconds. This is a limitation of the filesystems, not of utime().  

How do I print to more than one file at once?

If you only have to do this once, you can print individually to each filehandle.

        for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }

How can I read in an entire file all at once?

        use File::Slurp;

        $all_of_it = read_file($filename); # entire file in scalar
        @all_lines = read_file($filename); # one line perl element

The customary Perl approach for processing all the lines in a file is to do so one line at a time:

        open (INPUT, $file)     || die "can't open $file: $!";
        while (<INPUT>) {
                chomp;
                # do something with $_
                }
        close(INPUT)            || die "can't close $file: $!";

This is tremendously more efficient than reading the entire file into memory as an array of lines and then processing it one element at a time, which is often---if not almost always---the wrong approach. Whenever you see someone do this:

        @lines = <INPUT>;

you should think long and hard about why you need everything loaded at once. It's just not a scalable solution. You might also find it more fun to use the standard Tie::File module, or the DB_File module's $DB_RECNO bindings, which allow you to tie an array to a file so that accessing an element the array actually accesses the corresponding line in the file.

You can read the entire filehandle contents into a scalar.

        {
        local(*INPUT, $/);
        open (INPUT, $file)     || die "can't open $file: $!";
        $var = <INPUT>;
        }

That temporarily undefs your record separator, and will automatically close the file at block exit. If the file is already open, just use this:

        $var = do { local $/; <INPUT> };

For ordinary files you can also use the read function.

        read( INPUT, $var, -s INPUT );

The third argument tests the byte size of the data on the INPUT filehandle and reads that many bytes into the buffer $var.  

How can I read in a file by paragraphs?

Note that a blank line must have no blanks in it. Thus "fred\n \nstuff\n\n" is one paragraph, but "fred\n\nstuff\n\n" is two.  

How can I read a single character from a file? From the keyboard?

If your system supports the portable operating system programming interface (POSIX), you can use the following code, which you'll note turns off echo processing as well.

        #!/usr/bin/perl -w
        use strict;
        $| = 1;
        for (1..4) {
                my $got;
                print "gimme: ";
                $got = getone();
                print "--> $got\n";
                }
    exit;

        BEGIN {
        use POSIX qw(:termios_h);

        my ($term, $oterm, $echo, $noecho, $fd_stdin);

        $fd_stdin = fileno(STDIN);

        $term     = POSIX::Termios->new();
        $term->getattr($fd_stdin);
        $oterm     = $term->getlflag();

        $echo     = ECHO | ECHOK | ICANON;
        $noecho   = $oterm & ~$echo;

        sub cbreak {
                $term->setlflag($noecho);
                $term->setcc(VTIME, 1);
                $term->setattr($fd_stdin, TCSANOW);
                }

        sub cooked {
                $term->setlflag($oterm);
                $term->setcc(VTIME, 0);
                $term->setattr($fd_stdin, TCSANOW);
                }

        sub getone {
                my $key = '';
                cbreak();
                sysread(STDIN, $key, 1);
                cooked();
                return $key;
                }

        }

        END { cooked() }

The Term::ReadKey module from CPAN may be easier to use. Recent versions include also support for non-portable systems as well.

        use Term::ReadKey;
        open(TTY, "</dev/tty");
        print "Gimme a char: ";
        ReadMode "raw";
        $key = ReadKey 0, *TTY;
        ReadMode "normal";
        printf "\nYou said %s, char number %03d\n",
                $key, ord $key;

How can I tell whether there's a character waiting on a filehandle?

You should also check out the Frequently Asked Questions list in comp.unix.* for things like this: the answer is essentially the same. It's very system dependent. Here's one solution that works on BSD systems:

        sub key_ready {
                my($rin, $nfd);
                vec($rin, fileno(STDIN), 1) = 1;
                return $nfd = select($rin,undef,undef,0);
                }

If you want to find out how many characters are waiting, there's also the FIONREAD ioctl call to be looked at. The h2ph tool that comes with Perl tries to convert C include files to Perl code, which can be "require"d. FIONREAD ends up defined as a function in the sys/ioctl.ph file:

        require 'sys/ioctl.ph';

        $size = pack("L", 0);
        ioctl(FH, FIONREAD(), $size)    or die "Couldn't call ioctl: $!\n";
        $size = unpack("L", $size);

If h2ph wasn't installed or doesn't work for you, you can grep the include files by hand:

        % grep FIONREAD /usr/include/*/*
        /usr/include/asm/ioctls.h:#define FIONREAD      0x541B

Or write a small C program using the editor of champions:

        % cat > fionread.c
        #include <sys/ioctl.h>
        main() {
            printf("%#08x\n", FIONREAD);
        }
        ^D
        % cc -o fionread fionread.c
        % ./fionread
        0x4004667f

And then hard code it, leaving porting as an exercise to your successor.

