add u, the most awesome program

custom-pkgs/u/default.nix

@@ -0,0 +1,32 @@
+{ pkgs, ... }: pkgs.writeShellApplication {
+  name = "u";
+
+  text = ''
+    programs=(cargo git)
+
+    if [ -z "''${1:-}" ]; then
+      echo "u: universial command executor"
+      echo "programs: ''${programs[*]}"
+      exit 0
+    fi
+
+    program_to_use=""
+
+    for program in "''${programs[@]}"; do
+      if PAGER=true "$program" "$1" --help >/dev/null 2>/dev/null; then
+        if [ -n "$program_to_use" ]; then
+          echo "u: conflict: $1 is provided by both $program_to_use and $program" >&2
+          exit 1
+        fi
+        program_to_use="$program"
+      fi
+    done
+
+    if [ -n "$program_to_use" ]; then
+      exec "$program_to_use" "$@"
+    fi
+
+    echo "error: cannot find program for $1" >&2
+    exit 1
+  '';
+}