fix https://github.com/koalaman/shellcheck/wiki/SC2251

commands/is-headless

@@ -3,7 +3,11 @@
 function is_headless() (
 	source "$DOROTHY/sources/bash.bash"
 
-	! is-headful
+	if is-headful; then
+		return 1
+	else
+		return 0
+	fi
 )
 
 # fire if invoked standalone

commands/is-interactive

@@ -35,8 +35,11 @@ function is_interactive() (
 	# =====================================
 	# Action
 
-	is-tty
-	! is-ci
+	if is-tty && ! is-ci; then
+		return 0
+	else
+		return 1
+	fi
 )
 
 # fire if invoked standalone

commands/is-missing

@@ -3,7 +3,32 @@
 function is_missing() (
 	source "$DOROTHY/sources/bash.bash"
 
-	! is-present "$1"
+	# help
+	function help {
+		cat <<-EOF >/dev/stderr
+			ABOUT:
+			Check if a path is missing (not a file/directory/symlink).
+			Opposite of [is-present].
+
+			USAGE:
+			is-missing <input>
+		EOF
+		if test "$#" -ne 0; then
+			echo-error "$@"
+		fi
+		return 22 # EINVAL 22 Invalid argument
+	}
+
+	# process
+	if test "$#" -eq 0; then
+		help 'No <input> provided.'
+	elif test "$*" = '--help'; then
+		help
+	elif test -e "$1" -o -L "$1"; then # just -e is faulty, as -e fails on symlinks
+		return 1
+	else
+		return 0
+	fi
 )
 
 # fire if invoked standalone

commands/is-nonempty-string

@@ -3,7 +3,11 @@
 function is_nonempty_string() (
 	source "$DOROTHY/sources/bash.bash"
 
-	! is-empty-string "$1"
+	if is-empty-string "$1"; then
+		return 1
+	else
+		return 0
+	fi
 )
 
 # fire if invoked standalone

commands/is-present

@@ -3,8 +3,32 @@
 function is_present() (
 	source "$DOROTHY/sources/bash.bash"
 
-	# just -e is faulty, as -e fails on symlinks
-	test -e "$1" -o -L "$1"
+	# help
+	function help {
+		cat <<-EOF >/dev/stderr
+			ABOUT:
+			Check if a path is present (is a file/directory/symlink).
+			Opposite of [is-missing].
+
+			USAGE:
+			is-present <input>
+		EOF
+		if test "$#" -ne 0; then
+			echo-error "$@"
+		fi
+		return 22 # EINVAL 22 Invalid argument
+	}
+
+	# process
+	if test "$#" -eq 0; then
+		help 'No <input> provided.'
+	elif test "$*" = '--help'; then
+		help
+	elif test -e "$1" -o -L "$1"; then # just -e is faulty, as -e fails on symlinks
+		return 0
+	else
+		return 1
+	fi
 )
 
 # fire if invoked standalone

commands/is-value

@@ -3,7 +3,11 @@
 function is_value() (
 	source "$DOROTHY/sources/bash.bash"
 
-	! is-empty-value "${1-}"
+	if is-empty-value "${1-}"; then
+		return 1
+	else
+		return 0
+	fi
 )
 
 # fire if invoked standalone

docs/bash/errors.md

@@ -500,3 +500,7 @@ For more information on this, refer to:
 
 -   https://gist.github.com/balupton/21ded5cefc26dc20833e6ed606209e1b
 -   https://github.com/bevry/dorothy/blob/master/sources/bash.bash
+
+### a final gotcha
+
+https://github.com/koalaman/shellcheck/wiki/SC2251