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gcloud compute instance-groups managed delete-instances

NAME
gcloud compute instance-groups managed delete-instances - delete instances that are managed by a managed instance group
SYNOPSIS
gcloud compute instance-groups managed delete-instances NAME --instances=INSTANCE,[INSTANCE,…] [--skip-instances-on-validation-error] [--region=REGION     | --zone=ZONE] [GCLOUD_WIDE_FLAG …]
DESCRIPTION
gcloud compute instance-groups managed delete-instances is used to delete one or more instances from a managed instance group. Once the instances are deleted, the size of the group is automatically reduced to reflect the changes.

The command returns the operation status per instance, which might be FAIL, SUCCESS, SKIPPED, or MEMBER_NOT_FOUND. MEMBER_NOT_FOUND is returned only for regional groups when the gcloud command-line tool wasn't able to resolve the zone from the instance name. SKIPPED is returned only when the --skip-instances-on-validation-error flag is used and the instance is not a member of the group or is already being deleted or abandoned.

If you want to keep the underlying virtual machines but still remove them from the managed instance group, use the abandon-instances command instead.
POSITIONAL ARGUMENTS
NAME
Name of the managed instance group to operate on.
REQUIRED FLAGS
--instances=INSTANCE,[INSTANCE,…]
Names of instances to delete.
OPTIONAL FLAGS
--skip-instances-on-validation-error
Specifies whether the request should proceed even if the request includes instances that are not members of the group or that are already being deleted or abandoned. By default, if you omit this flag and such an instance is specified in the request, the operation fails. The operation always fails if the request contains a badly formatted instance name or a reference to an instance that exists in a zone or region other than the group's zone or region.
At most one of these can be specified:
--region=REGION
Region of the managed instance group to operate on. If not specified, you might be prompted to select a region (interactive mode only).

A list of regions can be fetched by running: 

gcloud compute regions list
 Overrides the default compute/region property value for this command invocation.
--zone=ZONE
Zone of the managed instance group to operate on. If not specified, you might be prompted to select a zone (interactive mode only).

A list of zones can be fetched by running: 

gcloud compute zones list
 Overrides the default compute/zone property value for this command invocation.
GCLOUD WIDE FLAGS
These flags are available to all commands: --access-token-file, --account, --billing-project, --configuration, --flags-file, --flatten, --format, --help, --impersonate-service-account, --log-http, --project, --quiet, --trace-token, --user-output-enabled, --verbosity.

Run $ gcloud help for details.

NOTES
These variants are also available: 
gcloud alpha compute instance-groups managed delete-instances
 
gcloud beta compute instance-groups managed delete-instances
 
gcloud preview compute instance-groups managed delete-instances