diff --git a/internal/cli/config/set.go b/internal/cli/config/set.go
index fd88f17..3a9dafc 100644
--- a/internal/cli/config/set.go
+++ b/internal/cli/config/set.go
@@ -4,8 +4,6 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
-	"reflect"
-	"strconv"
 
 	"git.avsdev.uk/Go/job-search/internal/cli"
 	"git.avsdev.uk/Go/job-search/internal/config"
@@ -13,65 +11,6 @@ import (
 	"github.com/spf13/viper"
 )
 
-func setConfig(conf *config.JobSearchConfig, key string, value string) error {
-	var confValue any = nil
-	var confType reflect.Kind
-	var err error
-
-	confType, err = conf.TypeOf(key)
-	if err != nil {
-		return err
-	}
-
-	switch confType {
-	case reflect.Bool:
-		confValue, err = strconv.ParseBool(value)
-	case reflect.String:
-		confValue = value
-	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-		confValue, err = strconv.ParseInt(value, 10, 64)
-	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
-		confValue, err = strconv.ParseUint(value, 10, 64)
-	case reflect.Float32, reflect.Float64:
-		confValue, err = strconv.ParseFloat(value, 64)
-	default:
-		err = errors.ErrUnsupported
-	}
-	if err != nil {
-		return fmt.Errorf("config value type not accepted, expected <%s>", confType)
-	}
-
-	viper.Set(key, confValue)
-	if err := viper.WriteConfig(); err != nil {
-		return err
-	}
-	fmt.Print(key, " = ", confValue, " <", confType, ">\n")
-
-	return nil
-}
-func unsetConfig(conf *config.JobSearchConfig, key string) error {
-	if _, err := conf.TypeOf(key); err != nil {
-		return err
-	}
-
-	v := viper.New()
-	v.SetConfigFile(viper.ConfigFileUsed())
-
-	keys := viper.AllKeys()
-	for i := range keys {
-		k := keys[i]
-		if k != key {
-			v.Set(k, viper.Get(k))
-		}
-	}
-
-	if err := v.WriteConfig(); err != nil {
-		return err
-	}
-
-	return nil
-}
-
 func commandSet(opts *cli.Options) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "set [config_key] [config_value]",
@@ -111,10 +50,17 @@ func commandSet(opts *cli.Options) *cobra.Command {
 			}
 
 			if unset {
-				return unsetConfig(conf, args[0])
-			} else {
-				return setConfig(conf, args[0], args[1])
+				return config.PurgeValue(conf, args[0])
 			}
+
+			confValue, confType, err := config.StoreValue(conf, args[0], args[1])
+			if err != nil {
+				return err
+			}
+
+			fmt.Print(args[0], " = ", confValue, " <", confType, ">\n")
+
+			return nil
 		},
 	}
 	cmd.Flags().BoolP("unset", "x", false, "clear configuration setting")
diff --git a/internal/config/config.go b/internal/config/config.go
index aa5321c..1814b19 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -54,16 +54,20 @@ func LoadFromViper(conf *JobSearchConfig) {
 		if keyType == reflect.Invalid {
 			continue
 		}
+		var err error
 		switch keyType {
 		case reflect.String:
-			conf.SetValue(key, viper.GetString(key))
+			err = conf.SetValue(key, viper.GetString(key))
 		case reflect.Int:
-			conf.SetValue(key, viper.GetInt(key))
+			err = conf.SetValue(key, viper.GetInt(key))
 		case reflect.Bool:
-			conf.SetValue(key, viper.GetBool(key))
+			err = conf.SetValue(key, viper.GetBool(key))
 		default:
 			slog.Error(fmt.Sprintf("unknown key type: %s", keyType))
 		}
+		if err != nil {
+			slog.Warn("ignoring invalid config value", "key", key, "err", err)
+		}
 	}
 }
 
diff --git a/internal/config/load.go b/internal/config/load.go
index 17c5515..ccda30a 100644
--- a/internal/config/load.go
+++ b/internal/config/load.go
@@ -3,6 +3,7 @@ package config
 import (
 	"fmt"
 	"log/slog"
+	"strings"
 
 	"github.com/joho/godotenv"
 	"github.com/spf13/viper"
@@ -33,6 +34,7 @@ func loadConfig(cfgFile string) error {
 	LoadDefaultConfig()
 
 	viper.SetEnvPrefix(EnvPrefix)
+	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
 	viper.AutomaticEnv() // read in environment variables that match
 
 	err := viper.ReadInConfig()
diff --git a/internal/config/modify.go b/internal/config/modify.go
new file mode 100644
index 0000000..7ed9499
--- /dev/null
+++ b/internal/config/modify.go
@@ -0,0 +1,83 @@
+package config
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"strconv"
+
+	"github.com/spf13/viper"
+)
+
+// StoreValue parses value as the type of key, checks conf accepts it, sets it
+// in conf and Viper and writes the config file Viper is using. It returns the
+// parsed value and its kind.
+func StoreValue(
+	conf *JobSearchConfig, key string, value string,
+) (any, reflect.Kind, error) {
+	var confValue any = nil
+	var confType reflect.Kind
+	var err error
+
+	confType, err = conf.TypeOf(key)
+	if err != nil {
+		return nil, confType, err
+	}
+
+	switch confType {
+	case reflect.Bool:
+		confValue, err = strconv.ParseBool(value)
+	case reflect.String:
+		confValue = value
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		confValue, err = strconv.ParseInt(value, 10, 64)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		confValue, err = strconv.ParseUint(value, 10, 64)
+	case reflect.Float32, reflect.Float64:
+		confValue, err = strconv.ParseFloat(value, 64)
+	default:
+		err = errors.ErrUnsupported
+	}
+	if err != nil {
+		return nil, confType, fmt.Errorf(
+			"config value type not accepted, expected <%s>", confType,
+		)
+	}
+
+	// Reject values the configuration itself would not accept, e.g. an
+	// unknown log level or an unparseable duration.
+	if err := conf.SetValue(key, confValue); err != nil {
+		return nil, confType, err
+	}
+
+	viper.Set(key, confValue)
+	if err := viper.WriteConfig(); err != nil {
+		return nil, confType, err
+	}
+
+	return confValue, confType, nil
+}
+
+// PurgeValue rewrites the config file Viper is using without key.
+func PurgeValue(conf *JobSearchConfig, key string) error {
+	if _, err := conf.TypeOf(key); err != nil {
+		return err
+	}
+
+	v := viper.New()
+	v.SetConfigFile(viper.ConfigFileUsed())
+
+	keys := viper.AllKeys()
+	for i := range keys {
+		k := keys[i]
+		if k != key {
+			v.Set(k, viper.Get(k))
+		}
+	}
+
+	if err := v.WriteConfig(); err != nil {
+		return err
+	}
+
+	return nil
+}