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"
 
@@ -899,3 +900,31 @@ 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
+}

completions_test.go

@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"os"
 	"strings"
 	"sync"
 	"testing"
@@ -3517,3 +3518,78 @@ func TestGetFlagCompletion(t *testing.T) {
 		})
 	}
 }
+
+func TestGetEnvConfig(t *testing.T) {
+	testCases := []struct {
+		use       string
+		suffix    string
+		cmdVar    string
+		globalVar string
+		cmdVal    string
+		globalVal string
+		expected  string
+	}{
+		{
+			use:       "root",
+			suffix:    "test",
+			cmdVar:    "ROOT_TEST",
+			globalVar: "COBRA_TEST",
+			cmdVal:    "cmd",
+			globalVal: "global",
+			expected:  "cmd",
+		},
+		{
+			use:       "root",
+			suffix:    "test",
+			cmdVar:    "ROOT_TEST",
+			globalVar: "COBRA_TEST",
+			cmdVal:    "",
+			globalVal: "global",
+			expected:  "global",
+		},
+		{
+			use:       "root",
+			suffix:    "test",
+			cmdVar:    "ROOT_TEST",
+			globalVar: "COBRA_TEST",
+			cmdVal:    "",
+			globalVal: "",
+			expected:  "",
+		},
+		{
+			use:       "foo.bar",
+			suffix:    "test",
+			cmdVar:    "FOO_BAR_TEST",
+			globalVar: "COBRA_TEST",
+			cmdVal:    "cmd",
+			globalVal: "global",
+			expected:  "cmd",
+		},
+		{
+			use:       "quux-BAZ",
+			suffix:    "test",
+			cmdVar:    "QUUX_BAZ_TEST",
+			globalVar: "COBRA_TEST",
+			cmdVal:    "cmd",
+			globalVal: "global",
+			expected:  "cmd",
+		},
+	}
+
+	for _, tc := range testCases {
+		// Could make env handling cleaner with t.Setenv with Go >= 1.17
+		func() {
+			err := os.Setenv(tc.cmdVar, tc.cmdVal)
+			defer assertNoErr(t, os.Unsetenv(tc.cmdVar))
+			assertNoErr(t, err)
+			err = os.Setenv(tc.globalVar, tc.globalVal)
+			defer assertNoErr(t, os.Unsetenv(tc.globalVar))
+			assertNoErr(t, err)
+			cmd := &Command{Use: tc.use}
+			got := GetEnvConfig(cmd, tc.suffix)
+			if got != tc.expected {
+				t.Errorf("expected: %q, got: %q", tc.expected, got)
+			}
+		}()
+	}
+}