Bash Scripting and bc

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.

ghostdog74 · Accepted Answer · 2010-02-25 08:31:22Z

12

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

edited Feb 25, 2010 at 8:31

answered Feb 25, 2010 at 5:59

ghostdog74

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4 Comments

Jonathan Leffler Over a year ago

Remember that the successful echo of 'NUM is $NUM' will have overwritten the value in $? from the '$(echo ... | bc)' command.

LandonSchropp Over a year ago

I clarified my post above. Thanks for the tip on awk. If I can't get bc to work, I'll have to check it out.

ghostdog74 Over a year ago

another way, is to sanitize your input $1 to be a decimal or number before passing to bc. see my edit.

LandonSchropp Over a year ago

That's a cleaver check, but is there any way to make sure there's only one '.' character present? Thanks.

Dennis Williamson · Accepted Answer · 2010-02-25 07:46:35Z

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"

answered Feb 25, 2010 at 7:46

Dennis Williamson

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Comments

stefanB · Accepted Answer · 2010-02-25 06:01:54Z

1

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

answered Feb 25, 2010 at 6:01

stefanB

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1 Comment

LandonSchropp Over a year ago

I was trying to get the status return from the system call. Sorry, I should have been more explicit.

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