        $FIONREAD = 0x4004667f;         # XXX: opsys dependent

        $size = pack("L", 0);
        ioctl(FH, $FIONREAD, $size)     or die "Couldn't call ioctl: $!\n";
        $size = unpack("L", $size);

FIONREAD requires a filehandle connected to a stream, meaning that sockets, pipes, and tty devices work, but not files.  

How do I do a tail -f in perl?

        seek(GWFILE, 0, 1);

The statement "seek(GWFILE, 0, 1)" doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next "<GWFILE>" makes Perl try again to read something.

If that doesn't work (it relies on features of your stdio implementation), then you need something more like this:

        for (;;) {
          for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
            # search for some stuff and put it into files
          }
          # sleep for a while
          seek(GWFILE, $curpos, 0);  # seek to where we had been
        }

If this still doesn't work, look into the "clearerr" method from "IO::Handle", which resets the error and end-of-file states on the handle.

There's also a "File::Tail" module from CPAN.  

How do I dup() a filehandle in Perl?

        open(LOG, ">>/foo/logfile");
        open(STDERR, ">&LOG");

Or even with a literal numeric descriptor:

   $fd = $ENV{MHCONTEXTFD};
   open(MHCONTEXT, "<&=$fd");   # like fdopen(3S)

Note that ``<&STDIN'' makes a copy, but ``<&=STDIN'' make an alias. That means if you close an aliased handle, all aliases become inaccessible. This is not true with a copied one.

Error checking, as always, has been left as an exercise for the reader.  

How do I close a file descriptor by number?

        use POSIX ();
        
        POSIX::close( $fd );

This should rarely be necessary, as the Perl "close()" function is to be used for things that Perl opened itself, even if it was a dup of a numeric descriptor as with "MHCONTEXT" above. But if you really have to, you may be able to do this:

        require 'sys/syscall.ph';
        $rc = syscall(&SYS_close, $fd + 0);  # must force numeric
        die "can't sysclose $fd: $!" unless $rc == -1;

Or, just use the fdopen(3S) feature of "open()":

        {
        open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
        close $fh;
        }

Why can't I use C:\temp\foo in DOS paths? Why doesn't `C:\temp\foo.exe` work?

Either single-quote your strings, or (preferably) use forward slashes. Since all DOS and Windows versions since something like MS-DOS 2.0 or so have treated "/" and "\" the same in a path, you might as well use the one that doesn't clash with Perl---or the POSIX shell, ANSI C and C++, awk, Tcl, Java, or Python, just to mention a few. POSIX paths are more portable, too.  

Why doesn't glob(*.*) get all the files?

Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?

The executive summary: learn how your filesystem works. The permissions on a file say what can happen to the data in that file. The permissions on a directory say what can happen to the list of files in that directory. If you delete a file, you're removing its name from the directory (so the operation depends on the permissions of the directory, not of the file). If you try to write to the file, the permissions of the file govern whether you're allowed to.  

How do I select a random line from a file?

        srand;
        rand($.) < 1 && ($line = $_) while <>;

This has a significant advantage in space over reading the whole file in. You can find a proof of this method in The Art of Computer Programming, Volume 2, Section 3.4.2, by Donald E. Knuth.

You can use the File::Random module which provides a function for that algorithm:

        use File::Random qw/random_line/;
        my $line = random_line($filename);

Another way is to use the Tie::File module, which treats the entire file as an array. Simply access a random array element.  

Why do I get weird spaces when I print an array of lines?

        print "@lines\n";

joins together the elements of @lines with a space between them. If @lines were "("little", "fluffy", "clouds")" then the above statement would print

        little fluffy clouds

but if each element of @lines was a line of text, ending a newline character "("little\n", "fluffy\n", "clouds\n")" then it would print:

        little
         fluffy
         clouds

If your array contains lines, just print them:

        print @lines;

REVISION

Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $

See perlfaq for source control details and availability.  

AUTHOR AND COPYRIGHT

This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.

Irrespective of its distribution, all code examples here are in the public domain. You are permitted and encouraged to use this code and any derivatives thereof in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit to the FAQ would be courteous but is not required.