Take -a and -p from environment as default

Take the values for the flags `-a` and `-p` from environment variables
as the default, still allowing them to be overridden on the command
line.

Sometimes using `servedir`, you may always want to listen on all
interfaces but the default invocation of `servedir` in a Makefile recipe
does not specify `-a`. You can now set `SERVEDIR_ALL_INTERFACES=true` in
the environment and these pre-canned `servedir` invocations can work
with your defaults.

Makefile

@@ -3,7 +3,7 @@
 
 # --- Global -------------------------------------------------------------------
 O = out
-COVERAGE = 59
+COVERAGE = 66
 VERSION ?= $(shell git describe --tags --dirty  --always)
 
 all: build test check-coverage lint ## build, test, check coverage and lint

main.go

@@ -7,9 +7,9 @@
 //
 //	Simple HTTP server, serving files from given directory.
 //
-//	  -a	listen on all interfaces not just localhost
+//	  -a	listen on all interfaces not just localhost (env: SERVEDIR_ALL_INTERFACES)
 //	  -p int
-//	        port number (default: os chosen free port)
+//	        port number (default: os chosen free port) (env: SERVEDIR_PORT)
 //	  <dir> defaults to current directory if not specified
 package main
 
@@ -20,6 +20,7 @@ import (
 	"net"
 	"net/http"
 	"os"
+	"strconv"
 	"strings"
 )
 
@@ -67,9 +68,23 @@ type config struct {
 }
 
 func parseFlags(args ...string) config {
+	var portDefault int = 0
+	if s, ok := os.LookupEnv("SERVEDIR_PORT"); ok {
+		if v, err := strconv.ParseInt(s, 0, strconv.IntSize); err == nil {
+			portDefault = int(v)
+		}
+	}
+
+	var allInterfacesDefault bool = false
+	if s, ok := os.LookupEnv("SERVEDIR_ALL_INTERFACES"); ok {
+		if v, err := strconv.ParseBool(s); err == nil {
+			allInterfacesDefault = v
+		}
+	}
+
 	fs := flag.NewFlagSet(os.Args[0], flag.ExitOnError)
-	port := fs.Int("p", 0, "port number (default: os chosen free port)")
-	allInterfaces := fs.Bool("a", false, "listen on all interfaces not just localhost")
+	port := fs.Int("p", portDefault, "port number (default: os chosen free port)")
+	allInterfaces := fs.Bool("a", allInterfacesDefault, "listen on all interfaces not just localhost")
 	fs.Usage = func() { usage(fs) }
 	fs.Parse(args) //nolint:errcheck // ExitOnError means this does not return an error
 	return config{

main_test.go

@@ -61,3 +61,14 @@ func TestParseFlags(t *testing.T) {
 	got := parseFlags()
 	require.Equal(t, want, got)
 }
+
+func TestParseFlagsFromEnvironment(t *testing.T) {
+	want := config{
+		listenAddr: ":1",
+		dir:        ".",
+	}
+	t.Setenv("SERVEDIR_ALL_INTERFACES", "true")
+	t.Setenv("SERVEDIR_PORT", "1")
+	got := parseFlags()
+	require.Equal(t, want, got)
+}