Use the pager given by the environment to display long output

Unix systems usually have a PAGER environment variable set.
If it is set, mercurial will use the pager application to display
output.

Two configuration variables are available to influence the behaviour of the
pager. ui.pager sets the pager application. The pager is
only used if ui.usepager is true. By default ui.usepager is disabled.

doc/hgrc.5.txt

@@ -522,6 +522,12 b' ui::'
     Print debugging information.  True or False.  Default is False.
   editor;;
     The editor to use during a commit.  Default is $EDITOR or "vi".
+  pager;;
+    The pager that is used when displaying long output.
+    Default is $PAGER. If not set, the output is written to the
+    stdandard output.
+  usepager;;
+    If set to true, the system pager is used. True or False. Default is False.
   fallbackencoding;;
     Encoding to try if it's not possible to decode the changelog using
     UTF-8.  Default is ISO-8859-1.

mercurial/ui.py

@@ -31,6 +31,7 b' class ui(object):'
                  parentui=None):
         self.overlay = None
         self.buffers = []
+        self.pager = None
         if parentui is None:
             # this is the parent of all ui children
             self.parentui = None
@@ -64,6 +65,15 b' class ui(object):'
     def __getattr__(self, key):
         return getattr(self.parentui, key)
 
+    def __del__(self):
+        if self.pager:
+            try:
+                self.pager.close()
+            except IOException:
+                # we might get into an broken pipe if the users quit
+                # the pager before we finished io
+                pass
+
     def isatty(self):
         if ui._isatty is None:
             ui._isatty = sys.stdin.isatty()
@@ -370,9 +380,14 b' class ui(object):'
         return "".join(self.buffers.pop())
 
     def write(self, *args):
+        """Write to a pager if available, otherwise to stdout"""
         if self.buffers:
             self.buffers[-1].extend([str(a) for a in args])
         else:
+            if self.getpager() and not self.pager:
+                self.pager = os.popen(self.getpager(), "w")
+                sys.stderr = self.pager
+                sys.stdout = self.pager
             for a in args:
                 sys.stdout.write(str(a))
 
@@ -478,3 +493,8 b' class ui(object):'
                 os.environ.get("VISUAL") or
                 os.environ.get("EDITOR", "vi"))
 
+    def getpager(self):
+        '''return a pager'''
+        if self.configbool("ui", "usepager", False):
+            return (self.config("ui", "pager")
+                    or os.environ.get("PAGER"))