34

I am trying to compare two decimal values but I am getting errors. I used 

if [ "$(echo $result1 '>' $result2 | bc -l)" -eq 1 ];then

as suggested by the other Stack Overflow thread.

I am getting errors.

What is the correct way to go about this?

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4
  • What are the errors? misspelled one variable name. Commented Jun 28, 2012 at 4:01
  • actually the errors are due to some other issues. This works fine. Commented Jun 28, 2012 at 4:21
  • 1
    What do you mean by 'decimal values'? Do you mean integers in base 10, or do you mean strings that represent non-integer real values? Commented Jun 28, 2012 at 14:16
  • 1
    @WilliamPursell: I'm assuming that since the OP is using bc especially with -l that it's floats that are being compared. The -l isn't needed for comparisons though. Commented Jun 28, 2012 at 14:31

8 Answers 8

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52

You can do it using Bash's numeric context:

if (( $(echo "$result1 > $result2" | bc -l) )); then

bc will output 0 or 1 and the (( )) will interpret them as false or true respectively.

The same thing using AWK:

if (( $(echo "$result1 $result2" | awk '{print ($1 > $2)}') )); then
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8 Comments

+1 for the awk solution, but: if echo $result1 $result2 | awk '{exit !( $1 > $2)}'; then ...
I like to retract my down vote as Dennis's reply is correct with decimal points. For expanded floating point arithmetic using bash, see my reply below.
@Samir: If you haven't already, you can click again on the down arrow to reverse the downvote.
@DennisWilliamson It says that my vote is locked and can not be edited unless someone edits this answer.
+1 @WilliamPursell but using awk "BEGIN {exit !(${result1} > ${result2})}" instead of piping echo into awk.
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11 
if awk 'BEGIN{exit ARGV[1]>ARGV[2]}' "$z" "$y"
then
  echo z not greater than y
else
  echo z greater than y
fi
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1 Comment

To those confused as I was with this, in AWK language, value of boolean statement is 1 if true and 0 if not, oppositely to UNIX shell. So in shell scripts that use above AWK program, one has to revert the result, for example use {exit !(ARGV[1]>ARGV[2])} to get shell exit code corresponding to comparison ARGV[1]>ARGV[2].
9 
if [[ `echo "$result1 $result2" | awk '{print ($1 > $2)}'` == 1 ]]; then
  echo "$result1 is greater than $result2"
fi
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Comments

5

Following up on Dennis's reply:

Although his reply is correct for decimal points, bash throws (standard_in) 1: syntax error with floating point arithmetic.

result1=12
result2=1.27554e-05


if (( $(echo "$result1 > $result2" | bc -l) )); then
    echo "r1 > r2"
else
    echo "r1 < r2"
fi

This returns incorrect output with a warning although with an exit code of 0.

(standard_in) 1: syntax error
r1 < r2

While there is no clear solution to this (discussion thread 1 and thread 2), I used following partial fix by rounding off floating point results using awk followed by use of bc command as in Dennis's reply and this thread

Round off to a desired decimal place: Following will get recursive directory space in TB with rounding off at the second decimal place.

result2=$(du -s "/home/foo/videos" | tail -n1 | awk '{$1=$1/(1024^3); printf "%.2f", $1;}')

You can then use bash arithmetic as above or using [[ ]] enclosure as in following thread.

if (( $(echo "$result1 > $result2" | bc -l) )); then
    echo "r1 > r2"
else
    echo "r1 < r2"
fi

or using -eq operator where bc output of 1 is true and 0 is false

if [[ $(bc <<< "$result1 < $result2") -eq 1 ]]; then
    echo "r1 < r2"
else
    echo "r1 > r2"
fi
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Comments

1

For shell script I couldn't use double brackets (()). So, what helped me was to split it in two rows and do the comparison in the classic way.

low_limit=4.2
value=3.9
        
result=$(echo "${value}<${low_limit}" | bc)
    
if [ $result = 1 ]; then
  echo too low; 
else 
  echo not too low; 
fi
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Comments

0

You can also echo an if...else statement to bc.

- echo $result1 '>' $result2
+ echo "if (${result1} > ${result2}) 1 else 0"

(
#export IFS=2  # example why quoting is important
result1="2.3" 
result2="1.7" 
if [ "$(echo $result1 '>' $result2 | bc -l)" -eq 1 ]; then echo yes; else echo no;fi
if [ "$(echo "if (${result1} > ${result2}) 1 else 0" | bc -l)" -eq 1 ];then echo yes; else echo no; fi
if echo $result1 $result2 | awk '{exit !( $1 > $2)}'; then echo yes; else echo no; fi
)
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Comments

-1

Can't bash force type conversion? For example:

($result1 + 0) < ($result2 + 0)
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1 Comment

() construct runs the enclosed text as a command, it doesn't do any arithmetic. Looks like you are confusing it with the ((...)) operator.
-2

Why use bc ?

for i in $(seq -3 0.5 4) ; do echo $i ; if [[ (( "$i" < 2 )) ]] ; then echo "... is < 2";fi; done

The only problem : the comparison "<" doesn't work with negative numbers : they are taken as their absolute value.

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1 Comment

< inside [[...]] is for string comparison (so 20 is less than 3, and 0.2 is less than 1e-50). The ('s are redundant, you can add as many as you want, they're only for grouping.