From eed9c34ebf20d264ddbc0128dfe1606a28290766 Mon Sep 17 00:00:00 2001
From: brent saner <brent.saner@gmail.com>
Date: Thu, 11 Apr 2024 12:46:13 -0400
Subject: [PATCH] checking in before refactoring interpolation

---
 envs/consts.go               |  22 +
 envs/funcs.go                | 642 ++++++++++++++++++++++++++-
 envs/funcs.go.old            | 825 +++++++++++++++++++++++++++++++++++
 envs/funcs_test.go           | 182 ++++++++
 envs/types.go                |  10 +
 envs/utils_nonwindows.go     |  18 +
 envs/utils_windows.go        |  15 +
 errs/errs.go                 |  10 +
 exec_extra/consts.go         |  34 ++
 exec_extra/consts_windows.go |  45 ++
 exec_extra/func.go           |  31 --
 exec_extra/funcs.go          |  86 ++++
 exec_extra/types.go          |   9 +
 exec_extra/utils.go          |  33 ++
 go.mod                       |   7 +-
 go.sum                       |  10 +
 internal/utils.go            |  83 ++++
 17 files changed, 2014 insertions(+), 48 deletions(-)
 create mode 100644 envs/funcs.go.old
 create mode 100644 envs/funcs_test.go
 create mode 100644 envs/types.go
 create mode 100644 envs/utils_nonwindows.go
 create mode 100644 envs/utils_windows.go
 create mode 100644 errs/errs.go
 create mode 100644 exec_extra/consts.go
 create mode 100644 exec_extra/consts_windows.go
 delete mode 100644 exec_extra/func.go
 create mode 100644 exec_extra/funcs.go
 create mode 100644 exec_extra/types.go
 create mode 100644 exec_extra/utils.go

diff --git a/envs/consts.go b/envs/consts.go
index 6ee386f..e4cedd4 100644
--- a/envs/consts.go
+++ b/envs/consts.go
@@ -9,3 +9,25 @@ var (
 	reMaybeInt   *regexp.Regexp = regexp.MustCompile(`^(?P<sign>\+|-)[0-9]+$`)
 	reMaybeFloat *regexp.Regexp = regexp.MustCompile(`(?P<sign>\+|-)?[0-9]+\.[0-9]+$`)
 )
+
+var (
+	StructTagInterpolate string = "envsub"
+	StructTagPopulate    string = "envpop"
+)
+
+var (
+	defaultInterpolateOpts interpolateOpts = interpolateOpts{
+		noMapKey: false,
+		noMapVal: false,
+	}
+	// InterpolateOptNoMapKey is the equivalent of the struct tag `no_map_key` for Interpolate.
+	InterpolateOptNoMapKey optInterpolate = func(o *interpolateOpts) (err error) {
+		o.noMapKey = true
+		return
+	}
+	// InterpolateOptNoMapValue is the equivalent of the struct tag `no_map_value` for Interpolate.
+	InterpolateOptNoMapValue optInterpolate = func(o *interpolateOpts) (err error) {
+		o.noMapVal = true
+		return
+	}
+)
diff --git a/envs/funcs.go b/envs/funcs.go
index 8bdeb58..073b672 100644
--- a/envs/funcs.go
+++ b/envs/funcs.go
@@ -6,28 +6,16 @@ import (
 	`fmt`
 	`io/ioutil`
 	`os`
+	`reflect`
 	`strings`
+	`sync`
 
+	`r00t2.io/goutils/multierr`
+	`r00t2.io/sysutils/errs`
 	`r00t2.io/sysutils/internal`
 	`r00t2.io/sysutils/paths`
 )
 
-// GetPathEnv returns a slice of the PATH variable's items.
-func GetPathEnv() (pathList []string, err error) {
-
-	var pathVar string = internal.GetPathEnvName()
-
-	pathList = make([]string, 0)
-
-	for _, p := range strings.Split(os.Getenv(pathVar), string(os.PathListSeparator)) {
-		if err = paths.RealPath(&p); err != nil {
-			return
-		}
-		pathList = append(pathList, p)
-	}
-	return
-}
-
 // GetEnvMap returns a map of all environment variables. All values are strings.
 func GetEnvMap() (envVars map[string]string) {
 
@@ -114,6 +102,22 @@ func GetFirstWithRef(varNames []string) (val string, ok bool, idx int) {
 	return
 }
 
+// GetPathEnv returns a slice of the PATH variable's items.
+func GetPathEnv() (pathList []string, err error) {
+
+	var pathVar string = internal.GetPathEnvName()
+
+	pathList = make([]string, 0)
+
+	for _, p := range strings.Split(os.Getenv(pathVar), string(os.PathListSeparator)) {
+		if err = paths.RealPath(&p); err != nil {
+			return
+		}
+		pathList = append(pathList, p)
+	}
+	return
+}
+
 /*
 	GetPidEnvMap will only work on *NIX-like systems with procfs.
 	It gets the environment variables of a given process' PID.
@@ -185,3 +189,609 @@ func HasEnv(key string) (envIsSet bool) {
 
 	return
 }
+
+/*
+	Interpolate takes one of:
+
+		- a string (pointer only)
+		- a struct (pointer only)
+		- a map
+		- a slice
+
+	and performs variable substitution on strings from environment variables.
+
+	It supports both UNIX/Linux/POSIX syntax formats (e.g. $VARNAME, ${VARNAME}) and,
+	if on Windows, it *additionally* supports the EXPAND_SZ format (e.g. %VARNAME%).
+
+	For structs, the tag name used can be changed by setting the StructTagInterpolate
+	variable in this submodule; the default is `envsub`.
+	If the tag value is "-", the field will be skipped.
+	For map fields within structs, the default is to apply interpolation to both keys and values;
+	this can be changed with the `no_map_key` and `no_map_value` options (tag values).
+	Any other tag value(s) are ignored.
+
+	For maps and slices, Interpolate will recurse into values (e.g. [][]string will work as expected).
+
+	Supported struct tag options:
+
+		* `no_map_key` - Do not operate on map keys if they are strings or string pointers.
+						 See also InterpolateOptNoMapKey.
+		* `no_map_value` - Do not operate on map values if they are strings or string pointers.
+						   See also InterpolateOptNoMapValue.
+
+	If s is nil, no interpolation will be performed. No error will be returned.
+	If s is not a valid/supported type, no interpolation will be performed. No error will be returned.
+*/
+func Interpolate[T any](s T, opts ...optInterpolate) (err error) {
+
+	var sVal reflect.Value = reflect.ValueOf(s)
+	var sType reflect.Type = sVal.Type()
+	var kind reflect.Kind = sType.Kind()
+	var ptrVal reflect.Value
+	var ptrType reflect.Type
+	var ptrKind reflect.Kind
+
+	switch kind {
+	case reflect.Ptr:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		ptrVal = sVal.Elem()
+		ptrType = ptrVal.Type()
+		ptrKind = ptrType.Kind()
+		if ptrKind == reflect.String {
+			err = interpolateStringReflect(ptrVal, opts, nil)
+		} else {
+			// Otherwise, it should be a struct ptr.
+			if ptrKind != reflect.Struct {
+				return
+			}
+			err = interpolateStruct(ptrVal, opts, nil)
+		}
+	case reflect.Map:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		err = interpolateMap(sVal, opts, nil)
+	case reflect.Slice:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		err = interpolateSlice(sVal, opts, nil)
+		/*
+			case reflect.Struct:
+				if sVal.IsZero() || !sVal.IsValid() {
+					return
+				}
+				err = interpolateStruct(sVal, opts, nil)
+
+		*/
+	}
+
+	return
+}
+
+/*
+	InterpolateString takes (a pointer to) a struct or string and performs variable substitution on it
+	from environment variables.
+
+	It supports both UNIX/Linux/POSIX syntax formats (e.g. $VARNAME, ${VARNAME}) and,
+	if on Windows, it *additionally* supports the EXPAND_SZ format (e.g. %VARNAME%).
+
+	If s is nil, nothing will be done and err will be ErrNilPtr.
+
+	This is a standalone function that is much more performant than Interpolate
+	at the cost of rigidity.
