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NAME ^

perlfaq5 - Files and Formats ($Revision: 1.22 $, $Date: 1997/04/24 22:44:02 $)

DESCRIPTION ^

This section deals with I/O and the "f" issues: filehandles, flushing, formats, and footers.

How do I flush/unbuffer a filehandle? Why must I do this?

The C standard I/O library (stdio) normally buffers characters sent to devices. This is done for efficiency reasons, so that there isn't a system call for each byte. Any time you use print() or write() in Perl, you go though this buffering. syswrite() circumvents stdio and buffering.

In most stdio implementations, the type of buffering and the size of the buffer varies according to the type of device. Disk files are block buffered, often with a buffer size of more than 2k. Pipes and sockets are often buffered with a buffer size between 1/2 and 2k. Serial devices (e.g. modems, terminals) are normally line-buffered, and stdio sends the entire line when it gets the newline.

Perl does not support truly unbuffered output (except insofar as you can syswrite(OUT, $char, 1)). What it does instead support is "command buffering", in which a physical write is performed after every output command. This isn't as hard on your system as unbuffering, but does get the output where you want it when you want it.

If you expect characters to get to your device when you print them there, you'll want to autoflush its handle, as in the older:

    use FileHandle;
    open(DEV, "<+/dev/tty");      # ceci n'est pas une pipe
    DEV->autoflush(1);

or the newer IO::* modules:

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

or even this:

    use IO::Socket;               # this one is kinda a pipe?
    $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
                                  PeerPort => 'http(80)',
                                  Proto    => 'tcp');
    die "$!" unless $sock;

    $sock->autoflush();
    $sock->print("GET /\015\012");
    $document = join('', $sock->getlines());
    print "DOC IS: $document\n";

Note the hardcoded carriage return and newline in their octal equivalents. This is the ONLY way (currently) to assure a proper flush on all platforms, including Macintosh.

You can use select() and the $| variable to control autoflushing (see perlvar/$ and "select" in perlfunc):

    $oldh = select(DEV);
    $| = 1;
    select($oldh);

You'll also see code that does this without a temporary variable, as in

    select((select(DEV), $| = 1)[0]);

How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?

Although humans have an easy time thinking of a text file as being a sequence of lines that operates much like a stack of playing cards -- or punch cards -- computers usually see the text file as a sequence of bytes. In general, there's no direct way for Perl to seek to a particular line of a file, insert text into a file, or remove text from a file.

(There are exceptions in special circumstances. Replacing a sequence of bytes with another sequence of the same length is one. Another is using the $DB_RECNO array bindings as documented in DB_File. Yet another is manipulating files with all lines the same length.)

The general solution is to create a temporary copy of the text file with the changes you want, then copy that over the original.

    $old = $file;
    $new = "$file.tmp.$$";
    $bak = "$file.bak";

    open(OLD, "< $old")         or die "can't open $old: $!";
    open(NEW, "> $new")         or die "can't open $new: $!";

    # Correct typos, preserving case
    while (<OLD>) {
        s/\b(p)earl\b/${1}erl/i;
        (print NEW $_)          or die "can't write to $new: $!";
    }

    close(OLD)                  or die "can't close $old: $!";
    close(NEW)                  or die "can't close $new: $!";

    rename($old, $bak)          or die "can't rename $old to $bak: $!";
    rename($new, $old)          or die "can't rename $new to $old: $!";

Perl can do this sort of thing for you automatically with the -i command-line switch or the closely-related $^I variable (see perlrun for more details). Note that -i may require a suffix on some non-Unix systems; see the platform-specific documentation that came with your port.

    # Renumber a series of tests from the command line
    perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t

    # form a script
    local($^I, @ARGV) = ('.bak', glob("*.c"));
    while (<>) {
        if ($. == 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 $.
    }

If you need to seek to an arbitrary line of a file that changes infrequently, you could build up an index of byte positions of where the line ends are in the file. If the file is large, an index of every tenth or hundredth line end would allow you to seek and read fairly efficiently. If the file is sorted, try the look.pl library (part of the standard perl distribution).

