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I want to compare two floating point numbers in a shell script. The following code is not working:

#!/bin/bash   
min=12.45
val=10.35    
if (( $val < $min )) ; then    
  min=$val
fi
echo $min 
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6 Answers 6

up vote 3 down vote accepted

You could check separately the integer and fractional parts:

#!/bin/bash
min=12.45
val=12.35    
if (( ${val%%.*} < ${min%%.*} || ( ${val%%.*} == ${min%%.*} && ${val##*.} < ${min##*.} ) )) ; then    
    min=$val
fi
echo $min

As fered says in the comments, it works only if both numbers have fractional parts and both fractional parts have the same number of digits. Here's a version that works for integer or fractional and any bash operator:

#!/bin/bash
shopt -s extglob
fcomp() {
    local oldIFS="$IFS" op=$2 x y digitx digity
    IFS='.' x=( ${1##+([0]|[-]|[+])}) y=( ${3##+([0]|[-]|[+])}) IFS="$oldIFS"
    while [[ "${x[1]}${y[1]}" =~ [^0] ]]; do
        digitx=${x[1]:0:1} digity=${y[1]:0:1}
        (( x[0] = x[0] * 10 + ${digitx:-0} , y[0] = y[0] * 10 + ${digity:-0} ))
        x[1]=${x[1]:1} y[1]=${y[1]:1} 
    done
    [[ ${1:0:1} == '-' ]] && (( x[0] *= -1 ))
    [[ ${3:0:1} == '-' ]] && (( y[0] *= -1 ))
    (( ${x:-0} $op ${y:-0} ))
}

for op in '==' '!=' '>' '<' '<=' '>='; do
    fcomp $1 $op $2 && echo "$1 $op $2"
done
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+1 because it works and is simple (moreso than mine), but it will fail if either number is an integer... and just a minor note: It doesn't need the double versions (##, %%) ... # and % are enough as there won't ever be more than one '.' –  Peter.O Nov 16 '11 at 18:38
2  
This can't be fixed without a lot of work (try comparing 0.5 and 0.06). You'd better use a tool that already understands the decimal notation. –  Gilles Nov 16 '11 at 23:47
    
Thanks Gilles, updated it to work more generally than the earlier version. –  ata Nov 17 '11 at 5:33
    
Note that it says that 1.00000000000000000000000001 is greater than 2. –  Stéphane Chazelas Jul 23 at 12:57
    
Stéphane is right. It's so because of bit limits in the number representation of bash. Of course, if you want more suffering you could use your own representation.... :) –  ata Jul 24 at 12:55

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.

Use awk or bc instead.

#!/bin/bash

min=12.45
val=10.35

if [ 1 -eq `echo "${val} < ${min}" | bc` ]
then  
    min=${val}
fi    

echo $min

If you intend to do a lot of math operations, it's probably better to rely on python or perl.

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You can use package num-utils for simple manipulations...

For more serious maths, see this link... It describes several options, eg.

  • R / Rscript (GNU R statistical computation and graphics system)
  • octave (mostly Matlab compatible)
  • bc (The GNU bc arbitrary precision calculator language)

An example of numprocess

echo "123.456" | numprocess /+33.267,%2.33777/
# 67.0395291239087  

A programs for dealing with numbers from the command line

The 'num-utils' are a set of programs for dealing with numbers from the
Unix command line. Much like the other Unix command line utilities like
grep, awk, sort, cut, etc. these utilities work on data from both
standard in and data from files.

Includes these programs:
 * numaverage: A program for calculating the average of numbers.
 * numbound: Finds the boundary numbers (min and max) of input.
 * numinterval: Shows the numeric intervals between each number in a sequence.
 * numnormalize: Normalizes a set of numbers between 0 and 1 by default.
 * numgrep: Like normal grep, but for sets of numbers.
 * numprocess: Do mathematical operations on numbers.
 * numsum: Add up all the numbers.
 * numrandom: Generate a random number from a given expression.
 * numrange: Generate a set of numbers in a range expression.
 * numround: Round each number according to its value.

Here is a bash hack...It adds leading 0's to the integer to make a string left-to-right comparison meaningful. This particular piece of code requires that both min and val actually have a decimal point and at least one decimal digit.

min=12.45
val=10.35

MIN=0; VAL=1 # named array indexes, for clarity
IFS=.; tmp=($min $val); unset IFS 
tmp=($(printf -- "%09d.%s\n" ${tmp[@]}))
[[ ${tmp[VAL]} < ${tmp[MIN]} ]] && min=$val
echo min=$min

output:

min=10.35
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For simple calculations on floating point numbers (+-*/ and comparisons), you can use awk.

min=$(echo 12.45 10.35 | awk '{if ($1 < $2) print $1; else print $2}')

Or, if you have ksh93 or zsh (not bash), you can use your shell's built-in arithmetic, which supports floating point numbers.

if ((min>val)); then ((val=min)); fi

For more advanced floating point calculations, look up bc. It actually works on arbitrary-precision fixpoint numbers.

To work on tables of numbers, look up R (example).

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Just use ksh (ksh93 precisely) or zsh, which both natively support floating point arithmetics:

$ cat test.ksh
#!/bin/ksh 
min=12.45
val=10.35    
if (( $val < $min )) ; then    
  min=$val
fi
echo $min
$ ./test.ksh
10.35

Edit: Sorry, I missed ksh93 was already suggested. Keeping my answer just to make clear the script posted in the opening question can be used with no change outside the shell switch.

Edit2: Note that ksh93 requires the variable content to be consistent with your locale, i.e. with a French locale, a comma instead of a dot must be used:

...
min=12,45
val=10,35
...

A more robust solution is to set the locale at the beginning of the script to make sure it will work regardless of the user's locale:

...
export LC_ALL=C
min=12.45
val=10.35
...
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Note that the above ksh93 script only works in locales where the decimal separator is . (so not in half the world where the decimal separator is ,). zsh doesn't have that issue. –  Stéphane Chazelas Jul 23 at 6:20
    
Indeed, answer edited to clarify that point. –  jlliagre Jul 23 at 12:13
    
Setting LC_NUMERIC won't work if the user has set LC_ALL, that also means that numbers will not be displayed (or input) in the user's preferred format. See unix.stackexchange.com/questions/87745/what-does-lc-all-c-do/… for a potentially better approach. –  Stéphane Chazelas Jul 23 at 12:20
    
@StéphaneChazelas fixed the LC_NUMERIC issue. Given the OP script syntax, I'm assuming his preferred separator is . anyway. –  jlliagre Jul 23 at 15:20
    
Yes, but it's the locale of the script user, not the locale of the script author that matters. As a script author, you should take localisation and its side effects into account. –  Stéphane Chazelas Jul 23 at 15:29
min=$(echo "${min}sa ${val}d la <a p" | dc)

That uses the dc calculator to store the value for $min in register a and duplicates the value of $val onto the top of its main execution stack. It then lists the contents of a onto the top of the stack, at which point it looks like:

${min} ${val} ${val}

The < pops the top two entries off of the stack and compares them. So the stack then looks like:

${val}

If the top entry was less than the second to top it pushes the contents of a onto the top, so the stack looks like:

${min} ${val}

Else it does nothing and the stack still looks like:

${val} 

Then it just prints the top stack entry.

So for your problem:

min=12.45
val=12.35
echo "${min}sa ${val}d la <a p" | dc

###OUTPUT

12.35

But:

min=12.45
val=12.55
echo "${min}sa ${val}d la <a p" | dc

###OUTPUT

12.45
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