通过不同的设置api 版本环境变量，支持的命令行不同。
Interacting with etcd： https://coreos.com/etcd/docs/latest/dev-guide/interacting_v3.html
1. 未设置 ETCDCTL_API 时，支持的命令
缺省使用 ETCDCTL_API=2
[root@vStack etcd-v3.0.15-linux-amd64]# ./etcdctl help
NAME:
etcdctl - A simple command line client for etcd.
USAGE:
etcdctl [global options] command [command options] [arguments...]
VERSION:
3.0.15
COMMANDS:
1. backup backup an etcd directory
2. cluster-health check the health of the etcd cluster
3. mk make a new key with a given value
4. mkdir make a new directory
5. rm remove a key or a directory
6. rmdir removes the key if it is an empty directory or a key-value pair
7. get retrieve the value of a key
8. ls retrieve a directory
9. set set the value of a key
10. setdir create a new directory or update an existing directory TTL
11. update update an existing key with a given value
12. updatedir update an existing directory
13. watch watch a key for changes
14. exec-watch watch a key for changes and exec an executable
15. member member add, remove and list subcommands
16. import import a snapshot to a cluster
17. user user add, grant and revoke subcommands
18. role role add, grant and revoke subcommands
19. auth overall auth controls
GLOBAL OPTIONS:
--debug output cURL commands which can be used to reproduce the request
--no-sync don't synchronize cluster information before sending request
--output simple, -o simple output response in the given format (simple, `extended` or `json`) (default: "simple")
--discovery-srv value, -D value domain name to query for SRV records describing cluster endpoints
--insecure-discovery accept insecure SRV records describing cluster endpoints
--peers value, -C value DEPRECATED - "--endpoints" should be used instead
--endpoint value DEPRECATED - "--endpoints" should be used instead
--endpoints value a comma-delimited list of machine addresses in the cluster (default: "http://127.0.0.1:2379,http://127.0.0.1:4001")
--cert-file value identify HTTPS client using this SSL certificate file
--key-file value identify HTTPS client using this SSL key file
--ca-file value verify certificates of HTTPS-enabled servers using this CA bundle
--username value, -u value provide username[:password] and prompt if password is not supplied.
--timeout value connection timeout per request (default: 1s)
--total-timeout value timeout for the command execution (except watch) (default: 5s)
--help, -h show help
--version, -v print the version
2. 设置 ETCDCTL_API=3 时，支持的命令
export ETCDCTL_API=3
[root@vStack etcd-v3.0.15-linux-amd64]# ./etcdctl --help
NAME:
etcdctl - A simple command line client for etcd3.
USAGE:
etcdctl
VERSION:
3.0.15
COMMANDS:
1. get Gets the key or a range of keys
etcdctl get [options] <key> [range_end]
OPTIONS:
--consistency="l" Linearizable(l) or Serializable(s)
--from-key[=false] Get keys that are greater than or equal to the given key
--keys-only[=false] Get only the keys
--limit=0 Maximum number of results
--order="" Order of results; ASCEND or DESCEND
--prefix[=false] Get keys with matching prefix
--rev=0 Specify the kv revision
--sort-by="" Sort target; CREATE, KEY, MODIFY, VALUE, or VERSION
2. put Puts the given key into the store
3. del Removes the specified key or range of keys [key, range_end)
4. txn Txn processes all the requests in one transaction
5. compaction Compacts the event history in etcd 。 etcd支持历史数据的读取，为了避免积累无限历史数据。compacting操作后，etcd将删除历史的版本数据，释放资源为后面使用。在compacted 版本前的数据都将无效。
6. alarm disarm Disarms all alarms
7. alarm list Lists all alarms
8. defrag Defragments the storage of the etcd members with given endpoints
9. endpoint health Checks the healthiness of endpoints specified in `--endpoints` flag
10. endpoint status Prints out the status of endpoints specified in `--endpoints` flag
11. watch Watches events stream on keys or prefixes
12. version Prints the version of etcdctl
13. lease grant Creates leases
14. lease revoke Revokes leases
15. lease keep-alive Keeps leases alive (renew)
16. member add Adds a member into the cluster
17. member remove Removes a member from the cluster
18. member update Updates a member in the cluster
19. member list Lists all members in the cluster
20. snapshot save Stores an etcd node backend snapshot to a given file
21. snapshot restore Restores an etcd member snapshot to an etcd directory
22. snapshot status Gets backend snapshot status of a given file
23. make-mirror Makes a mirror at the destination etcd cluster
24. migrate Migrates keys in a v2 store to a mvcc store
25. lock Acquires a named lock
26. elect Observes and participates in leader election
27. auth enable Enables authentication
28. auth disable Disables authentication
29. user add Adds a new user
30. user delete Deletes a user
31. user get Gets detailed information of a user
32. user list Lists all users
33. user passwd Changes password of user
34. user grant-role Grants a role to a user
35. user revoke-role Revokes a role from a user
36. role add Adds a new role
37. role delete Deletes a role
38. role get Gets detailed information of a role
39. role list Lists all roles
40. role grant-permission Grants a key to a role
41. role revoke-permission Revokes a key from a role
42. help Help about any command
OPTIONS:
--cacert="" verify certificates of TLS-enabled secure servers using this CA bundle
--cert="" identify secure client using this TLS certificate file
--command-timeout=5s timeout for short running command (excluding dial timeout)
--dial-timeout=2s dial timeout for client connections
--endpoints=[127.0.0.1:2379] gRPC endpoints
--hex[=false] print byte strings as hex encoded strings
--insecure-skip-tls-verify[=false] skip server certificate verification
--insecure-transport[=true] disable transport security for client connections
--key="" identify secure client using this TLS key file
--user="" username[:password] for authentication (prompt if password is not supplied)
-w, --write-out="simple" set the output format (simple, json, etc..)