Added parallel notes to the knowledge base

knowledge base/parallel.md

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+# GNU Parallel
+
+## TL;DR
+
+```shell
+# group output (--group)
+# use all cpu threads (--jobs 0 or --jobs 100%)
+# use newline as delimiter for the arguments in input
+# simulate and print to output the command that would have been executed
+find . -type f \
+| parallel --group --jobs 0 --delimiter '\n' --dry-run clamscan {}
+
+# get the exit status of all subjobs (--joblog $outfile)
+find . -type d -name .git -exec dirname "{}" + \
+| parallel --group --jobs 100% --tagstring {/} --joblog - 'git -C {} pull --recurse-submodules'
+
+# inject istio to all deployments in a namespace in (GNU) parallel
+# aka _I have a death wish_
+kubectl -n ${NAMESPACE:-default} get deployments -o jsonpath='{.items[*].metadata.name}' \
+| parallel --group --jobs 0 'kubectl -n ${NAMESPACE:-default} apply -f <(istioctl kube-inject -f <(kubectl -n ${NAMESPACE:-default} get deployments,services {} -o json))'
+
+# given a list of namespaces get pods and their nodes
+parallel --group --jobs 100% --tag \
+  "kubectl --context $KUBE_CONTEXT --namespace {} get pods --output json | jq -r '.items[] | .metadata.name + \"\t\" + .spec.nodeName' -" \
+  ::: "${NAMESPACES}" \
+| column -t
+```
+
+## Further readings
+
+- GNU Parallel's [man page]
+- GNU Parallel's [tutorial]
+- [Obtaining exit status values from GNU parallel]
+
+[man page]: https://www.gnu.org/software/parallel/man.html
+[tutorial]: https://www.gnu.org/software/parallel/parallel_tutorial.html
+
+[Obtaining exit status values from GNU parallel]: https://stackoverflow.com/questions/6310181/obtaining-exit-status-values-from-gnu-parallel#6789085