+*/
+func InterpolateString(s *string) (err error) {
+
+	var newStr string
+
+	if s == nil {
+		err = errs.ErrNilPtr
+		return
+	}
+
+	if newStr, err = interpolateString(*s); err != nil {
+		return
+	}
+	*s = newStr
+
+	return
+}
+
+/*
+	PopulateStruct takes (a pointer to) a struct and performs *population* on it.
+	Unlike the InterpolateStruct function, this *completely populates* (or *replaces*)
+	a field's value with the specified environment variable; no *substitution* is performed.
+
+	You can change the tag name used by changing the StructTagPopulate variable in this module;
+	the default is `envpop`.
+
+	Tag value format:
+		<tag>:"<VAR NAME>[,<option>,<option>...]"
+	e.g.
+		envpop:"SOMEVAR"
+		envpop:"OTHERVAR,force"
+		envpop:"OTHERVAR,allow_empty"
+		envpop:"OTHERVAR,force,allow_empty"
+
+	If the tag value is "-", or <VAR NAME> is not provided, the field will be explicitly skipped.
+	(This is the default behavior for struct fields not tagged with `envpop`.)
+
+	Recognized options:
+
+	* force - Existing field values that are non-empty strings or non-nil pointers are normally skipped; this option always replaces them.
+	* allow_empty - Normally no replacement will be performed if the specified variable is undefined/not found.
+					This option allows an empty	string to be used instead.
+					Not very useful for string fields, but potentially useful for string pointer fields.
+
+	e.g.:
+
+		struct{
+			// If this is an empty string, it will be replaced with the value of $CWD.
+			CurrentDir string `envpop:"CWD"`
+			// This would only populate with $USER if the pointer is nil.
+			UserName *string `envpop:"USER"`
+			// This will *always* replace the field's value with the value of $DISPLAY,
+			// even if not an empty string.
+			// Note the `force` option.
+			Display string `envpop:"DISPLAY,force"`
+			// Likewise, even if not nil, this field's value would be replaced with the value of $SHELL.
+			Shell *string `envpop:"SHELL,force"`
+			// This field will be untouched if non-nil, otherwise it will be a pointer to an empty string
+			// if FOOBAR is undefined.
+			NonExistentVar *string `envpop:"FOOBAR,allow_empty"`
+		}
+
+	If s is nil, nothing will be done and err will be errs.ErrNilPtr.
+	If s is not a pointer to a struct, nothing will be done and err will be errs.ErrBadType.
+*/
+func PopulateStruct[T any](s T) (err error) {
+
+	var structVal reflect.Value
+	var structType reflect.Type
+	var field reflect.StructField
+	var fieldVal reflect.Value
+	var tagVal string
+	var valSplit []string
+	var varNm string
+	var varVal string
+	var optsMap map[string]bool
+	var force bool
+	var allowEmpty bool
+	var defined bool
+
+	if reflect.TypeOf(s).Kind() != reflect.Ptr {
+		err = errs.ErrBadType
+		return
+	}
+
+	structVal = reflect.ValueOf(s)
+	if structVal.IsNil() || structVal.IsZero() || !structVal.IsValid() {
+		err = errs.ErrNilPtr
+		return
+	}
+
+	structVal = reflect.ValueOf(s).Elem()
+	structType = structVal.Type()
+
+	if structType.Kind() != reflect.Struct {
+		err = errs.ErrBadType
+		return
+	}
+
+	for i := 0; i < structVal.NumField(); i++ {
+		field = structType.Field(i)
+		fieldVal = structVal.Field(i)
+
+		// Skip explicitly skipped or non-tagged fields.
+		tagVal = field.Tag.Get(StructTagPopulate)
+		if tagVal == "" || strings.TrimSpace(tagVal) == "-" || strings.HasPrefix(tagVal, "-,") {
+			continue
+		}
+
+		fieldVal = structVal.Field(i)
+		if fieldVal.Kind() != reflect.Ptr && fieldVal.Kind() != reflect.String {
+			continue
+		}
+
+		optsMap = make(map[string]bool)
+		valSplit = strings.Split(tagVal, ",")
+		if valSplit == nil || len(valSplit) == 0 {
+			continue
+		}
+		varNm = valSplit[0]
+		if strings.TrimSpace(varNm) == "" {
+			continue
+		}
+		if len(valSplit) >= 2 {
+			for _, o := range valSplit[1:] {
+				optsMap[o] = true
+			}
+		}
+		force = optsMap["force"]
+		allowEmpty = optsMap["allow_empty"]
+
+		// if !force && (!fieldVal.IsNil() && !fieldVal.IsZero()) {
+		if !force && !fieldVal.IsZero() {
+			continue
+		}
+
+		if fieldVal.Kind() == reflect.Ptr {
+			if field.Type.Elem().Kind() != reflect.String {
+				continue
+			}
+		}
+		if !fieldVal.CanSet() {
+			continue
+		}
+
+		varVal, defined = os.LookupEnv(varNm)
+		if !defined && !allowEmpty {
+			continue
+		}
+
+		switch fieldVal.Kind() {
+		case reflect.Ptr:
+			fieldVal.Set(reflect.ValueOf(&varVal))
+		case reflect.String:
+			fieldVal.SetString(varVal)
+		}
+	}
+
+	return
+}
+
+// interpolateMap is used by Interpolate and interpolateReflect for maps. v should be a reflect.Value of a map.
+func interpolateMap(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var kVal reflect.Value
+	var newMap reflect.Value
+	var wg sync.WaitGroup
+	var numJobs int
+	var errChan chan error
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	if kind != reflect.Map {
+		err = errs.ErrBadType
+		return
+	}
+
+	if v.IsNil() || v.IsZero() || !v.IsValid() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+	if tagOpts != nil && len(tagOpts) > 0 {
+		for _, opt := range tagOpts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+	if valOpts.noMapKey && valOpts.noMapVal {
+		return
+	}
+
+	numJobs = v.Len()
+	errChan = make(chan error, numJobs)
+	wg.Add(numJobs)
+
+	newMap = reflect.MakeMap(reflect.TypeOf(v.Interface()))
+
+	for _, e := range v.MapKeys() {
+		kVal = e
+
+		go func(mapK reflect.Value) {
+			var mapErr error
+			var newKey reflect.Value
+			var newVal reflect.Value
+			var vVal reflect.Value = v.MapIndex(mapK)
+
+			defer wg.Done()
+
+			if !valOpts.noMapKey {
+				newKey = reflect.New(reflect.TypeOf(mapK.Interface()))
+				newKey.Set(vVal)
+				if mapK.Kind() == reflect.String {
+					if mapErr = interpolateStringReflect(newKey, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				} else {
+					if mapErr = interpolateValue(newKey, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				}
+			} else {
+				newKey = mapK
+			}
+			if !valOpts.noMapVal {
+				newVal = reflect.New(vVal.Type())
+				newVal.Set(vVal)
+				if vVal.Kind() == reflect.String {
+					if mapErr = interpolateStringReflect(newVal, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				} else {
+					if mapErr = interpolateValue(newVal, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				}
+			} else {
+				newVal = vVal
+			}
+			newMap.SetMapIndex(reflect.ValueOf(newKey), reflect.ValueOf(newVal))
+		}(kVal)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	v.Set(newMap)
+
+	return
+}
+
+// interpolateSlice is used by Interpolate and interpolateReflect for slices. v should be a reflect.Value of a slice.
+func interpolateSlice(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var wg sync.WaitGroup
+	var errChan chan error
+	var numJobs int
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	return
+
+	if kind != reflect.Slice {
+		err = errs.ErrBadType
+		return
+	}
+
+	if v.IsNil() || v.IsZero() || !v.IsValid() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+
+	numJobs = v.Len()
+	errChan = make(chan error, numJobs)
+	wg.Add(numJobs)
+
+	for i := 0; i < v.Len(); i++ {
+		go func(idx int) {
+			var sErr error
+			var newVal reflect.Value
+
+			defer wg.Done()
+
+			newVal = reflect.New(v.Index(idx).Type())
+			newVal.Set(v.Index(idx))
+			if v.Index(idx).Kind() == reflect.String {
+				if sErr = interpolateStringReflect(newVal, opts, tagOpts); sErr != nil {
+					errChan <- sErr
+					return
+				}
+			} else {
+				if sErr = interpolateValue(newVal, opts, tagOpts); sErr != nil {
+					errChan <- sErr
+					return
+				}
+			}
+			v.Index(idx).Set(reflect.ValueOf(newVal))
+		}(i)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	return
+}
+
+// interpolateStringReflect is used for structs/nested strings using reflection.
