Takes int or float seconds as input will output in format hh:mm:ss.ms where ms is rounded to 3 decimals

sec2ts.sh

#!/bin/bash

function padnum {
        NUM=$1
        if [ $(echo "$NUM < 10" | bc) -ne 0 ]
        then
                echo 0$NUM
        else
                echo $NUM
        fi
}

function sec2ts {
        SEC=$1
        HR=$(echo - | awk "{print $SEC / 3600}")
        HR=${HR%.*}
        SEC=$(echo - | awk "{print $SEC - ($HR * 3600)}")
        MIN=$(echo - | awk "{print $SEC / 60}")
        MIN=${MIN%.*}
        SEC=$(echo - | awk "{print $SEC - ($MIN * 60)}")
        SEC=$(printf "%0.3f" $SEC)

        echo $(padnum $HR):$(padnum $MIN):$(padnum $SEC)
}

sec2ts $1