In the unique case of deleting lines at the end of a file, you can use tell() and truncate(). The following code snippet deletes the last line of a file without making a copy or reading the whole file into memory:

        open (FH, "+< $file");
        while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
        truncate(FH, $addr);

Error checking is left as an exercise for the reader.

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

One fairly efficient way is to count newlines in the file. The following program uses a feature of tr///, as documented in perlop. If your text file doesn't end with a newline, then it's not really a proper text file, so this may report one fewer line than you expect.

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

How do I make a temporary file name?

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 IO::File;
        use Fcntl;
        my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
        my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
        sub temp_file {
            my $fh = undef;
            my $count = 0;
            until (defined($fh) || $count > 100) {
                $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
                $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
            }
            if (defined($fh)) {
                return ($fh, $base_name);
            } else {
                return ();
            }
        }
    }

Or you could simply use IO::Handle::new_tmpfile.

How can I manipulate fixed-record-length files?

The most efficient way is using pack() and unpack(). This is faster than using substr(). 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
    $PS_T = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    $_ = <PS>; print;
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
        for $var (qw!pid tt stat time command!) {
            print "$var: <$$var>\n";
        }
        print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
                "\n";
    }

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?

You may have some success with typeglobs, as we always had to use in days of old:

    local(*FH);

But while still supported, that isn't the best to go about getting local filehandles. Typeglobs have their drawbacks. You may well want to use the FileHandle module, which creates new filehandles for you (see FileHandle):

    use FileHandle;
    sub findme {
        my $fh = FileHandle->new();
        open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
        while (<$fh>) {
            print if /\b127\.(0\.0\.)?1\b/;
        }
        # $fh automatically closes/disappears here
    }

Internally, Perl believes filehandles to be of class IO::Handle. You may use that module directly if you'd like (see IO::Handle), or one of its more specific derived classes.

Once you have IO::File or FileHandle objects, you can pass them between subroutines or store them in hashes as you would any other scalar values:

    use FileHandle;

    # Storing filehandles in a hash and array
    foreach $filename (@names) {
        my $fh = new FileHandle($filename)              or die;
        $file{$filename} = $fh;
        push(@files, $fh);
    }

    # Using the filehandles in the array
    foreach $file (@files) {
        print $file "Testing\n";
    }

    # You have to do the { } ugliness when you're specifying the
    # filehandle by anything other than a simple scalar variable.
    print { $files[2] } "Testing\n";

    # Passing filehandles to subroutines
    sub debug {
        my $filehandle = shift;
        printf $filehandle "DEBUG: ", @_;
    }

    debug($fh, "Testing\n");

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

There's no builtin way to do this, but perlform has a couple of techniques to make it possible for the intrepid hacker.

How can I write() into a string?

See perlform for an swrite() function.

How can I output my numbers with commas added?

This one will do it for you:

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

    $n = 23659019423.2331;
    print "GOT: ", commify($n), "\n";

    GOT: 23,659,019,423.2331

You can't just:

    s/^(-?\d+)(\d{3})/$1,$2/g;

because you have to put the comma in and then recalculate your position.

Alternatively, this commifies all numbers in a line regardless of whether they have decimal portions, are preceded by + or -, or whatever:

    # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
    sub commify {
       my $input = shift;
        $input = reverse $input;
        $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
        return reverse $input;
    }

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

Use the <> (glob()) operator, documented in perlfunc. This requires that you have a shell installed that groks tildes, meaning csh or tcsh or (some versions of) ksh, and thus may have portability problems. The Glob::KGlob module (available from CPAN) gives more portable glob functionality.

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 the file read-write it wipes it out?

