Ruby Forum Ruby > Re: ruby tail

Actually, I am not sure how this would work.  If I do this, I get no 
output from the file:

`tail -f sim.out`

So there is something basic I am missing here.  Any help?

TIA,
Phy

----- Original Message ----
From: Kevin Jackson <foamdino@gmail.com>
To: ruby-talk ML <ruby-talk@ruby-lang.org>
Sent: Thursday, February 22, 2007 1:33:19 AM
Subject: Re: ruby tail

> Is there a gem or perhaps a method I am not aware of that does the similar function of tail?  Or is there a way to use tail in ruby?   I have a file that I want to process as new data is appended to the file.  This is easy enough using tail and tail has the added advantage that it can follow a file if it is "rolled".

Just call out to tail

def tail(f)
  `tail f`
end

or similar - if tail does what you want, use it

Kev

> Actually, I am not sure how this would work.  If I do this, I get no output from the file:
>
> `tail -f sim.out`
>
> So there is something basic I am missing here.  Any help?

it shells out to the process and waits for it to return - with -f does
tail ever return? if not you may not be able to use it - AFAIK ` and
exec are the same - someone with better ruby knowledge may be able to
offer a solution - perhaps using threads or something

Kev

Kevin Jackson wrote:
> offer a solution - perhaps using threads or something
Use IO.popen:

IO.popen('tail -f sim.out'){|f|
  while line = f.gets
    p line
  end
}

>> exec are the same - someone with better ruby knowledge may be able to
>> offer a solution - perhaps using threads or something
> Use IO.popen:
>
> IO.popen('tail -f sim.out'){|f|
>   while line = f.gets
>     p line
>   end
> }

Out of interest, the blocking call is "gets" here, right? Can you use
Kernel.select on a bunch of those handles?

Cheers,
  Benj

benjohn@fysh.org wrote:
>>> offer a solution - perhaps using threads or something
> 
I haven't ever done that, but it looks like it from the docs...

On 22.02.2007 10:59, Alex Young wrote:
>> exec are the same - someone with better ruby knowledge may be able to
>> offer a solution - perhaps using threads or something
> Use IO.popen:
> 
> IO.popen('tail -f sim.out'){|f|
>     while line = f.gets
>         p line
>     end
> }

You can do it in pure Ruby:

11:14:14 [Temp]: rm x; for ((i=0;i<30;++i)); do echo $i; sleep 0.2; done
 > x & ruby tail.rb
removed `x'
[1] 2252
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
[1]+  Done                    for ((i=0; i<30; ++i))
do
     echo $i; sleep 0.2;
done > x
tail.rb:12:in `sleep': Interrupt
         from tail.rb:12:in `tail'
         from tail.rb:5:in `loop'
         from tail.rb:5:in `tail'
         from tail.rb:16
11:14:31 [Temp]: cat tail.rb
# we never return!
def tail(file)
   io = File.open(file)

   loop do
     while ( line = io.gets )
       puts line
     end

     # uncomment next to watch what is happening
     # puts "-"
     sleep 1
   end
end

tail "x"
11:14:35 [Temp]:

Kind regards

  robert

This version is safer and a tad more flexible, just in case there is an
exception (e.g. because of thread interruption):

def tail(file, interval=1)
   raise "Illegal interval #{interval}" if interval < 0

   File.open(file) do |io|
     loop do
       while ( line = io.gets )
         puts line
       end

       # uncomment next to watch what is happening
       # puts "-"
       sleep interval
     end
   end
end

Cheers

  robert