Add general purpose config env var getter

Similar to the existing one for active help, but generalized.

completions.go

@@ -17,6 +17,7 @@ package cobra
 import (
 	"fmt"
 	"os"
+	"regexp"
 	"strings"
 	"sync"
 
@@ -904,3 +905,31 @@ func CompError(msg string) {
 func CompErrorln(msg string) {
 	CompError(fmt.Sprintf("%s\n", msg))
 }
+
+// configEnvVarGlobalPrefix should not be changed: users will be using it explicitly.
+const configEnvVarGlobalPrefix = "COBRA"
+
+var configEnvVarPrefixSubstRegexp = regexp.MustCompile(`[^A-Z0-9_]`)
+
+// configEnvVar returns the name of the program-specific configuration environment
+// variable.  It has the format <PROGRAM>_<SUFFIX> where <PROGRAM> is the name of the
+// root command in upper case, with all non-ASCII-alphanumeric characters replaced by `_`.
+func configEnvVar(name, suffix string) string {
+	// This format should not be changed: users will be using it explicitly.
+	v := strings.ToUpper(fmt.Sprintf("%s_%s", name, suffix))
+	v = configEnvVarPrefixSubstRegexp.ReplaceAllString(v, "_")
+	return v
+}
+
+// GetEnvConfig returns the value of the configuration environment variable
+// <PROGRAM>_<SUFFIX> where <PROGRAM> is the name of the root command in upper
+// case, with all non-ASCII-alphanumeric characters replaced by `_`.
+// If the value is empty or not set, the value of the environment variable
+// COBRA_<SUFFIX> is returned instead.
+func GetEnvConfig(cmd *Command, suffix string) string {
+	v := os.Getenv(configEnvVar(cmd.Root().Name(), suffix))
+	if v == "" {
+		v = os.Getenv(configEnvVar(configEnvVarGlobalPrefix, suffix))
+	}
+	return v
+}