In general, that depends on your shell, but if you use bash, zsh, ksh or sh (as provided by dash), the following should work:
if ! type "$foobar_command_name" > /dev/null; then
# install foobar here
fi
For a real installation script, you'd probably want to be sure that type
doesn't return successfully in the case when there is an alias foobar
. In bash you could do something like this:
if ! foobar_loc="$(type -p "$foobar_command_name")" || [[ -z $foobar_loc ]]; then
# install foobar here
fi