Because you're using something like this, which truncates the file and then gives you read-write access:

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

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

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

If this is an issue, try:

    sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);

Error checking is left as an exercise for the reader.

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

The <> operator performs a globbing operation (see above). By default glob() forks csh(1) to do the actual glob expansion, but csh can't handle more than 127 items and so gives the error message Argument list too long. People who installed tcsh as csh won't have this problem, but their users may be surprised by it.

To get around this, either do the glob yourself with Dirhandles and patterns, or use a module like Glob::KGlob, one that doesn't use the shell to do globbing.

Is there a leak/bug in glob()?

Due to the current implementation on some operating systems, when you use the glob() function or its angle-bracket alias in a scalar context, you may cause a leak and/or unpredictable behavior. It's best therefore to use glob() only in list context.

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

Normally perl ignores trailing blanks in filenames, and interprets certain leading characters (or a trailing "|") to mean something special. To avoid this, you might want to use a routine like this. It makes incomplete pathnames into explicit relative ones, and tacks a trailing null byte on the name to make perl leave it alone:

    sub safe_filename {
        local $_  = shift;
        return m#^/#
                ? "$_\0"
                : "./$_\0";
    }

    $fn = safe_filename("<<<something really wicked   ");
    open(FH, "> $fn") or "couldn't open $fn: $!";

You could also use the sysopen() function (see "sysopen" in perlfunc).

How can I reliably rename a file?

Well, usually you just use Perl's rename() function. But that may not work everywhere, in particular, renaming files across file systems. If your operating system supports a mv(1) program or its moral equivalent, this works:

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

It may be more compelling 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 semantics as a real rename(), though, which preserves metainformation like permissions, timestamps, inode info, etc.

How can I lock a file?

Perl's builtin flock() function (see perlfunc for details) will call flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and later), and lockf(3) if neither of the two previous system calls exists. On some systems, it may even use a different form of native locking. Here are some gotchas with Perl's flock():

  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. See the flock entry of perlfunc, and the INSTALL file in the source distribution for information on building Perl to do this.

The CPAN module File::Lock offers similar functionality and (if you have dynamic loading) won't require you to rebuild perl if your flock() can't lock network files.

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

A common bit of code NOT TO USE is this:

    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, 0644)
                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 involving link() have been suggested, but these tend to involve busy-wait, which is also subdesirable.

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

Didn't anyone ever tell you web-page hit counters were useless?

Anyway, this is what to do:

    use Fcntl;
    sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
    flock(FH, 2)                                 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: $!";
    # DO NOT UNLOCK THIS UNTIL YOU CLOSE
    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. :-)

How do I randomly update a binary file?

If you're just trying to patch a binary, in many cases something as simple as this works:

    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, $recno * $RECSIZE, 0);
    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.

Don't forget to set binmode() under DOS-like platforms when operating on files that have anything other than straight text in them. See the docs on open() and on binmode() for more details.

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

If you want to retrieve the time at which the file was last read, written, or had its meta-data (owner, etc) changed, you use the -M, -A, or -C filetest operations as documented in perlfunc. These retrieve the age of the file (measured against the start-time of your program) in days as a floating point number. To retrieve the "raw" time in seconds since the epoch, you would call the stat function, then use localtime(), gmtime(), or POSIX::strftime() to convert this into human-readable form.

Here's an example:

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

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

    use File::stat;
    use Time::localtime;
    $date_string = ctime(stat($file)->mtime);
    print "file $file updated at $date_string\n";

Error checking is left as an exercise for the reader.

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

You use the utime() function documented in "utime" in perlfunc. By way of example, here's a little program that copies the read and write times from its first argument to all the rest of them.

    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 left as an exercise for the reader.

Note that utime() currently doesn't work correctly with Win95/NT ports. A bug has been reported. Check it carefully before using it on those platforms.

