Comma-separated list of GCP Projects

PROJECTS=$(\
  gcloud projects list \
  --format='csv[no-heading,terminator=","](projectId)') && \
PROJECTS="${PROJECTS%,}" && \  
echo ${PROJECTS}

From:

gcloud projects list
PROJECT_ID    NAME          PROJECT_NUMBER
foo           foo           1234567890123
bar           bar           1234567890123
baz           baz           1234567890123

To:

foo,bar,baz

Which you may find useful when you need to pass a list of Project IDs to some command-line flag.