From cdfdb277f69d2543f10fc929d8beaf2fc0b91a91 Mon Sep 17 00:00:00 2001
From: payonel <payonel@hotmail.com>
Date: Thu, 22 Jun 2017 16:46:52 -0700
Subject: [PATCH 1/3] replacing yield_all with yield_past to control stack
 yielding

This change should allow popen, pipes, and threads to specify with exactness how many coroutines back they want to yield. This makes popen and threads immune to internal coroutine scenarios

Also using a code cleanup provided by @SDPhantom
---
 .../loot/openos/boot/01_process.lua           | 11 ++--
 .../opencomputers/loot/openos/lib/pipe.lua    | 53 ++++++++++++-------
 .../opencomputers/loot/openos/lib/thread.lua  |  4 +-
 3 files changed, 42 insertions(+), 26 deletions(-)

diff --git a/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua b/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
index 9bab80513..8d1929bbc 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
@@ -6,7 +6,11 @@ local _coroutine = coroutine -- real coroutine backend
 _G.coroutine = setmetatable(
   {
     resume = function(co, ...)
-      return assert(process.info(co), "thread has no proc").data.coroutine_handler.resume(co, ...)
+      local proc = process.info(co)
+      -- proc is nil if the process closed, natural resume will likely complain the coroutine is dead
+      -- but if proc is dead and an aborted coroutine is alive, it doesn't have any proc data like stack info
+      -- if the user really wants to resume it, let them
+      return (proc and proc.data.coroutine_handler.resume or _coroutine.resume)(co, ...)
     end
   },
   {
@@ -51,10 +55,7 @@ end
 _coroutine.wrap = function(f)
   local thread = coroutine.create(f)
   return function(...)
-    local result_pack = table.pack(coroutine.resume(thread, ...))
-    local result, reason = result_pack[1], result_pack[2]
-    assert(result, reason)
-    return select(2, table.unpack(result_pack))
+    return select(2, coroutine.resume(thread, ...))
   end
 end
 
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/pipe.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/pipe.lua
index 4f16688cd..80904f055 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/pipe.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/pipe.lua
@@ -22,17 +22,18 @@ function pipe.createCoroutineStack(root, env, name)
   function pco.yield(...)
     return _root_co.yield(nil, ...)
   end
-  function pco.yield_all(...)
-    return _root_co.yield(true, ...)
+  function pco.yield_past(co, ...)
+    return _root_co.yield(co, ...)
   end
   function pco.resume(co, ...)
     checkArg(1, co, "thread")
     local args = table.pack(...)
     while true do -- for consecutive sysyields
       local result = table.pack(_root_co.resume(co, table.unpack(args, 1, args.n)))
+      local target = result[2] == true and pco.root or result[2]
       if not result[1] or _root_co.status(co) == "dead" then
         return table.unpack(result, 1, result.n)
-      elseif result[2] and pco.root ~= co then
+      elseif target and target ~= co then
         args = table.pack(_root_co.yield(table.unpack(result, 2, result.n)))
       else
         return true, table.unpack(result, 3, result.n)
@@ -70,7 +71,8 @@ local pipe_stream =
         return self
       end
       -- not reading, it is requesting a yield
-      result = table.pack(coroutine.yield_all(table.unpack(result, 2, result.n)))
+      -- yield_past(true) will exit this coroutine stack
+      result = table.pack(coroutine.yield_past(true, table.unpack(result, 2, result.n)))
       result = table.pack(coroutine.resume(self.next, table.unpack(result, 1, result.n))) -- the request was for an event
     end
   end,
@@ -113,7 +115,7 @@ local pipe_stream =
       -- natural yield (i.e. for events). To differentiate this yield from natural
       -- yields we set read_mode here, which the pipe_stream write detects
       self.read_mode = true
-      coroutine.yield_all()
+      coroutine.yield_past(self.next) -- next is the first croutine in this stack
       self.read_mode = false
     end
     local result = string.sub(self.buffer, 1, n)
@@ -158,16 +160,24 @@ end
 
 local chain_stream =
 {
-  read = function(self, value)
+  read = function(self, value, ...)
     if self.io_stream.closed then return nil end
-    -- handler is currently on yield all [else we wouldn't have control here]
-    local read_ok, ret = self.pco.resume(self.pco.root, value)
-    -- ret can be non string when a process ends
-    ret = type(ret) == "string" and ret or nil
-    return select(read_ok and 2 or 1, nil, ret)
+    -- wake up prog
+    self.ready = false -- the pipe proc sets this true when ios completes
+    local ret = table.pack(coroutine.resume(self.pco.root, value, ...))
+    if coroutine.status(self.pco.root) == "dead" then
+      return nil
+    elseif not ret[1] then
+      return table.unpack(ret, 1, ret.n)
+    end
+    if not self.ready then
+      -- prog yielded back without writing/reading
+      return self:read(coroutine.yield())
+    end
+    return ret[2]
   end,
   write = function(self, ...)
-    return self:read(table.concat({...}))
+    return self:read(...)
   end,
   close = function(self)
     self.io_stream:close()
@@ -181,8 +191,8 @@ function pipe.popen(prog, mode, env)
   end
 
   local r = mode == "r"
-  local key = r and "read" or "write"
 
+  local chain = {}
   -- to simplify the code - shell.execute is run within a function to pass (prog, env)
   -- if cmd_proc where to come second (mode=="w") then the pipe_proc would have to pass
   -- the starting args. which is possible, just more complicated
@@ -194,22 +204,27 @@ function pipe.popen(prog, mode, env)
   -- the stream needs its own process for io
   local pipe_proc = process.load(function()
     local n = r and 0 or ""
+    local key = r and "read" or "write"
     local ios = stream.io_stream
     while not ios.closed do
-      n = coroutine.yield_all(ios[key](ios, n))
+      -- read from pipe
+      local ret = table.pack(ios[key](ios, n))
+      stream.ready = true
+      -- yield outside the chain now
+      n = coroutine.yield_past(chain[1], table.unpack(ret, 1, ret.n))
     end
   end, nil, nil, "pipe_handler")
 
