* Option `-B' to always show the output of builders, regardless of

  verbosity level.

doc/manual/nix-env.xml

@@ -9,6 +9,8 @@
       <command>nix-env</command>
       <arg rep='repeat'><option>--verbose</option></arg>
       <arg rep='repeat'><option>-v</option></arg>
+      <arg><option>--build-output</option></arg>
+      <arg><option>-B</option></arg>
       <arg><option>--keep-failed</option></arg>
       <arg><option>-K</option></arg>
       <arg>

doc/manual/nix-store.xml

@@ -9,6 +9,8 @@
       <command>nix-store</command>
       <arg rep='repeat'><option>--verbose</option></arg>
       <arg rep='repeat'><option>-v</option></arg>
+      <arg><option>--build-output</option></arg>
+      <arg><option>-B</option></arg>
       <arg><option>--keep-failed</option></arg>
       <arg><option>-K</option></arg>
       <arg choice='plain'><replaceable>operation</replaceable></arg>

doc/manual/opt-verbose.xml

@@ -71,3 +71,17 @@
 
   </listitem>
 </varlistentry>
+
+<varlistentry>
+  <term><option>--build-output</option> / <option>-B</option></term>
+  <listitem>
+    <para>
+      Causes the output written by build actions to standard output
+      and standard error to be echoed to standard error, regardless of
+      verbosity level.  By default, it is only echoed at a verbosity
+      level of at least 4 (<quote>Debug</quote>), and is suppressed at
+      lower levels.  Note that it is always written to a log file in
+      <filename><replaceable>prefix</replaceable>/nix/var/log/nix</filename>.
+    </para>
+  </listitem>
+</varlistentry>

src/libmain/shared.cc

@@ -51,6 +51,8 @@ static void initAndRun(int argc, char * * argv)
         string arg = *i;
         if (arg == "--verbose" || arg == "-v")
             verbosity = (Verbosity) ((int) verbosity + 1);
+        else if (arg == "--build-output" || arg == "-B")
+            buildVerbosity = lvlError; /* lowest */
         else if (arg == "--help") {
             printHelp();
             return;

src/libstore/exec.cc

@@ -22,7 +22,7 @@ void runProgram(const string & program,
 {
     /* Create a log file. */
     string logCommand = 
-	verbosity >= lvlDebug 
+	verbosity >= buildVerbosity 
 	? "tee "  + logFileName + " >&2"
 	: "cat > " + logFileName;
     /* !!! auto-pclose on exit */

src/libstore/globals.cc

@@ -7,3 +7,5 @@ string nixStateDir = "/UNINIT";
 string nixDBPath = "/UNINIT";
 
 bool keepFailed = false;
+
+Verbosity buildVerbosity = lvlDebug;

src/libstore/globals.hh

@@ -2,6 +2,7 @@
 #define __GLOBALS_H
 
 #include <string>
+#include "util.hh"
 
 using namespace std;
 
@@ -28,5 +29,8 @@ extern string nixDBPath;
 /* Whether to keep temporary directories of failed builds. */
 extern bool keepFailed;
 
+/* Verbosity level for build output. */
+extern Verbosity buildVerbosity;
+
 
 #endif /* !__GLOBALS_H */