Added env-related notes to the knowledge base

knowledge base/envsubst.md

@@ -0,0 +1,18 @@
+# Envsubst
+
+Substitutes environment variables in shell format strings.
+
+## TL;DR
+
+```shell
+envsubst < input.file
+envsubst < input.file > output.file
+```
+
+```shell
+$ cat hello.file
+hello $NAME
+
+$ NAME='mek' envsubst < hello.file
+hello mek
+```

knowledge base/export all variables in a envfile.md

@@ -0,0 +1,15 @@
+# Export all variables in an envfile
+
+## TL;DR
+
+```shell
+set -o allexport
+source envfile
+set +o allexport
+```
+
+## Sources
+
+- [Set environment variables from file of key/value pairs]
+
+[set environment variables from file of key/value pairs]: https://stackoverflow.com/questions/19331497/set-environment-variables-from-file-of-key-value-pairs#30969768

knowledge base/get the environment of a process running in a container.md

@@ -0,0 +1,19 @@
+# Get the environment of a process running in a container
+
+## TL;DR
+
+```shell
+cat /proc/${PID}/environ
+
+# Container in kubernetes.
+kubectl exec pod-name -- cat /proc/1/environ
+
+# Only works if the onboard `ps` is not from busybox.
+ps e -p $PID
+```
+
+## Sources
+
+- [Get the environment variables of running process in container]
+
+[get the environment variables of running process in container]: https://unix.stackexchange.com/a/412730