+func interpolateStringReflect(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var strVal string
+
+	if strVal, err = interpolateString(v.String()); err != nil {
+		return
+	}
+
+	v.Set(reflect.ValueOf(strVal).Convert(v.Type()))
+
+	return
+}
+
+// interpolateStruct is used by Interpolate and interpolateReflect for structs. v should be a reflect.Value of a struct.
+func interpolateStruct(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var field reflect.StructField
+	var fieldVal reflect.Value
+	var wg sync.WaitGroup
+	var errChan chan error
+	var numJobs int
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+
+	if kind != reflect.Struct {
+		err = errs.ErrBadType
+		return
+	}
+
+	numJobs = v.NumField()
+	wg.Add(numJobs)
+	errChan = make(chan error, numJobs)
+
+	for i := 0; i < v.NumField(); i++ {
+		field = t.Field(i)
+		fieldVal = v.Field(i)
+
+		go func(f reflect.StructField, fv reflect.Value) {
+			var fErr error
+
+			defer wg.Done()
+
+			if fErr = interpolateStructField(f, fv, opts, nil); fErr != nil {
+				errChan <- fErr
+				return
+			}
+		}(field, fieldVal)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	return
+}
+
+// interpolateStructField interpolates a struct field.
+func interpolateStructField(field reflect.StructField, v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var tagVal string
+	// var ftKind reflect.Kind = field.Type.Kind()
+	var parsedTagOpts map[string]bool
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	if !v.CanSet() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+
+	// Skip if explicitly instructed to do so.
+	tagVal = field.Tag.Get(StructTagInterpolate)
+	parsedTagOpts = internal.StringToMapBool(tagVal)
+	if parsedTagOpts["-"] {
+		return
+	}
+
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+
+	if v.Kind() == reflect.Ptr {
+		err = interpolateStructField(field, v.Elem(), opts, tagOpts)
+	} else {
+		err = interpolateValue(v, opts, tagOpts)
+	}
+
+	return
+}
+
+// interpolateValue is a dispatcher for a reflect value.
+func interpolateValue(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var kind reflect.Kind = v.Kind()
+
+	switch kind {
+	case reflect.Ptr:
+		if v.IsNil() || v.IsZero() || !v.IsValid() {
+			return
+		}
+		v = v.Elem()
+		if err = interpolateValue(v, opts, tagOpts); err != nil {
+			return
+		}
+	case reflect.String:
+		if err = interpolateStringReflect(v, opts, tagOpts); err != nil {
+			return
+		}
+		return
+	case reflect.Slice:
+		if err = interpolateSlice(v, opts, tagOpts); err != nil {
+		}
+	case reflect.Map:
+		if err = interpolateMap(v, opts, tagOpts); err != nil {
+			return
+		}
+	case reflect.Struct:
+		if err = interpolateStruct(v, opts, tagOpts); err != nil {
+			return
+		}
+	}
+
+	return
+}
diff --git a/envs/funcs.go.old b/envs/funcs.go.old
new file mode 100644
index 0000000..33cf3ce
--- /dev/null
+++ b/envs/funcs.go.old
@@ -0,0 +1,825 @@
+package envs
+
+import (
+	`bytes`
+	`errors`
+	`fmt`
+	`io/ioutil`
+	`os`
+	`reflect`
+	`strings`
+	`sync`
+
+	`r00t2.io/goutils/multierr`
+	`r00t2.io/sysutils/errs`
+	`r00t2.io/sysutils/internal`
+	`r00t2.io/sysutils/paths`
+)
+
+// GetEnvMap returns a map of all environment variables. All values are strings.
+func GetEnvMap() (envVars map[string]string) {
+
+	var envList []string = os.Environ()
+
+	envVars = envListToMap(envList)
+
+	return
+}
+
+/*
+	GetEnvMapNative returns a map of all environment variables, but attempts to "nativize" them.
+	All values are interfaces. It is up to the caller to typeswitch them to proper types.
+
+	Note that the PATH/Path environment variable (for *Nix and Windows, respectively) will be
+	a []string (as per GetPathEnv). No other env vars, even if they contain os.PathListSeparator,
+	will be transformed to a slice or the like.
+	If an error occurs during parsing the path env var, it will be rendered	as a string.
+
+	All number types will attempt to be their 64-bit version (i.e. int64, uint64, float64, etc.).
+
+	If a type cannot be determined for a value, its string form will be used
+	(as it would be found in GetEnvMap).
+*/
+func GetEnvMapNative() (envMap map[string]interface{}) {
+
+	var stringMap map[string]string = GetEnvMap()
+
+	envMap = nativizeEnvMap(stringMap)
+
+	return
+}
+
+/*
+	GetFirst gets the first instance if populated/set occurrence of varNames.
+
+	For example, if you have three potential env vars, FOO, FOOBAR, FOOBARBAZ,
+	and want to follow the logic flow of:
+
+		1.) Check if FOO is set. If not,
+		2.) Check if FOOBAR is set. If not,
+		3.) Check if FOOBARBAZ is set.
+
+	Then this would be specified as:
+
+		GetFirst([]string{"FOO", "FOOBAR", "FOOBARBAZ"})
+
+	If val is "" and ok is true, this means that one of the specified variable names IS
+	set but is set to an empty value. If ok is false, none of the specified variables
+	are set.
+
+	It is a thin wrapper around GetFirstWithRef.
+*/
+func GetFirst(varNames []string) (val string, ok bool) {
+
+	val, ok, _ = GetFirstWithRef(varNames)
+
+	return
+}
+
+/*
+	GetFirstWithRef behaves exactly like GetFirst, but with an additional returned value, idx,
+	which specifies the index in varNames in which a set variable was found. e.g. if:
+
+		GetFirstWithRef([]string{"FOO", "FOOBAR", "FOOBAZ"})
+
+	is called and FOO is not set but FOOBAR is, idx will be 1.
+
+	If ok is false, idx will always be -1 and should be ignored.
+*/
+func GetFirstWithRef(varNames []string) (val string, ok bool, idx int) {
+
+	idx = -1
+
+	for i, vn := range varNames {
+		if HasEnv(vn) {
+			ok = true
+			idx = i
+			val = os.Getenv(vn)
+			return
+		}
+	}
+
+	return
+}
+
+// GetPathEnv returns a slice of the PATH variable's items.
+func GetPathEnv() (pathList []string, err error) {
+
+	var pathVar string = internal.GetPathEnvName()
+
+	pathList = make([]string, 0)
+
+	for _, p := range strings.Split(os.Getenv(pathVar), string(os.PathListSeparator)) {
+		if err = paths.RealPath(&p); err != nil {
+			return
+		}
+		pathList = append(pathList, p)
+	}
+	return
+}
+
+/*
+	GetPidEnvMap will only work on *NIX-like systems with procfs.
+	It gets the environment variables of a given process' PID.
+*/
+func GetPidEnvMap(pid uint32) (envMap map[string]string, err error) {
+
+	var envBytes []byte
+	var envList []string
+	var envArr [][]byte
+	var procPath string
+	var exists bool
+
+	envMap = make(map[string]string, 0)
+
+	procPath = fmt.Sprintf("/proc/%v/environ", pid)
+
+	if exists, err = paths.RealPathExists(&procPath); err != nil {
+		return
+	}
+	if !exists {
+		err = errors.New(fmt.Sprintf("information for pid %v does not exist", pid))
+		return
+	}
+
+	if envBytes, err = ioutil.ReadFile(procPath); err != nil {
+		return
+	}
+
+	envArr = bytes.Split(envBytes, []byte{0x0})
+	envList = make([]string, len(envArr))
+	for idx, b := range envArr {
+		envList[idx] = string(b)
+	}
+
+	envMap = envListToMap(envList)
+
+	return
+}
+
+/*
+	GetPidEnvMapNative, like GetEnvMapNative, returns a map of all environment variables, but attempts to "nativize" them.
+	All values are interfaces. It is up to the caller to typeswitch them to proper types.
