kiss: initial unneeded [ (external command) removal

kiss

@@ -80,6 +80,10 @@ mkcd() {
     mkdir -p "$@" && cd "$1"
 }
 
+null() {
+    case $1 in '') return 1 ;; *) return 0; esac
+}
+
 fnr() {
     # Replace all occurrences of substrings with substrings. This
     # function takes pairs of arguments iterating two at a time
@@ -327,7 +331,7 @@ pkg_source_resolve() {
     set -- "$1" "$_fnr" "${3%"${3##*[!/]}"}" "$4"
 
     # Git repository.
-    if [ -z "${2##git+*}" ]; then
+    if ! null "${2##git+*}"; then
         _res=$2
 
     # Remote source (cached).
@@ -335,7 +339,7 @@ pkg_source_resolve() {
         _res=$src_dir/$1/${3:+"$3/"}${2##*/}
 
     # Remote source.
-    elif [ -z "${2##*://*}" ]; then
+    elif ! null "${2##*://*}"; then
         _res=url+$2
         _des=$src_dir/$1/${3:+"$3/"}${2##*/}
 
@@ -498,7 +502,7 @@ pkg_depends() {
     # dependencies are listed first and then the parents in reverse order.
     contains "$deps" "$1" || {
         # Filter out non-explicit, already installed packages.
-        [ -z "$3" ] || [ "$2" ] || contains "$explicit" "$1" ||
+        ! null "$3" || null "$2" || contains "$explicit" "$1" ||
             ! [ -d "$sys_db/$1" ] || return
 
         # Detect circular dependencies and bail out.
@@ -975,7 +979,7 @@ pkg_checksums() {
         esac
     done < "$repo_dir/sources"
 
-    sh256 "$@"
+    null "$1" || sh256 "$@"
 }
 
 pkg_verify() {
@@ -1703,7 +1707,7 @@ args() {
     # Rerun the script as root with a fixed environment if needed. We sadly
     # can't run singular functions as root so this is needed.
     case $action in a|alternatives|i|install|r|remove)
-        [ -z "$1" ] || [ -w "$KISS_ROOT/" ] || [ "$uid" = 0 ] || {
+        ! null "$1" || [ -w "$KISS_ROOT/" ] || [ "$uid" = 0 ] || {
             as_root env \
                 HOME="$HOME" \
                 XDG_CACHE_HOME="$XDG_CACHE_HOME" \