Setting-envars-docs opensearch-project#3582 (opensearch-project#7400)

* setting-envars-docs opensearch-project#3582

Signed-off-by: AntonEliatra <[email protected]>

* Update index.md

Signed-off-by: AntonEliatra <[email protected]>

* Apply suggestions from code review

Co-authored-by: Naarcha-AWS <[email protected]>
Signed-off-by: AntonEliatra <[email protected]>

* Update index.md

Signed-off-by: AntonEliatra <[email protected]>

---------

Signed-off-by: AntonEliatra <[email protected]>
Co-authored-by: Naarcha-AWS <[email protected]>
Signed-off-by: Sander van de Geijn <[email protected]>

_install-and-configure/configuring-opensearch/index.md

@@ -25,13 +25,56 @@ Certain operations are static and require you to modify the `opensearch.yml` [co
 
 ## Specifying settings as environment variables
 
+You can specify environment variables in the following ways.
+
+### Arguments at startup
+
 You can specify environment variables as arguments using `-E` when launching OpenSearch:
 
 ```bash
 ./opensearch -Ecluster.name=opensearch-cluster -Enode.name=opensearch-node1 -Ehttp.host=0.0.0.0 -Ediscovery.type=single-node
 ```
 {% include copy.html %}
 
+### Directly in the shell environment
+
+You can configure the environment variables directly in a shell environment before starting OpenSearch, as shown in the following example:
+
+```bash
+export OPENSEARCH_JAVA_OPTS="-Xms2g -Xmx2g"
+export OPENSEARCH_PATH_CONF="/etc/opensearch"
+./opensearch
+```
+{% include copy.html %}
+
+### Systemd service file
+
+When running OpenSearch as a service managed by `systemd`, you can specify environment variables in the service file, as shown in the following example:
+
+```bash
+# /etc/systemd/system/opensearch.service.d/override.conf
+[Service]
+Environment="OPENSEARCH_JAVA_OPTS=-Xms2g -Xmx2g"
+Environment="OPENSEARCH_PATH_CONF=/etc/opensearch"
+```
+After creating or modifying the file, reload the systemd configuration and restart the service using the following command:
+
+```bash
+sudo systemctl daemon-reload
+sudo systemctl restart opensearch
+```
+{% include copy.html %}
+
+### Docker environment variables
+
+When running OpenSearch in Docker, you can specify environment variables using the `-e` option with `docker run` command, as shown in the following command:
+
+```bash
+docker run -e "OPENSEARCH_JAVA_OPTS=-Xms2g -Xmx2g" -e "OPENSEARCH_PATH_CONF=/usr/share/opensearch/config" opensearchproject/opensearch:latest
+```
+{% include copy.html %}
+
+
 ## Updating cluster settings using the API
 
 The first step in changing a setting is to view the current settings by sending the following request:
@@ -113,4 +156,4 @@ If you are working on a client application running against an OpenSearch cluster
 - http.cors.enabled:true
 - http.cors.allow-headers:X-Requested-With,X-Auth-Token,Content-Type,Content-Length,Authorization
 - http.cors.allow-credentials:true
-```
+```