contains_word() {
    word=${1:?"target word required"}
    shift
    for elem in "${@}"; do
        if [ "${word}" = "${elem}" ]; then
            unset elem
            unset word
            return 0
        fi
    done
    unset elem
    unset word
    return 1
}

prog_name() {
    name=${1:?"Program name required"}
    name=$(basename -- ${name})
    if [ "-" = "$(printf %.1s "${name}")" ]; then
        name=${name#?}
    fi
    echo "${name}"
    unset name
}

# If ssh-agent is not available to the login shell, exec a new ssh-agent
# with a new login shell as its subshell. Whitelist known good shell
# names, because Xsession also runs .profile and dies if it's run as a
# subshell of ssh-agent. (Xsession has its own mechanism for getting an
# ssh-agent running.)
if [ -n "$(which ssh-agent)" ]; then
    ssh-add -l >/dev/null 2>&1
    if [ ${?} -eq 2 ]; then
        if contains_word $(prog_name ${0}) bash dash ksh sh; then
            exec ssh-agent $(prog_name ${0}) -l
        fi
    fi
fi