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Grabbing value from command output and monitoring for changes

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Top Forums Shell Programming and Scripting Grabbing value from command output and monitoring for changes
# 8  
Old 08-23-2010
Quote:
Originally Posted by monty77
..only thing that's not perfect now is that it (understandably) will always write a row to the log when you first fire it up. Any way to avoid this?
Cool -- glad you found something that works for you. Yes, change the if a bit to skip if last_value is 'x' or whatever you initialised it to. Something like:

Code:
 if [[ $last_value != "x"  &&   $last_value != $new_value ]]
   then
       echo "$(date) $new_value" >>$log_file
   fi
last_value=$new_value

You will also always have to capture the current value in last_value, so move that line following the fi. There are other ways, but I think this is pretty straight forward.

Cheers!
Last edited by agama; 08-24-2010 at 12:44 AM.. Reason: corrected 2nd typo in if statement
This User Gave Thanks to agama For This Post:
monty77
# 9  
Old 08-23-2010
Quote:
Originally Posted by agama
Cool -- glad you found something that works for you. Yes, change the if a bit to skip if last_value is 'x' or whatever you initialised it to. Something like:

Code:
 if [[$last_value != "x" ||  $last_value != $new_value ]]
   then
       echo "$(date) $new_value" >>$log_file
   fi
last_value=$new_value

You will also always have to capture the current value in last_value, so move that line following the fi. There are other ways, but I think this is pretty straight forward.

Cheers!
Can't seem to get that last change to work, I get this:

./adam.sh[8]: [[x: not found
./adam.sh[8]: x: not found

Any ideas?
Thanks!
Monty
# 10  
Old 08-23-2010
Oops, typo.

The [[ needs to have a trailing space.

Code:
if [[ $last_value != "x" ||  $last_value != $new_value ]]

Sorry bout that.
This User Gave Thanks to agama For This Post:
monty77
# 11  
Old 08-23-2010
Quote:
Originally Posted by agama
Try something like this:

Code:
last_value=x
log_file=/tmp/log.file
while true
do
   your-command-string | read new_value
   if [[ $last_value != $new_value ]]
   then
       echo "$(date) $new_value" >>$log_file
       last_value=$new_value
   fi
   sleep 300      # every 5 minutes
done

This assumes bash -- most modern UNIXes link /usr/bin/sh to bash -- the [[ expression on the if are much more efficient. If AIX doesn't link to bash, and/or ksh isn't available, you'll have to use if [...].
Quote:
Originally Posted by agama
Code:
echo "psc -i 10050 -s RELOAD_SERVICE" | tmadmin | grep -vE 'Name|------' | tail -1| read new_value;

If there is more output following the value you want, but it is always line seven, then try this:
Code:
echo "psc -i 10050 -s RELOAD_SERVICE" | tmadmin | grep -vE 'Name|------' | awk 'NR==7 {print}' | read new_value;

Have a go with one of these and see if that works.
Those suggestions will often fail, since some very popular shells (bash included) will run the read command at the end of the pipeline in a subshell. The value assigned to new_value will never be visible in the script's environment, since it will only ever exist in the subshell's environment (which is destroyed as soon as the read is completed).

Example using bash:
Code:
$ x=; echo hello | read x; echo $x

$ x=; echo hello | ( read x; echo SUBSHELL: $x ); echo CURRENT: $x
SUBSHELL: hello
CURRENT:

Regards,
Alister
# 12  
Old 08-24-2010
Quote:
Originally Posted by alister
Those suggestions will often fail, since some very popular shells (bash included) will run the read command at the end of the pipeline in a subshell. The value assigned to new_value will never be visible in the script's environment, since it will only ever exist in the subshell's environment (which is destroyed as soon as the read is completed).

Example using bash:
Code:
$ x=; echo hello | read x; echo $x

$ x=; echo hello | ( read x; echo $x ) 
hello

Regards,
Alister
Right about bash, however Ksh does the right thing and this will work in ksh.

Code:
$ cat t19
#!/usr/bin/env ksh
x= 
echo hello | read x 
echo $x

$ t19
hello

This User Gave Thanks to agama For This Post:
monty77
# 13  
Old 08-24-2010
Quote:
Originally Posted by agama
Oops, typo.

The [[ needs to have a trailing space.

Code:
if [[ $last_value != "x" ||  $last_value != $new_value ]]

Sorry bout that.
Thanks for the correction .. still doesn't work however, it just writes to the log on startup and after the sleep. Confused, looks like it should work to me.

Cheers,
Monty
# 14  
Old 08-24-2010
Bugger! I need to call it a night and go to bed.

Test should be AND rather than OR.

Code:
if [[ $last_value != "x"  &&  $last_value != $new_value ]]

This User Gave Thanks to agama For This Post:
monty77
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