Something

#!/bin/bashfunction format_number () {        rev <<< $1 | \        sed 's/\([0-9][0-9][0-9]\)/\1,/g' | rev | sed 's/^,//'}format_number 123format_number 1234format_number 123456format_number 1234567format_number 12345678format_number 1234567890format_number 12345678901234567890