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comp.lang.ruby

Re: ruby tail

Kev Jackson

2/22/2007 9:49:00 AM

> 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
5 Answers

Alex Young

2/22/2007 10:00:00 AM

0

Kevin Jackson wrote:
>> 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
Use IO.popen:

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

--
Alex

benjohn

2/22/2007 10:05:00 AM

0

> Kevin Jackson wrote:
>>> 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
> 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


>
> --
> Alex
>
>

Alex Young

2/22/2007 10:13:00 AM

0

benjohn@fysh.org wrote:
>> Kevin Jackson wrote:
>>>> 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
>> 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?
>

I haven't ever done that, but it looks like it from the docs...

--
Alex

Robert Klemme

2/22/2007 10:16:00 AM

0

On 22.02.2007 10:59, Alex Young wrote:
> Kevin Jackson wrote:
>>> 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
> 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

Robert Klemme

2/22/2007 1:12:00 PM

0


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