+
+	See the documentation for GetEnvMapNative for details.
+*/
+func GetPidEnvMapNative(pid uint32) (envMap map[string]interface{}, err error) {
+
+	var stringMap map[string]string
+
+	if stringMap, err = GetPidEnvMap(pid); err != nil {
+		return
+	}
+
+	envMap = nativizeEnvMap(stringMap)
+
+	return
+}
+
+/*
+	HasEnv is much like os.LookupEnv, but only returns a boolean for
+	if the environment variable key exists or not.
+
+	This is useful anywhere you may need to set a boolean in a func call
+	depending on the *presence* of an env var or not.
+*/
+func HasEnv(key string) (envIsSet bool) {
+
+	_, envIsSet = os.LookupEnv(key)
+
+	return
+}
+
+/*
+	Interpolate takes one of:
+
+		- a string (pointer only)
+		- a struct (pointer only)
+		- a map (comprised of the same requirements)
+		- a slice (comprised of the same requirements)
+
+	and performs variable substitution on strings from environment variables.
+
+	It supports both UNIX/Linux/POSIX syntax formats (e.g. $VARNAME, ${VARNAME}) and,
+	if on Windows, it *additionally* supports the EXPAND_SZ format (e.g. %VARNAME%).
+
+	For structs, the tag name used can be changed by setting the StructTagInterpolate
+	variable in this submodule; the default is `envsub`.
+	If the tag value is "-", the field will be skipped.
+	For map fields within structs, the default is to apply interpolation to both keys and values;
+	this can be changed with the `no_map_key` and `no_map_value` options (tag values).
+	Any other tag value(s) are ignored.
+
+	For maps and slices, Interpolate will recurse into values (e.g. [][]string will work as expected).
+
+	Supported struct tag options:
+
+		* `no_map_key` - Do not operate on map keys if they are strings or string pointers.
+						 See also InterpolateOptNoMapKey.
+		* `no_map_value` - Do not operate on map values if they are strings or string pointers.
+						   See also InterpolateOptNoMapValue.
+
+	If s is nil, no interpolation will be performed. No error will be returned.
+	If s is not a valid/supported type, no interpolation will be performed. No error will be returned.
+*/
+func Interpolate[T any](s T, opts ...optInterpolate) (err error) {
+
+	var sVal reflect.Value = reflect.ValueOf(s)
+	var sType reflect.Type = sVal.Type()
+	var kind reflect.Kind = sType.Kind()
+	var ptrVal reflect.Value
+	var ptrType reflect.Type
+	var ptrKind reflect.Kind
+
+	switch kind {
+	case reflect.Ptr:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		ptrVal = sVal.Elem()
+		ptrType = ptrVal.Type()
+		ptrKind = ptrType.Kind()
+		if ptrKind == reflect.String {
+			err = interpolateStringReflect(ptrVal, opts, nil)
+		} else {
+			// Otherwise, it should be a struct ptr.
+			if ptrKind != reflect.Struct {
+				return
+			}
+			err = interpolateStruct(ptrVal, opts, nil)
+		}
+	case reflect.Map:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		err = interpolateMap(sVal, opts, nil)
+	case reflect.Slice:
+		if sVal.IsNil() || sVal.IsZero() || !sVal.IsValid() {
+			return
+		}
+		err = interpolateSlice(sVal, opts, nil)
+		/*
+			case reflect.Struct:
+				if sVal.IsZero() || !sVal.IsValid() {
+					return
+				}
+				err = interpolateStruct(sVal, opts, nil)
+
+		*/
+	}
+
+	return
+}
+
+/*
+	InterpolateString takes (a pointer to) a struct or string and performs variable substitution on it
+	from environment variables.
+
+	It supports both UNIX/Linux/POSIX syntax formats (e.g. $VARNAME, ${VARNAME}) and,
+	if on Windows, it *additionally* supports the EXPAND_SZ format (e.g. %VARNAME%).
+
+	If s is nil, nothing will be done and err will be ErrNilPtr.
+
+	This is a standalone function that is much more performant than Interpolate
+	at the cost of rigidity.
+*/
+func InterpolateString(s *string) (err error) {
+
+	var newStr string
+
+	if s == nil {
+		err = errs.ErrNilPtr
+		return
+	}
+
+	if newStr, err = interpolateString(*s); err != nil {
+		return
+	}
+	*s = newStr
+
+	return
+}
+
+/*
+	PopulateStruct takes (a pointer to) a struct and performs *population* on it.
+	Unlike the InterpolateStruct function, this *completely populates* (or *replaces*)
+	a field's value with the specified environment variable; no *substitution* is performed.
+
+	You can change the tag name used by changing the StructTagPopulate variable in this module;
+	the default is `envpop`.
+
+	Tag value format:
+		<tag>:"<VAR NAME>[,<option>,<option>...]"
+	e.g.
+		envpop:"SOMEVAR"
+		envpop:"OTHERVAR,force"
+		envpop:"OTHERVAR,allow_empty"
+		envpop:"OTHERVAR,force,allow_empty"
+
+	If the tag value is "-", or <VAR NAME> is not provided, the field will be explicitly skipped.
+	(This is the default behavior for struct fields not tagged with `envpop`.)
+
+	Recognized options:
+
+	* force - Existing field values that are non-empty strings or non-nil pointers are normally skipped; this option always replaces them.
+	* allow_empty - Normally no replacement will be performed if the specified variable is undefined/not found.
+					This option allows an empty	string to be used instead.
+					Not very useful for string fields, but potentially useful for string pointer fields.
+
+	e.g.:
+
+		struct{
+			// If this is an empty string, it will be replaced with the value of $CWD.
+			CurrentDir string `envpop:"CWD"`
+			// This would only populate with $USER if the pointer is nil.
+			UserName *string `envpop:"USER"`
+			// This will *always* replace the field's value with the value of $DISPLAY,
+			// even if not an empty string.
+			// Note the `force` option.
+			Display string `envpop:"DISPLAY,force"`
+			// Likewise, even if not nil, this field's value would be replaced with the value of $SHELL.
+			Shell *string `envpop:"SHELL,force"`
+			// This field will be untouched if non-nil, otherwise it will be a pointer to an empty string
+			// if FOOBAR is undefined.
+			NonExistentVar *string `envpop:"FOOBAR,allow_empty"`
+		}
+
+	If s is nil, nothing will be done and err will be errs.ErrNilPtr.
+	If s is not a pointer to a struct, nothing will be done and err will be errs.ErrBadType.
+*/
+func PopulateStruct[T any](s T) (err error) {
+
+	var structVal reflect.Value
+	var structType reflect.Type
+	var field reflect.StructField
+	var fieldVal reflect.Value
+	var tagVal string
+	var valSplit []string
+	var varNm string
+	var varVal string
+	var optsMap map[string]bool
+	var force bool
+	var allowEmpty bool
+	var defined bool
+
+	if reflect.TypeOf(s).Kind() != reflect.Ptr {
+		err = errs.ErrBadType
+		return
+	}
+
+	structVal = reflect.ValueOf(s)
+	if structVal.IsNil() || structVal.IsZero() || !structVal.IsValid() {
+		err = errs.ErrNilPtr
+		return
+	}
+
+	structVal = reflect.ValueOf(s).Elem()
+	structType = structVal.Type()
+
+	if structType.Kind() != reflect.Struct {
+		err = errs.ErrBadType
+		return
+	}
+
+	for i := 0; i < structVal.NumField(); i++ {
+		field = structType.Field(i)
+		fieldVal = structVal.Field(i)
+
+		// Skip explicitly skipped or non-tagged fields.
