How do I compare float / real numbers in a shell script?

If you simply strip off the "0.", then "0.984" will appear to be a smaller integer than "0.6566", but if you compare the originals as strings (dictionary order), then the result would be correct. Unfortunately, neither of these approaches works when you have different numbers of digits before and after the decimal point. Then you'd have to "normalize" by adding leading or trailing zeroes depending on whether you want to use string comparison or integer comparison.

(sound of me slapping my head)

Doh! You're absolutely right, Paul. I'll dig into this and come up with a different solution, just for completion. :-)

This question touches on a limitation of the Bash shell--namely, that it does not understand floating point arithmetic and treats such numbers as strings.

Bash documentation even goes on to say this:

"When not to use shell scripts
...
* Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead)"

So the work-arounds are indirect and perhaps (for some) counter-intuitive. One simple way to fix your code in place is to pipe your input (presumably shell variables) to bc(1), an arbitrary precision calculator language.

An example follows below:

if [ $max -lt $i ]

becomes:

if [ $(echo "$max < $i"|bc) -eq 1 ]

Where you're using a temporary subshell invocation to calculate and return a value for the test brackets. The comparison against 1 ("-eq 1") is necessary.

This is the 'quickest' and smallest solution I can see to your problem, but I'm sure there are (better?) alternative methods. Hope that helps.

Hello. This might make a good interview question for a toolsmith SysAdmin. Here's a script I wrote in answer. It includes flt and fgt functions, that use sort -n, as well as a test harness. Seems to work okay on this LINUX box:

#!/bin/sh
#
# "Floating Point" less-than or greater-than shell functions.
#
# Written as "proof-of-concept" answer for
# http://www.askdavetaylor.com/compare_float_real_numbers_in_a_shell_script.html
#
# Copyright 2007 Danny Howard, anyone can publish or reuse this code
# with attribution.
#
# Danny Howard
# http://dannyman.toldme.com/

function fgt () {
LHS=$1
RHS=$2

min=`(echo $LHS ; echo $RHS) | sort -n | head -1`
if [ "$min" = "$LHS" ]; then
return 1
else
return 0
fi
}

function flt () {
LHS=$1
RHS=$2

min=`(echo $LHS ; echo $RHS) | sort -n | head -1`
if [ "$min" = "$LHS" ]; then
return 0
else
return 1
fi
}
function true_or_false () {
FUNC=$1
ARG1=$2
ARG2=$3

echo -n "$FUNC $ARG1 $ARG2 ... "
$FUNC $ARG1 $ARG2

if [ $? -eq 0 ]; then
echo TRUE
else
echo FALSE
fi
}

true_or_false fgt 1.2 3.4
true_or_false flt 1.2 3.4
true_or_false fgt 0.9997 0.9996

When I run it:

> ./fgt.sh
fgt 1.2 3.4 ... FALSE
flt 1.2 3.4 ... TRUE
fgt 0.9997 0.9996 ... TRUE

Dave perhaps you can re-insert the PRE tags. ;)

Cheers,
-danny

The one posted by Justin K is really a quick way of doing that, it helped me. Thanks Justin.

use simply awk,like:
a=1.2
b=2.3
echo $a|awk '{ if ($1 < '$b') {print "a is bigger then b"}}'

a=1.2
b=2.3
echo $a|awk '{ if ($1 > '$b') {print "a is bigger then b"}}'

The korn shell ( open source from at&t research ) has had
floating point arithmetic since 1993. So the original test
would become:

if (( max < i )); then ... ; fi

On the other hand, I would not use 'i' for a floating point
variable, as single character variables like 'i' .. 'n' are by
convention used for integers in mathematics.

Plug: ksh93 gets shipped with some versions of unix and linux. Source and binaries can be found at at&t via kornshell.com.

Henk

sir,
i have question to ask...
how do we compare the decimal values with the integer values in shell programming?????

and

how to find the maximum values among the no.s????

please give the answer with one or two examples.

We can use tr+sort+head or tail to get high and low values

#To get Highest value
echo 100 500 300 400 101 204 499|tr ' ' '\n'|sort|head -n1

#To get Lowest value
echo 100 500 300 400 101 204 499|tr ' ' '\n'|sort|head -n1

#To see for yourself
echo 100 500 300 400 101 204 499|tr ' ' '\n'|sort

sorry for the above typo: please use this for Lowest value:

#To get Lowest value
echo 100 500 300 400 101 204 499|tr ' ' '\n'|sort|tail -n1

How do I make complex floating operation in a linux script. If I try "100 / 500 | bc" it returns 0, instead of 0.20.

Right, Shashank. You need to be a bit more tricky because the default setting for "bc" is to have zero digits after the decimal point. What you need to do is feed the statement "scale=2" or similar to get any precision at all. I explain it -- with lots of examples -- in my book Wicked Cool Shell Scripts, which you can learn about at http://www.intuitive.com/wicked/

Thanks Justin, that short cut across comparing floating point numbers really helped me... doing that was such a torture.... however just for info is there an alternative way of doing it also ... ?