Lists upgradable packages comma separated on systems with apt package management (hopefully)

get_upgradable_packages.sh

#!/bin/bash
# Dennis Ullrich
#
# Lists upgradable packages comma separated on systems with apt package management (hopefully)
# Made for "Linux Helpdesk Deutschland - Hilfe für Linux Einsteiger" group
#
# 2018-09-14 v0.1: First release
# 2018-09-14 v0.2: Improved code... No read, while loops etc... Everything is processed with bash internals.

get_upgradable_packages() {
        #
        # Return code 0 + stdout = List of updates
        # Return code 1 = Something went wrong with apt
        # Return code 10 = No updates available
        #
        declare -a packages
        tmp=`mktemp`

        sudo apt list --upgradable 2>/dev/null >$tmp || { rm -f $tmp; return 1; }
        GLOBIGNORE='*' IFS=$'\r\n' command eval 'packages=($(<$tmp))'
        # remove 1st element (it's always "Listing..." from apt)
        packages=("${packages[@]:1}")
        # remove everything after 1st /
        packages=("${packages[@]%\/*}")
        rm -f $tmp

        [[ ${#packages[@]} -eq 0 ]] && return 10

        IFS=, eval 'echo "${packages[*]}"'
        return 0
}

get_upgradable_packages