b2/: add "print VAR" command in substitutions (for debugging/tracing)

b2/test/subprint

@@ -0,0 +1,78 @@
+#!/bin/bash
+. ./Common
+
+###############################################################################
+
+tst "substitutions: print input variable" -ds -q foo=bar <<EOF
+!-s
+print foo
+EOF
+
+expect <<EOF
+print foo
+foo(in) = "bar"
+EOF
+
+#------------------------------------------------------------------------------
+
+tst "substitutions: print output variable" -q <<EOF
+!-s
+foo = x
+print foo
+EOF
+
+expect <<EOF
+foo(out) = "x"
+foo=x
+EOF
+
+#------------------------------------------------------------------------------
+
+tst "substitutions: print output variable overriding input" -q x=y <<EOF
+!-s
+x = z
+print x
+EOF
+
+expect <<EOF
+x(out) = "z"
+x=z
+EOF
+
+#------------------------------------------------------------------------------
+
+tst "substitutions: print unknown variable" -q <<EOF
+!-s
+print x
+EOF
+
+expect <<EOF
+x = ?
+EOF
+
+#------------------------------------------------------------------------------
+
+tst "substitutions: print conditionally defined variable" -q x=y <<EOF
+!-s
+x=y { z=1 }
+print z
+EOF
+
+expect <<EOF
+z(out) = "1"
+z=1
+EOF
+
+#------------------------------------------------------------------------------
+
+tst "substitutions: print conditionally undefined variable" -q x=z <<EOF
+!-s
+x=y { z=1 }
+print z
+EOF
+
+expect <<EOF
+z = ?
+EOF
+
+###############################################################################