+		tagVal = field.Tag.Get(StructTagPopulate)
+		if tagVal == "" || strings.TrimSpace(tagVal) == "-" || strings.HasPrefix(tagVal, "-,") {
+			continue
+		}
+
+		fieldVal = structVal.Field(i)
+		if fieldVal.Kind() != reflect.Ptr && fieldVal.Kind() != reflect.String {
+			continue
+		}
+
+		optsMap = make(map[string]bool)
+		valSplit = strings.Split(tagVal, ",")
+		if valSplit == nil || len(valSplit) == 0 {
+			continue
+		}
+		varNm = valSplit[0]
+		if strings.TrimSpace(varNm) == "" {
+			continue
+		}
+		if len(valSplit) >= 2 {
+			for _, o := range valSplit[1:] {
+				optsMap[o] = true
+			}
+		}
+		force = optsMap["force"]
+		allowEmpty = optsMap["allow_empty"]
+
+		// if !force && (!fieldVal.IsNil() && !fieldVal.IsZero()) {
+		if !force && !fieldVal.IsZero() {
+			continue
+		}
+
+		if fieldVal.Kind() == reflect.Ptr {
+			if field.Type.Elem().Kind() != reflect.String {
+				continue
+			}
+		}
+		if !fieldVal.CanSet() {
+			continue
+		}
+
+		varVal, defined = os.LookupEnv(varNm)
+		if !defined && !allowEmpty {
+			continue
+		}
+
+		switch fieldVal.Kind() {
+		case reflect.Ptr:
+			fieldVal.Set(reflect.ValueOf(&varVal))
+		case reflect.String:
+			fieldVal.SetString(varVal)
+		}
+	}
+
+	return
+}
+
+// interpolateMap is used by Interpolate and interpolateReflect for maps. v should be a reflect.Value of a map.
+func interpolateMap(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var kVal reflect.Value
+	var vVal reflect.Value
+	var mIter *reflect.MapIter
+	var newMap reflect.Value
+	var wg sync.WaitGroup
+	var numJobs int
+	var errChan chan error
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	if kind != reflect.Map {
+		err = errs.ErrBadType
+		return
+	}
+
+	if v.IsNil() || v.IsZero() || !v.IsValid() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+	if tagOpts != nil && len(tagOpts) > 0 {
+		for _, opt := range tagOpts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+	if valOpts.noMapKey && valOpts.noMapVal {
+		return
+	}
+
+	numJobs = v.Len()
+	errChan = make(chan error, numJobs)
+	wg.Add(numJobs)
+
+	newMap = reflect.MakeMap(reflect.TypeOf(v.Interface()))
+
+	mIter = v.MapRange()
+	for mIter.Next() {
+		kVal = mIter.Key()
+		vVal = mIter.Value()
+		go func(mapK, mapV reflect.Value) {
+			var mapErr error
+			var newKey reflect.Value
+			var newVal reflect.Value
+
+			defer wg.Done()
+
+			if !valOpts.noMapKey {
+				/*
+					if mapK.Kind() == reflect.String {
+						if mapErr = interpolateStringReflect(mapK, opts, nil); mapErr != nil {
+							errChan <- mapErr
+							return
+						}
+					} else {
+						if mapErr = interpolateValue(mapK, opts, nil); mapErr != nil {
+							errChan <- mapErr
+							return
+						}
+					}
+				*/
+				newKey = reflect.New(mapK.Type())
+				newKey.Set(mapK)
+				if mapK.Kind() == reflect.String {
+					if mapErr = interpolateStringReflect(newKey, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				} else {
+					if mapErr = interpolateValue(newKey, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				}
+			} else {
+				newKey = mapK
+			}
+			if !valOpts.noMapVal {
+				/*
+					if mapV.Kind() == reflect.String {
+						if mapErr = interpolateStringReflect(mapV, opts, nil); mapErr != nil {
+							errChan <- mapErr
+							return
+						}
+					} else {
+						if mapErr = interpolateValue(mapV, opts, nil); mapErr != nil {
+							errChan <- mapErr
+							return
+						}
+					}
+				*/
+				newVal = reflect.New(mapV.Type())
+				newVal.Set(mapV)
+				if mapV.Kind() == reflect.String {
+					if mapErr = interpolateStringReflect(newVal, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				} else {
+					if mapErr = interpolateValue(newVal, opts, nil); mapErr != nil {
+						errChan <- mapErr
+						return
+					}
+				}
+			} else {
+				newVal = mapV
+			}
+			newMap.SetMapIndex(reflect.ValueOf(newKey), reflect.ValueOf(newVal))
+		}(kVal, vVal)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	v.Set(newMap)
+
+	return
+}
+
+// interpolateSlice is used by Interpolate and interpolateReflect for slices. v should be a reflect.Value of a slice.
+func interpolateSlice(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var wg sync.WaitGroup
+	var errChan chan error
+	var numJobs int
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	return
+
+	if kind != reflect.Slice {
+		err = errs.ErrBadType
+		return
+	}
+
+	if v.IsNil() || v.IsZero() || !v.IsValid() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+
+	numJobs = v.Len()
+	errChan = make(chan error, numJobs)
+	wg.Add(numJobs)
+
+	for i := 0; i < v.Len(); i++ {
+		go func(idx int) {
+			var sErr error
+			var newVal reflect.Value
+
+			defer wg.Done()
+
+			newVal = reflect.New(v.Index(idx).Type())
+			newVal.Set(v.Index(idx))
+			if v.Index(idx).Kind() == reflect.String {
+				if sErr = interpolateStringReflect(newVal, opts, tagOpts); sErr != nil {
+					errChan <- sErr
+					return
+				}
+			} else {
+				if sErr = interpolateValue(newVal, opts, tagOpts); sErr != nil {
+					errChan <- sErr
+					return
+				}
+			}
+			v.Index(idx).Set(reflect.ValueOf(newVal))
+		}(i)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	return
+}
+
+// interpolateStringReflect is used for structs/nested strings using reflection.
+func interpolateStringReflect(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var strVal string
+
+	if strVal, err = interpolateString(v.String()); err != nil {
+		return
+	}
+
+	v.Set(reflect.ValueOf(strVal).Convert(v.Type()))
+
+	return
+}
+
+// interpolateStruct is used by Interpolate and interpolateReflect for structs. v should be a reflect.Value of a struct.
+func interpolateStruct(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var field reflect.StructField
+	var fieldVal reflect.Value
+	var wg sync.WaitGroup
+	var errChan chan error
+	var numJobs int
+	var doneChan chan bool = make(chan bool, 1)
+	var mErr *multierr.MultiError = multierr.NewMultiError(nil)
+	var t reflect.Type = v.Type()
+	var kind reflect.Kind = t.Kind()
+
+	if kind != reflect.Struct {
+		err = errs.ErrBadType
+		return
+	}
+
+	numJobs = v.NumField()
+	wg.Add(numJobs)
+	errChan = make(chan error, numJobs)
+
+	for i := 0; i < v.NumField(); i++ {
+		field = t.Field(i)
+		fieldVal = v.Field(i)
+
+		go func(f reflect.StructField, fv reflect.Value) {
+			var fErr error
+
+			defer wg.Done()
+
+			if fErr = interpolateStructField(f, fv, opts, nil); fErr != nil {
+				errChan <- fErr
+				return
+			}
+		}(field, fieldVal)
+	}
+
+	go func() {
+		wg.Wait()
+		close(errChan)
+		doneChan <- true
+	}()
+
+	<-doneChan
+
+	for i := 0; i < numJobs; i++ {
+		if err = <-errChan; err != nil {
+			mErr.AddError(err)
+			err = nil
+		}
+	}
+
+	if !mErr.IsEmpty() {
+		err = mErr
+		return
+	}
+
+	return
+}
+
+// interpolateStructField interpolates a struct field.
+func interpolateStructField(field reflect.StructField, v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var tagVal string
+	// var ftKind reflect.Kind = field.Type.Kind()
+	var parsedTagOpts map[string]bool
+	var valOpts *interpolateOpts = new(interpolateOpts)
+
+	if !v.CanSet() {
+		return
+	}
+
+	*valOpts = defaultInterpolateOpts
+
+	// Skip if explicitly instructed to do so.
+	tagVal = field.Tag.Get(StructTagInterpolate)
+	parsedTagOpts = internal.StringToMapBool(tagVal)
+	if parsedTagOpts["-"] {
+		return
+	}
+
+	if opts != nil && len(opts) > 0 {
+		for _, opt := range opts {
+			if err = opt(valOpts); err != nil {
+				return
+			}
+		}
+	}
+
+	if v.Kind() == reflect.Ptr {
+		err = interpolateStructField(field, v.Elem(), opts, tagOpts)
+	} else {
+		err = interpolateValue(v, opts, tagOpts)
+	}
+
+	return
+}
+
+// interpolateValue is a dispatcher for a reflect value.
