feat: validate input

Gracefully handle invalid setting and environment.

usr/local/bin/wallpaperctl

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+pictureuri="picture-uri"
+
+validate_config() {
+    if [ -z "$WALLPAPER_DIR" ]; then
+        quit "Error: The wallpaper directory environment variable (WALLPAPER_DIR) has not been set."
+    fi
+
+    if [ ! -e "$WALLPAPER_DIR" ]; then
+        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" does not exist."
+    fi
+
+    if [ ! -d "$WALLPAPER_DIR" ]; then
+        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" is not a directory."
+    fi
+
+    if [ ! -r "$WALLPAPER_DIR"  ]; then
+        quit "Error: The wallpaper directory \"$WALLPAPER_DIR\" is not accessible. "
+    fi
+}
+prepare() {
+    get_wallpaper
+    validate_wallpaper
+}
+get_wallpaper() {
+    selection="$(find "$WALLPAPER_DIR" -type f -name "*.jpg" -o -name "*.png" | shuf -n1)"
+}
+validate_wallpaper() {
+    if [ ! -f "$selection" ]; then
+        quit "Error: The wallpaper \"$WALLPAPER_DIR\" is not a file."
+    fi
+
+    if [ ! -r "$selection" ]; then
+        quit "Error: The wallpaper \"$WALLPAPER_DIR\" is not accessible."
+    fi
+}
+set_wallpaper() {
+    gsettings set org.gnome.desktop.background "$pictureuri" "file://$selection"
+}
+quit() {
+    if [[ "$1" = 0 ]]
+    then
+            exit 0
+    else
+            printf "%b\n" "$1"
+            exit 1
+    fi
+}
+
+validate_config
+prepare
+set_wallpaper
+quit 0