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

If you only have to do this once, you can do this:

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

To connect up to one filehandle to several output filehandles, it's easiest to use the tee(1) program if you have it, and let it take care of the multiplexing:

    open (FH, "| tee file1 file2 file3");

Otherwise you'll have to write your own multiplexing print function -- or your own tee program -- or use Tom Christiansen's, at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is written in Perl.

In theory a IO::Tee class could be written, but to date we haven't seen such.

How can I read in a file by paragraphs?

Use the $\ variable (see perlvar for details). You can either set it to "" to eliminate empty paragraphs ("abc\n\n\n\ndef", for instance, gets treated as two paragraphs and not three), or "\n\n" to accept empty paragraphs.

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

You can use the builtin getc() function for most filehandles, but it won't (easily) work on a terminal device. For STDIN, either use the Term::ReadKey module from CPAN, or use the sample code in "getc" in perlfunc.

If your system supports 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:

    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;

For DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:

To put the PC in "raw" mode, use ioctl with some magic numbers gleaned from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes across the net every so often):

    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
    $old_ioctl &= 0xff;
    ioctl(STDIN,1,$old_ioctl | 32);    # Writes it back, setting bit 5

Then to read a single character:

    sysread(STDIN,$c,1);               # Read a single character

And to put the PC back to "cooked" mode:

    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.

So now you have $c. If ord($c) == 0, you have a two byte code, which means you hit a special key. Read another byte with sysread(STDIN,$c,1), and that value tells you what combination it was according to this table:

    # PC 2-byte keycodes = ^@ + the following:

    # HEX     KEYS
    # ---     ----
    # 0F      SHF TAB
    # 10-19   ALT QWERTYUIOP
    # 1E-26   ALT ASDFGHJKL
    # 2C-32   ALT ZXCVBNM
    # 3B-44   F1-F10
    # 47-49   HOME,UP,PgUp
    # 4B      LEFT
    # 4D      RIGHT
    # 4F-53   END,DOWN,PgDn,Ins,Del
    # 54-5D   SHF F1-F10
    # 5E-67   CTR F1-F10
    # 68-71   ALT F1-F10
    # 73-77   CTR LEFT,RIGHT,END,PgDn,HOME
    # 78-83   ALT 1234567890-=
    # 84      CTR PgUp

This is all trial and error I did a long time ago, I hope I'm reading the file that worked.

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

You should 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);
    }

You should look into getting the Term::ReadKey extension from CPAN.

How do I open a file without blocking?

You need to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with sysopen():

    use Fcntl;
    sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
    or die "can't open /tmp/somefile: $!":

How do I create a file only if it doesn't exist?

You need to use the O_CREAT and O_EXCL flags from the Fcntl module in conjunction with sysopen():

    use Fcntl;
    sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
                or die "can't open /tmp/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 successful create or unlink the same file!

How do I do a tail -f in perl?

First try

    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 POSIX module. POSIX defines the clearerr() method, which can remove the end of file condition on a filehandle. The method: read until end of file, clearerr(), read some more. Lather, rinse, repeat.

How do I dup() a filehandle in Perl?

If you check "open" in perlfunc, you'll see that several of the ways to call open() should do the trick. For example:

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

Or even with a literal numeric descriptor:

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

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

How do I close a file descriptor by number?

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;

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

Whoops! You just put a tab and a formfeed into that filename! Remember that within double quoted strings ("like\this"), the backslash is an escape character. The full list of these is in "Quote and Quote-like Operators" in perlop. Unsurprisingly, you don't have a file called "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on your DOS filesystem.

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.

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

Because even on non-Unix ports, Perl's glob function follows standard Unix globbing semantics. You'll need glob("*") to get all (non-hidden) files.

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

This is elaborately and painstakingly described in the "Far More Than You Every Wanted To Know" in http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .

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?

Here's an algorithm from the Camel Book:

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

This has a significant advantage in space over reading the whole file in.

AUTHOR AND COPYRIGHT ^

Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights reserved. See perlfaq for distribution information.

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