Bash comparing float numbers using POSIX compliance

Question

my case is very simple:

I am trying to compare two values:

a=0.2
b=0.1

The code that I am trying to execute is:

if [ "$a" -gt "$b" ]; then
        echo "You are running an outdated version"
fi

If you target bash, you don't need to worry about standards compliance. [[ $a > $b ]] will work fine. Otherwise you should use the sh tag, not bash. — oguz ismail
– oguz ismail, Commented Oct 12, 2021 at 15:04
You can't reliably do semver comparison as floats, because something like 1.2.12 is not a valid float. see: stackoverflow.com/q/4023830/1032785. — jordanm
– jordanm, Commented Oct 12, 2021 at 15:12
@oguzismail that compares in lexical order, not in numerical — jordanm
– jordanm, Commented Oct 12, 2021 at 15:12
Bash only supports integer math. See mywiki.wooledge.org/BashFAQ/022 — Shawn
– Shawn, Commented Oct 12, 2021 at 15:13

tshiono · Accepted Answer · 2021-10-13 02:27:31Z

Assuming you want to compare version numbers, would you please try the following:

#!/bin/bash -posix

# compares version numbers
# prints 0               if $a == $b
#        positive number if $a is newer than $b
#        negative number if $a is older than $b
vercmp() {
    local a=$1
    local b=$2
    local a1=${a%%.*}           # major number of $a
    local b1=${b%%.*}           # major number of $b

    if [[ $a = "" ]]; then
        if [[ $b = "" ]]; then
            echo 0              # both $a and $b are empty
        else
            vercmp "0" "$b"
        fi
    elif [[ $b = "" ]]; then
        vercmp "$a" "0"
    elif (( 10#$a1 == 10#$b1 )); then
        local a2=${a#*.}        # numbers after the 1st dot
        if [[ $a2 = $a ]]; then
            a2=""               # no more version numbers
        fi
        local b2=${b#*.}        # numbers after the 1st dot
        if [[ $b2 = $b ]]; then
            b2=""               # no more version numbers
        fi
        vercmp "$a2" "$b2"
    else
        echo $(( 10#$a1 - 10#$b1 ))
    fi
}

Examples:

vercmp 0.2 0.1
=> 1 (positive number: the former is newer)

vercmp 1.0.2 1.0.10
=> -8 (negative number: the latter is newer)

a=0.2
b=0.1
if (( $(vercmp "$a" "$b") > 0 )); then
    echo "You are running an outdated version"
fi

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