On Sun, 8 Apr 2018, Stephen M. Webb wrote: > On 2018-04-08 04:21 AM, Robert P. J. Day wrote: > > not sure if i'm reading a shell script properly ... here's a script > > for downloading and installing kubernetes: > > > > https://get.kubernetes.io/ > > > > but it seems to be doing strange (messy) things with parameter > > substitution. > > > > first, there's this bit of weirdness partway through the script: > > > > if [[ -x ./cluster/get-kube-binaries.sh ]]; then > > # Make sure to use the same download URL in get-kube-binaries.sh > > KUBERNETES_RELEASE_URL="${KUBERNETES_RELEASE_URL}" \ > > ./cluster/get-kube-binaries.sh > > fi > > > > i'm unsure of the value of assigning a variable to itself; is > > there something subtle or tricky about that assignment that isn't > > obvious? > > If the variable $KUBERNETES_RELEASE_URL is local (ie. not exported > to the global environment using the export statement), this > construct will export it to the environment of the subcommand, which > it otherwise wouldn't do. The author may not want it set in the > environment of other subcommands. Controlling unintended side > effects is a hallmark of good developer hygiene. yes, that occurred to me about 10 minutes after i posted. le *sigh*. > > then there are a number of constructs like this: > > > > if [[ -n "${KUBERNETES_SKIP_CREATE_CLUSTER-}" ]]; then > > exit 0 > > fi > > > > AIUI, the whole point of the construct ${VAR-default} > > > > is if VAR is not set, then the value of "default" will be used. so > > what does it mean to say: > > > > ${VAR-} > > > > that is, to have the empty string as the default value? is this some > > magic that i am unaware of? > > That construct tests the value of $KUBERNETES_SKIP_CREATE_CLUSTER > for an empty or unset value without side effects. Sometimes you > don't want side effects, like setting an environment variable that > was not already set if all you want to do is test it. that one you're going to have to explain in more detail. i've seen a lot of scripts in my day, and i've *never* seen that construct. i'm not sure what "side effects" you're referring to here. > > and there's this near the top: > > > > KUBERNETES_RELEASE_URL="${KUBERNETES_RELEASE_URL:-https://dl.k8s.io}"; > > > > ok, that will admittedly set that variable to a value if it does not > > already have a value, but i've always done that this way: > > > > : ${VAR:=defaultvalue} > > What would happen if I ran the script with, say, VAR="&& rm -rf /" ? > Consider that installer scripts are often meant to be run as root. i'm going to test this to see if that's actually what would happen. > > > the script clearly works, but it seems to have been written by > > someone who didn't quite grasp proper parameter substitution. > > thoughts? > > I'm more concerned that some dev at Kubernetes is trying to write a > portable script using bash-specific constructs like '[['. They > ought to be flayed alive and forced to repent. agreed. rday