Improves documentation

 * Describes better available source tree sharing method (between
xds-agent and xds-server), and which to prefer depending on the
deployment setup.
 * Gives indication about how to pass RSYNC_TARGET and RSYNC_PREFIX
 env vars to the helloworld-native-application build process.
 * Swap chapter order (build before config)

Bug: SPEC-1993

Change-Id: Ie4c22f277a59b0405744d71b0fadff67c5d5d025
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>

1 files changed, 57 insertions, 0 deletions

diff --git a/docs/part-2/3_xds-cli/2_config.md b/docs/part-2/3_xds-cli/2_config.md
new file mode 100644
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+# Configuration
+
+`xds-cli` configuration is defined either by environment variables or by
+setting command line options.
+
+Configuration through environment variables may also be defined in a file that
+will be sourced by `xds-cli` on start-up.
+
+Use `--config|-c` option or set `XDS_CONFIG` environment variable to specify the config file to use.
+
+So configuration is driven either by environment variables or by command line
+options or using a config file knowing that the following priority order is used:
+
+1. use option value (for example use project ID set by `--id` option),
+1. else use variable `XDS_xxx` (for example `XDS_PROJECT_ID` variable) when a
+   config file is specified with `--config|-c` option,
+1. else use `XDS_xxx` (for example `XDS_PROJECT_ID`) environment variable
+
+<!-- section-note -->
+**Note:**
+
+All parameters after a double dash (--) are considered as the command
+to execute on xds-server.
+<!-- end-section-note -->
+
+## Global Options / Configuration variables
+
+Following is the list of global options across all sub-commands.
+
+__`--config|-c` option or `XDS_CONFIG` env variable__
+
+Env config file to source on startup
+
+__`--log|-l` option or `XDS_LOGLEVEL` env variable__
+
+Logging level, supported levels are:
+
+- panic
+- fatal
+- error
+- warn
+- info
+- debug
+
+Default level is "error".
+
+**`--rpath` option or `XDS_PATH` env variable**
+
+Relative path into project
+
+**`timestamp|-ts` option or `XDS_TIMESTAMP` env variable**
+
+Prefix output with timestamp
+
+**`url` option or `XDS_AGENT_URL` env variable**
+
+Local XDS agent url (default: "localhost:8800")