Bash Scripting and bc - bash

Question

I'm trying to write a bash script and I needed to do some floating point math. Basically I want to do something like this:
NUM=$(echo "scale=25;$1/10" | bc)
if [ $? -ne 0 ]
then
echo bad
fi
The problem I'm running into is $? tends to hold the output from the echo program and not the bc call. Is there a way I save the output from the bc program into a variable?
EDIT:
Thanks for the quick replies. Here's another way of looking at the problem. Say I modified the script a little bit so it looks like this:
#!/bin/bash
NUM=$(echo "scale=25;$1/10" | bc)
if [ $? -ne 0 ]
then
echo bad
exit
fi
echo "$NUM"
When the user inputs a normal floating point value, it works fine:
bash script.sh 1.0
output:
.1000000000000000000000000
However, when the user enters an incorrect value, the script can't recover:
bash script.sh 1.0a
output:
(standard_in) 1: parse error
What I'm trying to do is get it to exit gracefully.

Answer 1

I don't see anything wrong. $NUM is supposed to hold your bc command results
see:
NUM=$(echo "scale=25;$1/10" | bc)
echo "\$? is $?"
echo "NUM is $NUM"
output
$ ./shell.sh 10
$? is 0
NUM is 1.0000000000000000000000000
another way is to use awk
NUM=$(awk -vinput="$1" 'BEGIN{printf "%.25f", input/10 }')
echo "\$? is $?"
echo "NUM is $NUM"
The other way, is to do the check of "$1" before you pass to bc. eg
shopt -s extglob
input="$1"
case "$input" in
+([0-9.]))
IFS="."; set -- $input
if [ $# -ne 2 ];then
echo "bad decimal"
else
NUM=$(echo "scale=25;$1/10" | bc )
echo "$NUM"
fi
esac
you don't have to check for $? from bc anymore

Answer 2

For GNU bc, an error similar to "(standard_in) 1: syntax error" will be output on stderr. You can capture this in your variable and check for it.
#!/bin/bash
NUM=$(echo "scale=25;$1/10" | bc 2>&1)
if [[ $NUM =~ error || $? -ne 0 ]]
then
echo bad
exit
fi
echo "$NUM"

Answer 3

Are you after the result of calculation from bc (which you store in NUM) or the status return from the system call?
As I said you have the result of calculation in $NUM:
#bctest.sh
NUM=$(echo "scale=25;$1/10" | bc)
if [ $? -ne 0 ]
then
echo bad
fi
echo "result: ", $NUM
Test:
bash ./bctest.sh 15
result: , 1.5000000000000000000000000