Allow setting the config path through an environment variable

The config path previously could only be specified through the command
line options. This made it very difficult to set a default config path
without using any option.

Now the environment variable can be set so the default configuration
path is set to a specific place, but can be overwritten using the
command line option.

The primary purpose of this is so the Docker container can have a
default configuration file, but not have to parse the command line
options to figure out if a different configuration file has been
specified while still allowing the user to only type `influxd` and have
the program start correctly.

This might also help #6392 as it would allow a default configuration
location to be included with the package by setting an environment
variable.

A default search path is also provided now with checking the following
paths for a config file when none is specified:

* `~/.influxdb/influxdb.conf`
* `/etc/influxdb/influxdb.conf`

CHANGELOG.md

@@ -14,6 +14,7 @@
 - [#3166](https://github.com/influxdata/influxdb/issues/3166): Sort the series keys inside of a tag set so output is deterministic.
 - [#1856](https://github.com/influxdata/influxdb/issues/1856): Add `elapsed` function that returns the time delta between subsequent points.
 - [#5502](https://github.com/influxdata/influxdb/issues/5502): Add checksum verification to TSM inspect tool
+- [#6444](https://github.com/influxdata/influxdb/pull/6444): Allow setting the config path through an environment variable and default config path.
 
 ### Bugfixes
 

cmd/influxd/run/command.go

@@ -87,7 +87,7 @@ func (cmd *Command) Run(args ...string) error {
 	runtime.SetBlockProfileRate(int(1 * time.Second))
 
 	// Parse config
-	config, err := cmd.ParseConfig(options.ConfigPath)
+	config, err := cmd.ParseConfig(options.GetConfigPath())
 	if err != nil {
 		return fmt.Errorf("parse config: %s", err)
 	}
@@ -239,3 +239,31 @@ type Options struct {
 	CPUProfile string
 	MemProfile string
 }
+
+var searchPath = []string{
+	os.ExpandEnv("${HOME}/.influxdb/influxdb.conf"),
+	"/etc/influxdb/influxdb.conf",
+}
+
+// GetConfigPath returns the config path from the options.
+// It will return a path by searching in this order:
+//   1. The CLI option in ConfigPath
+//   2. The environment variable INFLUXDB_CONFIG_PATH
+//   3. The first influxdb.conf file on the path:
+//        - ~/.influxdb
+//        - /etc/influxdb
+//        - /etc
+func (opt *Options) GetConfigPath() string {
+	if opt.ConfigPath != "" {
+		return opt.ConfigPath
+	} else if envVar := os.Getenv("INFLUXDB_CONFIG_PATH"); envVar != "" {
+		return envVar
+	}
+
+	for _, path := range searchPath {
+		if _, err := os.Stat(path); err == nil {
+			return path
+		}
+	}
+	return ""
+}