refactor: specific section for the gnu userland

knowledge base/gnu userland/README.md

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+# GNU userland
+
+Most operating systems have two distinct parts: the **kernel** and the **userland**.
+
+Most Linux distributions use the [GNU userland][software].<br/>
+It contains system daemons, user applications, the GUI system (X11, KDE, etc.) and various libraries (such as SDL, Mesa, GNUstep, glibc, etc.).
+
+## Table of contents <!-- omit in toc -->
+
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## Further readings
+
+- [Software]
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+- [GNU userland]
+
+<!--
+  References
+  -->
+
+<!-- Upstream -->
+[software]: https://www.gnu.org/software/
+
+<!-- In-article sections -->
+[further readings]: #further-readings
+
+<!-- Knowledge base -->
+[parallel]: parallel.md
+
+<!-- Others -->
+[gnu userland]: https://www.linux.org/threads/gnu-userland.11066/

knowledge base/gnu userland/parallel.md

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+# GNU Parallel
+
+## Table of contents <!-- omit in toc -->
+
+1. [TL;DR](#tldr)
+1. [Further readings](#further-readings)
+
+## TL;DR
+
+```sh
+# Group output ('--group', defaults to on).
+# Fill up 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 {}
+
+# Rsync all folders in a directory to a NAS.
+# So it one by one, and print and properly quote the command before execution.
+parallel -qt -j 1 \
+  rsync -a --info=stats2 {} nas.lan:/shares/backup/ \
+  ::: backup/.snapshots/*
+
+# Get the exit status of all subjobs ('--joblog $outfile').
+# Use all the threads you can (--jobs 0), hammering the CPU.
+find . -type d -name .git -exec dirname "{}" + \
+| parallel --jobs 0 --tagstring {/} --joblog - \
+    'git -C {} pull --recurse-submodules'
+
+# Inject Istio's sidecar to all Deployments in a Namespace.
+kubectl get deployments -o jsonpath='{.items[*].metadata.name}' \
+| parallel --jobs 0 'kubectl -n ${NAMESPACE:-default} apply -f \
+    <(istioctl kube-inject -f \
+        <(kubectl get deployments,services {} -o json))'
+
+# Given a list of Namespaces, get all Pods in them and the Nodes they are
+# running on.
+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
+
+# Given  a collection of images in the same working directory, find all those
+# that are at least 90% similar, but not equal, to another.
+parallel --tag "magick compare -metric NCC -quiet {} NULL: 2>&1; echo" \
+  ::: *.jpg *.jpeg *.png *.webp ::: *.jpg *.jpeg *.png *.webp \
+| grep '0\.9'
+```
+
+## Further readings
+
+- [`man` page][man page]
+- [Tutorial]
+- [Obtaining exit status values from GNU parallel]
+
+<!--
+  References
+  -->
+
+<!-- Upstream -->
+[man page]: https://www.gnu.org/software/parallel/man.html
+[tutorial]: https://www.gnu.org/software/parallel/parallel_tutorial.html
+
+<!-- Others -->
+[obtaining exit status values from gnu parallel]: https://stackoverflow.com/questions/6310181/obtaining-exit-status-values-from-gnu-parallel#6789085