perlfaq5 - Files and Formats

- NAME
- DESCRIPTION

- How do I flush/unbuffer an output filehandle? Why must I do this?
- How do I count the number of lines in a file?
- How can I use Perl's `-i` option from within a program?
- How can I copy a file?
- How do I make a temporary file name?
- How can I manipulate fixed-record-length files?
- 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?
- How can I use a filehandle indirectly?
- 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?
- How can I translate tildes (~) in a filename?
- How come when I open a file read-write it wipes it out?
- Why do I sometimes get an "Argument list too long" when I use <*>?
- Is there a leak/bug in glob()?
- How can I open a file with a leading ">" or trailing blanks?
- How can I reliably rename a file?
- How can I lock a file?
- Why can't I just open(FH, ">file.lock")?
- I still don't get locking. I just want to increment the number in the file. How can I do this?
- 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?
- How do I randomly update a binary file?
- How do I get a file's timestamp in perl?
- How do I set a file's timestamp in perl?
- How do I print to more than one file at once?
- How can I read in an entire file all at once?
- How can I read in a file by paragraphs?
- How can I read a single character from a file? From the keyboard?
- How can I tell whether there's a character waiting on a filehandle?
- How do I do a `tail -f` in perl?
- How do I `dup()` a filehandle in Perl?
- How do I close a file descriptor by number?
- Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
- 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?
- How do I select a random line from a file?
- Why do I get weird spaces when I print an array of lines?

- REVISION
- AUTHOR AND COPYRIGHT

# NAME

perlfaq5 - Files and Formats ($Revision: 10126 $)

# DESCRIPTION

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

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

Perl does not support truly unbuffered output (except insofar as you can `syswrite(OUT, $char, 1)`), although it does support is "command buffering", in which a physical write is performed after every output command.

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 the perlvar manpage and select in the perlfunc manpage).

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

    =head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
    X<file, editing>

(contributed by brian d foy)

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 C<next> in this example
    skips comment lines, and the C<last> stops all processing once it
    encounters either C<__END__> or C<__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 the perlrun manpage 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?

One fairly efficient way is to count newlines in the file. The following program uses a feature of tr///, as documented in the perlop manpage. 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;

This assumes no funny games with newline translations.

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

`-i` sets the value of Perl's `$^I` variable, which in turn affects the behavior of `<>`; see the perlrun manpage for more details. By modifying the appropriate variables directly, you can get the same behavior within a larger program. For example:

            # ...
            {
            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?

(contributed by brian d foy)

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?

If you don't need to know the name of the file, you can use `open()` with `undef` in place of the file name. The `open()` function creates an anonymous temporary file.

            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?

The most efficient way is using pack() and unpack(). This is faster than using substr() when taking many, many strings. It is slower for just a few.

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?

As of perl5.6, `open()` autovivifies file and directory handles as references if you pass it an uninitialized scalar variable. You can then pass these references just like any other scalar, and use them in the place of named handles.

            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?

An indirect filehandle is using something other than a symbol in a place that a filehandle is expected. Here are ways to get indirect filehandles:

            $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()?

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

## How can I `write()` into a string?

See Accessing Formatting Internals in the perlform manpage for an `swrite()` function.

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

(contributed by Peter J. Holzer, hjp-usenet2@hjp.at)

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.

    =head2 How can I output my numbers with commas added?
    X<number, commify>

(contributed by brian d foy and Benjamin Goldberg)

You can use the Number::Format manpage 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?

Use the <> (glob()) operator, documented in the perlfunc manpage. Older versions of Perl require that you have a shell installed that groks tildes. Recent perl versions have this feature built in. The File::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 a 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 (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 the perlopentut manpage if you have it (new for 5.6).

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

The `<>` operator performs a globbing operation (see above). In Perl versions earlier than v5.6.0, the internal `glob()` operator 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 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()?

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 memory 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?

(contributed by Brian McCauley)

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?

If your operating system supports a proper `mv(1)` utility or its functional equivalent, this works:

            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?

Perl's builtin `flock()` function (see the perlfunc manpage 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. But even this is dubious at best. See the flock entry of the perlfunc manpage 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 the perlopentut manpage if you have it (new for 5.6).

## Why 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)
                    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?

Didn't anyone ever tell you web-page hit counters were useless? They don't count number of hits, they're a waste of time, and they serve only to stroke the writer's vanity. It's better to pick a random number; they're more realistic.

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 are on a system that correctly implements `flock()` and you use the example appending code from "perldoc -f flock" everything will be OK even if the OS you are on doesn't implement append mode correctly (if such a system exists.) So if you are happy to restrict yourself to OSs that implement `flock()` (and that's not really much of a restriction) then that is what you should do.

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?

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: $!";
            # munge the record
            seek(FH, -$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?

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 -A, -M, or -C file test operations as documented in the perlfunc manpage. These retrieve the age of the file (measured against the start-time of your program) in days as a floating point number. Some platforms may not have all of these times. See the perlport manpage for details. 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];
            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 the perllocale manpage for details.

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

You use the `utime()` function documented in utime in the perlfunc manpage. 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, 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?

To connect one filehandle to several output filehandles, you can use the IO::Tee or Tie::FileHandle::Multiplex modules.

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?

You can use the File::Slurp module to do it in one step.

            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?

Use the `$/` variable (see the perlvar manpage 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.

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?

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 the perlfunc manpage.

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?

The very first thing you should do is look into getting the Term::ReadKey extension from CPAN. As we mentioned earlier, it now even has limited support for non-portable (read: not open systems, closed, proprietary, not POSIX, not Unix, etc) systems.

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?

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 `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?

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

            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?

If, for some reason, you have a file descriptor instead of a filehandle (perhaps you used `POSIX::open`), you can use the `close()` function from the `POSIX` module:

            use POSIX ();

            POSIX::close( $fd );

    This should rarely be necessary, as the Perl C<close()> function is to be
    used for things that Perl opened itself, even if it was a dup of a
    numeric descriptor as with C<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?

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 the perlop manpage. Unsurprisingly, you don't have a file called "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on your legacy 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. POSIX paths are more portable, too.

## 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. This makes `glob()` portable even to legacy systems. Your port may include proprietary globbing functions as well. Check its documentation for details.

## 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 file-dir-perms article in the "Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .

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. 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?

Saying

            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

Revision: $Revision: 10126 $

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

See the perlfaq manpage for source control details and availability.

# AUTHOR AND COPYRIGHT

Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved.

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.

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