bash loops with multi-statement conditions

March 3, 2022

I wanted to pause a script until the status of 2 Cloud Run services became ready. In this case, I wanted to check for the status condition RoutesReady to be True.

My initial attempt is ghastly:

while [[ "True"!=$(gcloud run services describe ...) && "True"!=$(gcloud run service describe ...)]]
do
  ...
loop

NOTE In actuality, is was worse than that because I had --project and --region flags and pumped the result through jq

I found an interesting comment by jonathan-leffler on this Stack overflow answer suggesting that the condition in bash loops could actually be an (arbitrarily?) complex sequence of commands as long as the final statement returns true. Thanks Jonathan!

As a result, I was able to rewrite the above in a better way:

FILTER=".status.conditions[]|select(.type==\"RoutesReady\").status"
until
  STATUS_ONE=$(\
    gcloud run services describe ${NAME_ONE} \
    --project=${PROJECT} \
    --region=${REGION} \
    --format=json \
    | jq -r "${FILTER}")
  STATUS_TWO=$(\
    gcloud run services describe ${NAME_TWO} \
    --project=${PROJECT} \
    --region=${REGION} \
    --format=json \
    | jq -r "${FILTER}")
  [[ ${STATUS_ONE}=="True" && ${STATUS_TWO}=="True" ]]
do
  sleep 5s
done

NOTE Notice in the above that the until condition is actually 3 statements (STATUS_ONE=...; STATUS_TWO=...; [[ ...]]). The first 2 statements allow me assign the filtered gcloud run describe commands output to variables that can then be queried in the condition. An advantage to this approach is that these 2 variables are only needed for the condition and don’t need to be smeared into the loop body.

Another improvement could be to turn the status acquisition into a function.

Aside

I wanted to be able to perform the query without resorting to jq but am unsuccessful.

I tried:

gcloud run services describe ${NAME} \
--project=${PROJECT} \
--region=${REGION} \
--format="value(status.conditions.filter(type=\"RoutesReady\").)"

This yields the correctly filtered condition but I’m unable to work out how to then grab the status from that result.

Bash