+func interpolateValue(v reflect.Value, opts []optInterpolate, tagOpts []optInterpolate) (err error) {
+
+	var kind reflect.Kind = v.Kind()
+
+	switch kind {
+	case reflect.Ptr:
+		if v.IsNil() || v.IsZero() || !v.IsValid() {
+			return
+		}
+		v = v.Elem()
+		if err = interpolateValue(v, opts, tagOpts); err != nil {
+			return
+		}
+	case reflect.String:
+		if err = interpolateStringReflect(v, opts, tagOpts); err != nil {
+			return
+		}
+		return
+	case reflect.Slice:
+		if err = interpolateSlice(v, opts, tagOpts); err != nil {
+		}
+	case reflect.Map:
+		if err = interpolateMap(v, opts, tagOpts); err != nil {
+			return
+		}
+	case reflect.Struct:
+		if err = interpolateStruct(v, opts, tagOpts); err != nil {
+			return
+		}
+	}
+
+	return
+}
diff --git a/envs/funcs_test.go b/envs/funcs_test.go
new file mode 100644
index 0000000..cdb1824
--- /dev/null
+++ b/envs/funcs_test.go
@@ -0,0 +1,182 @@
+package envs
+
+import (
+	`errors`
+	`os`
+	`testing`
+	`time`
+
+	`github.com/davecgh/go-spew/spew`
+	`r00t2.io/sysutils/errs`
+)
+
+type (
+	testCustom string
+	testStruct struct {
+		Hello         string  `envpop:"USER"`
+		HelloPtr      *string `envpop:"USER"`
+		HelloForce    string  `envpop:"USER,force"`
+		HelloPtrForce *string `envpop:"USER,force"`
+		HelloNo       string  `envsub:"-" envpop:"-"`
+		HelloNoPtr    *string `envsub:"-" envpop:"-"`
+		BadType       int
+		NilField      *string `envpop:"NONEXISTENTBADVAR,allow_empty"`
+		PtrInt        *int
+		Custom        testCustom
+		MapStr        map[string]string
+		SliceStr      []string
+		SliceSlice    [][]string
+		SliceMap      []map[string]string
+		SliceStruct   []testStruct
+	}
+)
+
+func TestInterpolateString(t *testing.T) {
+
+	var err error
+	var s string = "My username is ${USER}; hello!"
+	var sp *string = &s
+
+	if err = InterpolateString(&s); err != nil {
+		t.Fatalf("Failed interpolation: %v", err)
+	}
+	t.Logf("String test passed:\n%v", s)
+
+	if err = InterpolateString(sp); err != nil {
+		t.Fatalf("Failed interpolation: %v", err)
+	}
+	t.Logf("String pointer test passed:\n%v", *sp)
+}
+
+func TestInterpolateStruct(t *testing.T) {
+
+	var err error
+
+	for _, i := range []interface{}{
+		"i am ${USER}, it is ${CURDATETIME}",
+		new(string),
+		/*
+			testStruct{
+				Hello:         "i am ${USER}, it is ${CURDATETIME}",
+				HelloPtr:      new(string),
+				HelloForce:    "i am ${USER}, it is ${CURDATETIME}",
+				HelloPtrForce: new(string),
+				HelloNo:       "i am ${USER}, it is ${CURDATETIME}",
+				HelloNoPtr:    new(string),
+				BadType:       4,
+				NilField:      nil,
+				PtrInt:        new(int),
+				Custom:        testCustom("i am ${USER}, it is ${CURDATETIME}"),
+				SliceStr:      []string{"i am ${USER}, it is ${CURDATETIME}"},
+				SliceSlice:    [][]string{[]string{"i am ${USER}, it is ${CURDATETIME}"}},
+				SliceMap:      []map[string]string{map[string]string{"i am ${USER} key": "i am ${USER} value, it is ${CURDATETIME}"}},
+				SliceStruct: []testStruct{
+					{
+						Hello:         "i am nested ${USER}, it is ${CURDATETIME}",
+						HelloPtr:      nil,
+						HelloForce:    "i am nested ${USER}, it is ${CURDATETIME}",
+						HelloPtrForce: nil,
+						HelloNo:       "i am nested ${USER}, it is ${CURDATETIME}",
+						HelloNoPtr:    nil,
+						BadType:       0,
+						NilField:      nil,
+						PtrInt:        nil,
+						Custom:        testCustom("i am nested ${USER}, it is ${CURDATETIME}"),
+						SliceStr:      []string{"i am nested ${USER}, it is ${CURDATETIME}"},
+						SliceSlice:    [][]string{[]string{"i am nested ${USER}, it is ${CURDATETIME}"}},
+						SliceMap:      []map[string]string{map[string]string{"i am nested ${USER} key": "i am ${USER} value, it is ${CURDATETIME}"}},
+						SliceStruct:   nil,
+					},
+				},
+			},
+		*/
+		&testStruct{
+			Hello:         "i am ${USER}, it is ${CURDATETIME}",
+			HelloPtr:      new(string),
+			HelloForce:    "i am ${USER}, it is ${CURDATETIME}",
+			HelloPtrForce: new(string),
+			HelloNo:       "i am ${USER}, it is ${CURDATETIME}",
+			HelloNoPtr:    new(string),
+			BadType:       4,
+			NilField:      nil,
+			PtrInt:        new(int),
+			Custom:        testCustom("i am ${USER}, it is ${CURDATETIME}"),
+			MapStr:        map[string]string{"i am ${USER} key": "i am ${USER} value, it is ${CURDATETIME}"},
+			SliceStr:      []string{"i am ${USER}, it is ${CURDATETIME}"},
+			SliceSlice:    [][]string{[]string{"i am ${USER}, it is ${CURDATETIME}"}},
+			SliceMap:      []map[string]string{map[string]string{"i am ${USER} key": "i am ${USER} value, it is ${CURDATETIME}"}},
+			SliceStruct: []testStruct{
+				{
+					Hello:         "i am nested ${USER}, it is ${CURDATETIME}",
+					HelloPtr:      nil,
+					HelloForce:    "i am nested ${USER}, it is ${CURDATETIME}",
+					HelloPtrForce: nil,
+					HelloNo:       "i am nested ${USER}, it is ${CURDATETIME}",
+					HelloNoPtr:    nil,
+					BadType:       0,
+					NilField:      nil,
+					PtrInt:        nil,
+					Custom:        testCustom("i am nested ${USER}, it is ${CURDATETIME}"),
+					SliceStr:      []string{"i am nested ${USER}, it is ${CURDATETIME}"},
+					SliceSlice:    [][]string{[]string{"i am nested ${USER}, it is ${CURDATETIME}"}},
+					SliceMap:      []map[string]string{map[string]string{"i am nested ${USER} key": "i am ${USER} value, it is ${CURDATETIME}"}},
+					SliceStruct:   nil,
+				},
+			},
+		},
+	} {
+		if err = os.Setenv("CURDATETIME", time.Now().String()); err != nil {
+			t.Fatalf("Received error setting CURDATETIME: %v", err)
+		}
+		switch x := i.(type) {
+		case *string:
+			*x = "i am ${USER}, it is ${CURDATETIME}"
+		case testStruct:
+			*x.HelloPtr = x.Hello
+			*x.HelloPtrForce = x.HelloForce
+			*x.HelloNoPtr = x.HelloNo
+			*x.PtrInt = x.BadType
+		case *testStruct:
+			*x.HelloPtr = x.Hello
+			*x.HelloPtrForce = x.HelloForce
+			*x.HelloNoPtr = x.HelloNo
+			*x.PtrInt = x.BadType
+		}
+		t.Logf("Before (%T):\n%v", i, spew.Sdump(i))
+		if err = Interpolate(i); err != nil {
+			t.Fatalf("Failed interpolation: %v", err)
+		}
+		t.Logf("After (%T):\n%v\n", i, spew.Sdump(i))
+	}
+}
+
+func TestPopulateStruct(t *testing.T) {
+
+	var err error
+	var greet string = "My username is ${USER}; hello!"
