Help with Shell

GavMan

Hi all,

I'm a bit of a shell noob trying to write a little script.

I have a piece of SQL that gets the average time difference between 2 MySQL timestamps in a table.

My Shell outputs the result to a file and then I read the result from the file into the a variable.

Then I'm effectively want to say, if the variable less than or equal to 2, then output a message to that effect. Else, send a warning email.

However, there seems to be some issue with Shell dealing with decimals. Anybody have a solution for me. Here is the portion of my if statement:

export p=`cat /usr/local/tomcat/AvgMsgSendTime.out.$dt.$1 | tail -1`
echo $p

if [ $p < 2.00 ]
then
echo "Average Message sending time is less than 2 seconds."
else
mail -s "Average Message sending time greater than 2 seconds" glumsden@saadian.com < "/usr/local/tomcat/AvgMsgSendTime.out."$dt.$1
echo "Average Message sending time greater than 2 seconds"
echo `cat /usr/local/tomcat/AvgMsgSendTime.out.$dt.$1`
fi

croo

yeah I don't think shell script handles floats very well.
one work around I've seen is to pipe the calc to the bc calculator.

Try...

export p=`cat /usr/local/tomcat/AvgMsgSendTime.out.$dt.$1 | tail -1`
echo $p
ans=`echo "$p < 2.00" | bc`

if [ $ans ]
then
echo "Average Message sending time is less than 2 seconds."
else
mail -s "Average Message sending time greater than 2 seconds" glumsden@saadian.com < "/usr/local/tomcat/AvgMsgSendTime.out."$dt.$1
echo "Average Message sending time greater than 2 seconds"
echo `cat /usr/local/tomcat/AvgMsgSendTime.out.$dt.$1`
fi

GavMan

Croo,

Can you explain whats happening here:

ans=`echo "$p < 2.00" | bc`

if [ $ans ]

from my limited knowledge thats just outputting the result of p < 2.00. Then its just saying
if [ans]
then do something

there seems to be no logic to that if statement

GavMan

Croo,

Can you explain whats happening here:

ans=`echo "$p < 2.00" | bc`

if [ $ans ]

from my limited knowledge thats just outputting the result of p < 2.00. Then its just saying
if [ans]
then do something

there seems to be no logic to that if statement

croo

writing shells scripts is often a question of how you use the considerable number of utilities that constitute the unix/liunux OS... just like you did with cat & tail -1 to extract the last time from the log file!

In this case we use echo & bc...
we use echo to create our calculation and then pipe that through to bc which does the calculation.

In this case the answer to, is x < y, is true or false ... 1 or 0 is how this is represented here; 1= true, 0=false.

you should read "[ $ans ] " as, if ans is true then.
ans will be either 1 or 0 and you could check that too, i.e. [ $ans = 1 ] or [ $ans = 0 ], but the shell will evaluate 1 as true and 0 or false so why bother.

I think you might have missed the | bc bit?

It does work doesn't it?

GavMan

Thanks croo.

As I say I'm a real noob with shell. I didn't get thats what ans was.

I do believe it works. We're testing it out a good bit but so far so good

croo

Hmmm... seems bash doesn't evaluate 1 & 0 as true & false which is a real surprise!

I found this little script to test evaluations and the results are very surprising - to me anyway. But it's years since I did any serious scripting and then it was under unix with the korn shell.

#!/bin/bash

#  Tip:
#  If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.

echo

echo "Testing \"0\""
if [ 0 ]      # zero
then
  echo "0 is true."
else          # Or else ...
  echo "0 is false."
fi            # 0 is true.

echo

echo "Testing \"1\""
if [ 1 ]      # one
then
  echo "1 is true."
else
  echo "1 is false."
fi            # 1 is true.

echo

echo "Testing \"-1\""
if [ -1 ]     # minus one
then
  echo "-1 is true."
else
  echo "-1 is false."
fi            # -1 is true.

echo

echo "Testing \"NULL\""
if [ ]        # NULL (empty condition)
then
  echo "NULL is true."
else
  echo "NULL is false."
fi            # NULL is false.

echo

echo "Testing \"xyz\""
if [ xyz ]    # string
then
  echo "Random string is true."
else
  echo "Random string is false."
fi            # Random string is true.

echo

echo "Testing \"\$xyz\""
if [ $xyz ]   # Tests if $xyz is null, but...
              # it's only an uninitialized variable.
then
  echo "Uninitialized variable is true."
else
  echo "Uninitialized variable is false."
fi            # Uninitialized variable is false.

echo

echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]            # More pedantically correct.
then
  echo "Uninitialized variable is true."
else
  echo "Uninitialized variable is false."
fi            # Uninitialized variable is false.

echo


xyz=          # Initialized, but set to null value.

echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
  echo "Null variable is true."
else
  echo "Null variable is false."
fi            # Null variable is false.


echo


# When is "false" true?

echo "Testing \"false\""
if [ "false" ]              #  It seems that "false" is just a string.
then
  echo "\"false\" is true." #+ and it tests true.
else
  echo "\"false\" is false."
fi            # "false" is true.

echo

echo "Testing \"\$false\""  # Again, uninitialized variable.
if [ "$false" ]
then
  echo "\"\$false\" is true."
else
  echo "\"\$false\" is false."
fi            # "$false" is false.
              # Now, we get the expected result.

#  What would happen if we tested the uninitialized variable "$true"?

echo

exit 0

So, anyway, it seems with bash we need to use [ $ans = 1 ]

By the way, I used the $ans to make it easier to read/understand but you can drop it and use the the check directly

if [ 1 = `echo "$p < 2.00" | bc` ]

GavMan

Thanks croo.

I noticed it wasn't quite right later in the day when we looked at our result files from our testing.

if [ 1 = `echo "$p < 2.00" | bc` ]

The above code works a treat so far

GavMan

Thanks croo.

I noticed it wasn't quite right later in the day when we looked at our result files from our testing.

if [ 1 = `echo "$p < 2.00" | bc` ]

The above code works a treat so far