From: "James F.Hranicky" <jfh@...>
Date: 2002-08-01T21:50:03+09:00
Subject: Re: A very basic tail -f implementation

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On Thu, 1 Aug 2002 21:23:09 +0900
"Berger, Daniel" <djberge@qwest.com> wrote:
 
> > 	- Check for rotation by checking for changes in the inode number
> 
> This is proving difficult in practice because, as the Pickaxe says, the
> File::Stat info is recorded at the moment the File::Stat object is created;
> changes made to the file after that point will not be reflected.  This
> applies to filehandles as well, as experimentation has shown.  Simply
> calling the class method won't work either because that requires the
> filename which, by that point, has already changed.
> 
> If I'm doing something wrong, I'd love to know how to make this work!

That's true, you just have to stat the actual filename everytime and
check against the current filehandle.

I wrote my own very simple tail -f thingy in Ruby a few months back, 
and was recently working on improving it when I noticed that Florian
released his version. Here's what I have so far. Anyone can use any
of it however they want.

----------------------------------------------------------------------
| Jim Hranicky, Senior SysAdmin                   UF/CISE Department |
| E314D CSE Building                            Phone (352) 392-1499 |
| jfh@cise.ufl.edu                      http://www.cise.ufl.edu/~jfh |
----------------------------------------------------------------------

"Given a choice between a complex, difficult-to-understand, disconcerting
 explanation and a simplistic, comforting one, many prefer simplistic
 comfort if it's remotely plausible, especially if it involves blaming
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                                                -- Bob Lewis, _Infoworld_

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#!/local/bin/ruby

class Tail

    TAIL_TOP = 1
    TAIL_END = 2
    TAIL_SEEK = 3


    def initialize(filename, seek, whence)

        @filename = filename

        begin
            @fileh = File.open(filename)
        rescue Errno::ENOENT
            retry
        end

        @ino   = @fileh.stat.ino
        @ino_changed   = false
        
        case whence 
            when TAIL_TOP
                raise "Must specify non-negative number for TAIL_TOP" unless seek >= 0
                wind(seek)
            when TAIL_END
                raise "Must specify non-positive number for TAIL_END" unless seek <= 0
                rewind(seek)
            when TAIL_SEEK
                raise "Invalid file position #{seek} for file #{@filename}" unless
                    seek >= 0 && seek <= @fileh.stat.size
                @fileh.seek(seek, IO::SEEK_SET)
            else
                raise "Invalid value for whence #{whence}"
        end

    end

    def wind(lines)

        @fileh.seek(0, IO::SEEK_SET)

        numlines = 0
        0.upto(@fileh.stat.size) { |filepos|
            @fileh.seek(filepos, IO::SEEK_SET)
            return if (numlines == lines)
            i = @fileh.getc
            c = sprintf("%c", i)
            numlines += 1 if (c == "\n")
        }

    end

    def rewind(lines)

        @fileh.seek(0, IO::SEEK_END)

        lines = lines.abs
        numlines = 0
        size = @fileh.stat.size - 1

        size.downto(0) { |filepos|
            next if (size == filepos)
            @fileh.seek(filepos, IO::SEEK_SET)
            i = @fileh.getc
            c = sprintf("%c", i)
            numlines += 1 if (c == "\n")
            return if (numlines == lines)
        }
    end

    def gets

        loop { 
            while (@fileh.eof?) 
    
                begin
                    if ((ino = File.stat(@filename).ino) != @ino)
                        @ino_changed = true
                    end
                rescue Errno::ENOENT
                    retry
                end
    
                if (@ino_changed)
                    @fileh.close
                    initialize(@filename, 0, TAIL_TOP)
                    @ino_changed = false
                    next
                end
    
                sleep .5
                @fileh.seek(0, IO::SEEK_CUR)
    
            end
    
            line = @fileh.gets
            if block_given? 
                yield line
            else
                return line
            end
        }
    end
  
end

def Main
    
    tail = Tail.new(ARGV.shift, -10, Tail::TAIL_END)

    tail.gets { |line|
        print line
    }

#    while (line = tail.gets) 
#        print line
#    end
end

Main()

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