Strip output

[brononi]

Hey,

I'm trying to read a certain value from a power meter. The end result should be that i can read it with snmp from a monitoring machine.

When i now do following command on the power meter:

cat /dev/ttyS0

I recieve following output:

pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000801
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000801
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000801
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pls 285afa070a3fd4a5f2a69f55e0b8007c:0006413740
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000798
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000802
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000803
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pwr 285afa070a3fd4a5f2a69f55e0b8007c:0000000800
pwr 1151f07babce795d244a11e389ae0a36:0000000000
pls 285afa070a3fd4a5f2a69f55e0b8007c:0006413741
.......

My problem is that i only need to have the pwr values behind 7C. So in this list, this would be:

801
801
801
798
802
803
800

So i should have a script that returns only the lines with pwr 285afa070a3fd4a5f2a69f55e0b8007c, and that i only recieve the part behind the :


Anybody can help me out with this?

[Irithori]

Try this:

cat /dev/ttyS0  | sed -n 's/^pwr 285afa070a3fd4a5f2a69f55e0b8007c:\(0*\)\(.*\)/\2/p'

[brononi]

Great,

It's working!!!
I see nicely the output now...

Is there a way to stop it once he has read 1 value?
Because he keeps going on till i hit ctrl-c...

[atreyu]

maybe there's a way to do it in sed, but off the top of my head, i'd say pipe it to, well, to head, e.g.

cat /dev/ttyS0 | sed .... | head -n1

Using head here will just show the first n (in this case, 1) lines of output.

[brononi]

this isn't working: cat /dev/ttyS0 | sed .... | head -n1

But this is: cat /dev/ttyS0 | head -n1 | sed ....

In the second way, i'm not 100% sure that it's correct. It waits a while before it gives the result. But at first sight, it's correct. Or is it just luck that it sends back the correct value? :$

ps any reason why this is working in case B, but not in case A?

[atreyu]

If I were you, i'd put that 2nd command in an infinite loop (or finite, depending on what you want to do).

while :; do
  val=$(cat /dev/ttyS0 | head -n1 | sed ....)
  if [ -n "$val" ]; then
    echo "doing something with $val ... "
  else
    echo "nothing to do..."
  fi
  sleep 1
done