* nix-build: default to `./default.nix' if no paths are specified.

So when using Nix as a build tool, you can just say `nix-build' and
  it will build the top-level derivation defined in `default.nix'.

doc/manual/nix-build.xml

@@ -25,6 +25,10 @@ to multiple derivations, multiple sequentially numbered symlinks are
 created (<filename>result</filename>, <filename>result-2</filename>,
 and so on).</para>
 
+<para>If no <replaceable>paths</replaceable> are specified, then
+<command>nix-build</command> will use <filename>default.nix</filename>
+in the current directory, if it exists.</para>
+
 <note><para><command>nix-build</command> is essentially a wrapper
 around <link
 linkend="sec-nix-instantiate"><command>nix-instantiate</command></link>

doc/manual/package-management.xml

@@ -176,7 +176,7 @@ set.</para></footnote></para>
 <sect1 id="sec-profiles"><title>Profiles</title>
 
 <para>Profiles and user environments are Nix’s mechanism for
-implementing the ability to allow differens users to have different
+implementing the ability to allow different users to have different
 configurations, and to do atomic upgrades and rollbacks.  To
 understand how they work, it’s useful to know a bit about how Nix
 works.  In Nix, components are stored in unique locations in the

scripts/nix-build.in

@@ -2,11 +2,6 @@
 
 nixExpr=$1
 
-if test -z "$nixExpr"; then
-    echo "syntax: $0 NIX-EXPR..." >&2
-    exit 1
-fi
-
 extraArgs=
 addDrvLink=0
 addOutLink=1
@@ -16,9 +11,15 @@ trap 'rm -f ./.nix-build-tmp-*' EXIT
 
 
 # Process the arguments.
+exprs=
 for i in "$@"; do
     case "$i" in
 
+        --help)
+            echo "syntax: $0 [NIX-EXPR...]" >&2
+            exit 0
+            ;;
+
         --add-drv-link)
             addDrvLink=1
             ;;
@@ -32,6 +33,18 @@ for i in "$@"; do
             ;;
             
         *)
+            exprs="$exprs $i"
+            ;;
+    esac
+done
+
+if test -z "$exprs"; then
+    exprs="./default.nix"
+fi
+
+# Process the specified Nix expressions.
+for i in $exprs; do
+
     # Instantiate the Nix expression.
     prefix=
     if test "$addDrvLink" = 0; then prefix=.nix-build-tmp-; fi
@@ -53,7 +66,4 @@ for i in "$@"; do
     for j in $outPaths; do
         echo "$(readlink "$j")"
     done
-            
-            ;;
-    esac
 done