+	var num int = 4
+	var sp *testStruct = &testStruct{
+		Hello:         greet,
+		HelloPtr:      &greet,
+		HelloForce:    greet,
+		HelloPtrForce: &greet,
+		HelloNo:       greet,
+		HelloNoPtr:    &greet,
+		BadType:       4,
+		PtrInt:        &num,
+	}
+
+	if err = PopulateStruct(sp); err != nil {
+		if errors.Is(err, errs.ErrNilPtr) {
+			err = nil
+			t.Logf("Detected nil.")
+		} else if errors.Is(err, errs.ErrBadType) {
+			err = nil
+			t.Log("Detected bad type.")
+		} else {
+			t.Fatalf("Failed interpolation: %v", err)
+		}
+	}
+	t.Logf("Evaluated:\n%v", spew.Sdump(sp))
+
+}
diff --git a/envs/types.go b/envs/types.go
new file mode 100644
index 0000000..02059c8
--- /dev/null
+++ b/envs/types.go
@@ -0,0 +1,10 @@
+package envs
+
+type (
+	interpolateOpts struct {
+		noMapKey bool
+		noMapVal bool
+		isTagged bool
+	}
+	optInterpolate func(o *interpolateOpts) (err error)
+)
diff --git a/envs/utils_nonwindows.go b/envs/utils_nonwindows.go
new file mode 100644
index 0000000..36dd765
--- /dev/null
+++ b/envs/utils_nonwindows.go
@@ -0,0 +1,18 @@
+//go:build !windows
+
+package envs
+
+import (
+	`os`
+)
+
+/*
+	interpolateString takes string s and performs environment variable interpolation/substitution on it.
+	err will always be nil; it's here for compat with the Windows equivalent.
+*/
+func interpolateString(s string) (subbed string, err error) {
+
+	subbed = os.ExpandEnv(s)
+
+	return
+}
diff --git a/envs/utils_windows.go b/envs/utils_windows.go
new file mode 100644
index 0000000..3634560
--- /dev/null
+++ b/envs/utils_windows.go
@@ -0,0 +1,15 @@
+//go:build windows
+
+package envs
+
+import (
+	"golang.org/x/sys/windows/registry"
+)
+
+// interpolateString takes string s and performs environment variable interpolation/substitution on it.
+func interpolateString(s string) (subbed string, err error) {
+
+	subbed, err = registry.ExpandString(os.ExpandEnv(s))
+
+	return
+}
diff --git a/errs/errs.go b/errs/errs.go
new file mode 100644
index 0000000..f41c224
--- /dev/null
+++ b/errs/errs.go
@@ -0,0 +1,10 @@
+package errs
+
+import (
+	`errors`
+)
+
+var (
+	ErrBadType error = errors.New("a bad type was passed")
+	ErrNilPtr  error = errors.New("a nil pointer was passed")
+)
diff --git a/exec_extra/consts.go b/exec_extra/consts.go
new file mode 100644
index 0000000..8ba33f2
--- /dev/null
+++ b/exec_extra/consts.go
@@ -0,0 +1,34 @@
+package exec_extra
+
+var (
+	StructTagCmdArgs string = "cmdarg"
+)
+
+var (
+	/*
+		CmdArgsOptPreferShort, if specified, prefers the "short" argument over "long" if both are specified.
+		The default is to prefer long.
+
+		Can be specified per-field via the `prefer_short` option (no value/value ignored).
+	*/
+	CmdArgsOptPreferShort cmdArgOpt = func(opts *cmdArgsOpts) (err error) {
+		opts.preferShort = true
+		return
+	}
+
+	/*
+		CmdArgsOptShortEquals, if specified, renders short flags *with* an equals sign
+		(if using POSIX args).
+
+		Has no effect if using Windows traditional syntax or if there is no value for the field.
+	*/
+	CmdArgsOptShortEquals cmdArgOpt = func(opts *cmdArgsOpts) (err error) {
+		opts.preferShort = true
+		return
+	}
+
+	CmdArgsOptLongNoEquals cmdArgOpt = func(opts *cmdArgsOpts) (err error) {
+		opts.preferShort = true
+		return
+	}
+)
diff --git a/exec_extra/consts_windows.go b/exec_extra/consts_windows.go
new file mode 100644
index 0000000..c750ead
--- /dev/null
+++ b/exec_extra/consts_windows.go
@@ -0,0 +1,45 @@
+package exec_extra
+
+var (
+	/*
+		CmdArgsOptForcePosix forces the resulting command string to use "POSIX-style" flag notation.
+
+		Traditionally, Windows used flags like `/f` instead of POSIX `-f`, `/c:value` instead of `-c value`
+		or `-c=value`, etc.
+
+		If this option is passed, either to GetCmdFromStruct() or for a specific field via the
+		tag defined by StructTagCmdArgs (option `force_posix`, no value/value ignored), then the
+		POSIX-style flag syntax will be used instead.
+
+		Note that on Windows runtime, the default is to use the traditional slash-based syntax.
+		If you are generating command strings for Powershell or third-party software, you probably
+		want to use this option.
+
+		See also the inverse of this option, CmdArgsOptForceNoPosix.
+	*/
+	CmdArgsOptForcePosix cmdArgOpt = func(opts *cmdArgsOpts) (err error) {
+		opts.forcePosix = true
+		return
+	}
+
+	/*
+		CmdArgsOptForceNoPosix forces the resulting command string to use "traditional Windows" flag notation.
+
+		Traditionally, Windows used flags like `/f` instead of POSIX `-f`, `/c:value` instead of `-c value`
+		or `-c=value`, etc.
+
+		If this option is passed, either to GetCmdFromStruct() or for a specific field via the
+		tag defined by StructTagCmdArgs (option `force_no_posix`, no value/value ignored), then the
+		Windows-style flag syntax will be used instead.
+
+		Note that on Windows runtime, the default is to use the traditional slash-based syntax.
+		If you are generating command strings for Powershell or third-party software, you probably
+		want to use CmdArgsOptForcePosix instead.
+
+		See also the inverse of this option, CmdArgsOptForcePosix.
+	*/
+	CmdArgsOptForceNoPosix cmdArgOpt = func(opts *cmdArgsOpts) (err error) {
+		opts.forcePosix = false
+		return
+	}
+)
diff --git a/exec_extra/func.go b/exec_extra/func.go
deleted file mode 100644
index cf9cb3e..0000000
--- a/exec_extra/func.go
+++ /dev/null
@@ -1,31 +0,0 @@
-/*
-   SysUtils - a library to assist with various system-related functions
-   Copyright (C) 2020  Brent Saner
-
-   This program is free software: you can redistribute it and/or modify
-   it under the terms of the GNU General Public License as published by
-   the Free Software Foundation, either version 3 of the License, or
-   (at your option) any later version.
-
-   This program is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-   GNU General Public License for more details.
-
-   You should have received a copy of the GNU General Public License
-   along with this program.  If not, see <https://www.gnu.org/licenses/>.
-*/
-
-package exec_extra
-
-import (
-	"os/exec"
-)
-
-func ExecCmdReturn(cmd *exec.Cmd) (exitStatus int, err error) {
-	// https://stackoverflow.com/a/55055100/733214
-	err = cmd.Run()
-	exitErr, _ := err.(*exec.ExitError)
-	exitStatus = exitErr.ExitCode()
-	return
-}
diff --git a/exec_extra/funcs.go b/exec_extra/funcs.go
new file mode 100644
index 0000000..37c9077
--- /dev/null
+++ b/exec_extra/funcs.go
@@ -0,0 +1,86 @@
+/*
+   SysUtils - a library to assist with various system-related functions
+   Copyright (C) 2020  Brent Saner
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+package exec_extra
+
+import (
+	"os/exec"
+)
+
+func ExecCmdReturn(cmd *exec.Cmd) (exitStatus int, err error) {
+	// https://stackoverflow.com/a/55055100/733214
+	err = cmd.Run()
+	exitErr, _ := err.(*exec.ExitError)
+	exitStatus = exitErr.ExitCode()
+	return
+}
+
+/*
+	GetCmdFromStruct takes (a pointer to) a struct and returns a slice of
+	strings compatible with os/exec.Cmd.
+
+	The tag name used can be changed by setting the StructTagCmdArgs variable in this module;
+	the default is `cmdarg`.
+
+	If the tag value is "-", the field will be skipped. Any other tag value(s) are ignored.
+
+	Tag value format:
+		<tag>:"<option>=<value>[,<option>[=<value>],<option>[=<value>]...]"
+	e.g.
+		cmdarg:"short=l,long=list"
+		cmdarg:"short=l"
+		cmdarg:"long=list"
+
+	If the tag value is "-", or <VAR NAME> is not provided, the field will be explicitly skipped.
+	(This is the default behavior for struct fields not tagged with `cmdarg`.)
+
+	If a cmdarg tag is specified but has no `short` or `long` option value, the field will be skipped entirely.
+	If a field's value is nil, it will be skipped.
+	Otherwise if a field's value is the zero-value, it will be skipped.
+
+	Recognized options:
+
+	* short - A short flag for the argument
+
+	e.g.:
+
+		struct{
+			// If this is an empty string, it will be replaced with the value of $CWD.
+			CurrentDir string `envpop:"CWD"`
+			// This would only populate with $USER if the pointer is nil.
+			UserName *string `envpop:"USER"`
+			// This will *always* replace the field's value with the value of $DISPLAY,
+			// even if not an empty string.
+			// Note the `force` option.
+			Display string `envpop:"DISPLAY,force"`
+			// Likewise, even if not nil, this field's value would be replaced with the value of $SHELL.
+			Shell *string `envpop:"SHELL,force"`
+			// This field will be untouched if non-nil, otherwise it will be a pointer to an empty string
+			// if FOOBAR is undefined.
+			NonExistentVar *string `envpop:"FOOBAR,allow_empty"`
+		}
+
+	If s is nil, nothing will be done and err will be errs.ErrNilPtr.
+	If s is not a pointer to a struct, nothing will be done and err will be errs.ErrBadType.
+*/
+func GetCmdFromStruct[T any](s T, opts ...cmdArgOpt) (cmdSlice []string, err error) {
+
+	// TODO
+
+	return
+}
diff --git a/exec_extra/types.go b/exec_extra/types.go
new file mode 100644
index 0000000..57d8fa5
--- /dev/null
+++ b/exec_extra/types.go
@@ -0,0 +1,9 @@
+package exec_extra
+
+type cmdArgsOpts struct {
+	preferShort bool
+	forcePosix  bool
+	cmd         *string
+}
+
+type cmdArgOpt func(*cmdArgsOpts) (err error)
diff --git a/exec_extra/utils.go b/exec_extra/utils.go
new file mode 100644
index 0000000..55c925e
--- /dev/null
+++ b/exec_extra/utils.go
@@ -0,0 +1,33 @@
+package exec_extra
+
+import (
+	`r00t2.io/sysutils/paths`
+)
+
+/*
+	CmdArgsWithBin returns a cmdArgsOpt that specifies program/executable/binary path `bin`,
+	ensuring that the resulting cmdSlice from GetCmdFromStruct() will return a ready-to-use slice.
+	(Otherwise the executable would need to be prepended to the resulting slice.)
+
+	Path normalization/canonziation can be enabled/disabled via normalizePath.
+*/
+func CmdArgsWithBin(bin string, normalizePath bool) (opt cmdArgOpt, err error) {
+
+	if normalizePath {
+		if err = paths.RealPath(&bin); err != nil {
+			return
+		}
+	}
+
+	opt = func(opts *cmdArgsOpts) (err error) {
+		/*
+			if opts.cmd == nil {
+				opts.cmd = new(string)
+			}
+		*/
+		*opts.cmd = bin
+		return
+	}
+
+	return
+}
diff --git a/go.mod b/go.mod
index 4e6077b..0f27912 100644
--- a/go.mod
+++ b/go.mod
@@ -2,7 +2,12 @@ module r00t2.io/sysutils
 
 go 1.21
 
-require github.com/g0rbe/go-chattr v1.0.1
+require (
+	github.com/davecgh/go-spew v1.1.1
+	github.com/g0rbe/go-chattr v1.0.1
+	golang.org/x/sys v0.19.0
+	r00t2.io/goutils v1.4.0
+)
 
 // Pending https://github.com/g0rbe/go-chattr/pull/3
 replace github.com/g0rbe/go-chattr => github.com/johnnybubonic/go-chattr v0.0.0-20240126141003-459f46177b13
diff --git a/go.sum b/go.sum
index a347158..d5a6d04 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,12 @@
+github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf/go.mod h1:F5haX7vjVVG0kc13fIWeqUViNPyEJxv/OmvnBo0Yme4=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/johnnybubonic/go-chattr v0.0.0-20240126141003-459f46177b13 h1:tgEbuE4bNVjaCWWIB1u9lDzGqH/ZdBTg33+4vNW2rjg=
 github.com/johnnybubonic/go-chattr v0.0.0-20240126141003-459f46177b13/go.mod h1:yQc6VPJfpDDC1g+W2t47+yYmzBNioax/GLiyJ25/IOs=
+golang.org/x/sys v0.0.0-20211216021012-1d35b9e2eb4e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.19.0 h1:q5f1RH2jigJ1MoAWp2KTp3gm5zAGFUTarQZ5U386+4o=
+golang.org/x/sys v0.19.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+r00t2.io/goutils v1.4.0 h1:/x/etLpMFv3+j1aPtT7KK2G0uOk+gQkGvXIYBCdjn3E=
+r00t2.io/goutils v1.4.0/go.mod h1:9ObJI9S71wDLTOahwoOPs19DhZVYrOh4LEHmQ8SW4Lk=
+r00t2.io/sysutils v1.1.1/go.mod h1:Wlfi1rrJpoKBOjWiYM9rw2FaiZqraD6VpXyiHgoDo/o=
diff --git a/internal/utils.go b/internal/utils.go
index 657a4af..d49b2db 100644
--- a/internal/utils.go
+++ b/internal/utils.go
@@ -2,6 +2,7 @@ package internal
 
 import (
 	`runtime`
+	`strings`
 )
 
 // GetPathEnvName gets the OS-specific path environment variable name.
@@ -16,3 +17,85 @@ func GetPathEnvName() (envVarName string) {
 
 	return
 }
+
+/*
+	StringToMap takes string s, assumed to be in the form of
+		key=value[,key=value,key=value...]
+	and returns a map[string]string (map[key]value).
+	It is proccessed in order; later duplicate keys overwrite previous ones.
+
+	If s is an empty string or comprised only of whitespace, m will be nil.
+	If only a key is provided with no value, the value in the map will be an empty string.
+	(e.g. "foo,bar=baz" => map[string]string{"foo": "", "bar: "baz"}
+
+	Surrounding whitespace is trimmed.
+*/
+func StringToMap(s string) (m map[string]string) {
+
+	var kvSplit []string
+	var valSplit []string
+	var k string
+	var v string
+
+	if strings.TrimSpace(s) == "" {
+		return
+	}
+
+	kvSplit = strings.Split(s, ",")
+	if kvSplit == nil || len(kvSplit) == 0 {
+		return
+	}
+	for _, kv := range kvSplit {
+		valSplit = strings.SplitN(kv, "=", 2)
+		if valSplit == nil || len(valSplit) == 0 {
+			continue
+		}
+		k = valSplit[0]
+		switch len(valSplit) {
+		case 1:
+			v = ""
+		case 2:
+			v = valSplit[1]
+			// It's not possible to have more than 2.
+		}
+		if m == nil {
+			m = make(map[string]string)
+		}
+		k = strings.TrimSpace(k)
+		v = strings.TrimSpace(v)
+		m[k] = v
+	}
+
+	return
+}
+
+/*
+	StringToMapBool is like StringToMap but designed for a map of booleans.
+	It takes string s, assumed to be in the form of
+		option[,option,option...]
+	and returns a map[string]bool (map[option]true).
+
+	If s is an empty string or comprised only of whitespace, m will be nil.
+
+	Surrounding whitespace is trimmed.
+*/
+func StringToMapBool(s string) (m map[string]bool) {
+
+	var optSplit []string
+
+	if strings.TrimSpace(s) == "" {
+		return
+	}
+
+	optSplit = strings.Split(s, ",")
+	if optSplit == nil || len(optSplit) == 0 {
+		return
+	}
+	m = make(map[string]bool)
+	for _, o := range optSplit {
+		o = strings.TrimSpace(o)
+		m[o] = true
+	}
+
+	return
+}