-  local pipe_index = r and 2 or 1
-  local cmd_index = r and 1 or 2
-  local chain = {[cmd_index]=cmd_proc, [pipe_index]=pipe_proc}
+  chain[r and 1 or 2] = cmd_proc
+  chain[r and 2 or 1] = pipe_proc
 
   -- link the cmd and pipe proc io
   pipe.buildPipeChain(chain)
-  local cmd_stack = process.info(chain[1]).data.coroutine_handler
+  local cmd_data = process.info(chain[1]).data
+  local cmd_stack = cmd_data.coroutine_handler
 
   -- store handle to io_stream from easy access later
-  stream.io_stream = process.info(chain[1]).data.io[1].stream
+  stream.io_stream = cmd_data.io[1].stream
   stream.pco = cmd_stack
 
   -- popen commands start out running, like threads
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/thread.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/thread.lua
index f602e7b77..317b28a7e 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/thread.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/thread.lua
@@ -213,11 +213,11 @@ function thread.create(fp, ...)
 
   function mt.process.data.pull(_, timeout)
     mt.register(timeout)
-    -- yield_all will yield this pco stack
+    -- yield_past(root) will yield until out of this thread
     -- the callback will resume this stack
     local event_data
     repeat
-      event_data = table.pack(t.pco.yield_all(timeout))
+      event_data = table.pack(t.pco.yield_past(t.pco.root, timeout))
       -- during sleep, we may have been suspended
     until t:status() ~= "suspended"
     return table.unpack(event_data, 1, event_data.n)

From f9f37897296ec5699718f9d723832ef63774e46f Mon Sep 17 00:00:00 2001
From: SDPhantom <SDPhantom@users.noreply.github.com>
Date: Fri, 23 Jun 2017 15:52:47 -0700
Subject: [PATCH 2/3] Optimized sleep.lua interval parsing (#2436)

* Optimized sleep.lua interval parsing

Modified regex to use captures, consolidated invalid interval checks.
Note: tonumber(nil) safely returns nil

* Update sleep.lua

Updated pattern matching
---
 .../assets/opencomputers/loot/openos/bin/sleep.lua | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/src/main/resources/assets/opencomputers/loot/openos/bin/sleep.lua b/src/main/resources/assets/opencomputers/loot/openos/bin/sleep.lua
index 95701a159..43e2d5222 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/bin/sleep.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/bin/sleep.lua
@@ -39,23 +39,15 @@ end
 local total_time = 0
 
 for _,v in ipairs(args) do
-  local interval = v:match('^%d+%.?%d*[smhd]?$')
-  if not interval then
-    help(v)
-    return 1
-  end
-
-  local time_type = interval:match('[smhd]') or ''
-  interval = interval:sub(1, -#time_type-1)
+  local interval, time_type = v:match('^([%d%.]+)([smhd]?)$')
   interval = tonumber(interval)
 
-  if interval < 0 then
+  if not interval or interval < 0 then
     help(v)
     return 1
   end
 
-  interval = time_type_multiplier(time_type) * interval
-  total_time = total_time + interval
+  total_time = total_time + time_type_multiplier(time_type) * interval
 end
 
 os.sleep(total_time)

From 90fdcc19c541b5bfed2a4e3b1c93cb5172828051 Mon Sep 17 00:00:00 2001
From: payonel <payonel@hotmail.com>
Date: Sat, 24 Jun 2017 09:04:11 -0700
Subject: [PATCH 3/3] keep env clean with intercepted loads

also remove unused vars and some light code clean up
moving some methods out of tty into delay loaded space for memory
fix print from overwriting vbuf settings on stdout
fix regression for expanding ${VAR}
closes #2434
---
 .../opencomputers/loot/openos/bin/rc.lua      | 13 ++---
 .../loot/openos/boot/00_base.lua              |  3 +-
 .../loot/openos/boot/01_process.lua           | 26 +++++-----
 .../loot/openos/lib/filesystem.lua            |  8 +--
 .../opencomputers/loot/openos/lib/sh.lua      |  2 +-
 .../opencomputers/loot/openos/lib/text.lua    |  4 +-
 .../opencomputers/loot/openos/lib/tty.lua     | 49 ++-----------------
 .../loot/openos/opt/core/full_tty.lua         | 41 +++++++++++++++-
 .../opencomputers/loot/openos/usr/man/sh      |  7 ++-
 9 files changed, 76 insertions(+), 77 deletions(-)

diff --git a/src/main/resources/assets/opencomputers/loot/openos/bin/rc.lua b/src/main/resources/assets/opencomputers/loot/openos/bin/rc.lua
index 488553626..ebb35364f 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/bin/rc.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/bin/rc.lua
@@ -1,6 +1,5 @@
 local rc = require("rc")
 local fs = require("filesystem")
-local shell = require("shell")
 
 local function loadConfig()
   local env = {}
@@ -117,9 +116,7 @@ local function allRunCommand(cmd, ...)
   return results
 end
 
-local args = table.pack(...)
-
-if #args == 0 then  
+if select("#", ...) == 0 then  
   local results,reason = allRunCommand("start")
   if not results then
     local msg = "rc failed to start:"..tostring(reason)
@@ -127,16 +124,16 @@ if #args == 0 then
     require("event").onError(msg)
     return
   end
-  for name, result in pairs(results) do
+  for _, result in pairs(results) do
     local ok, reason = table.unpack(result)
     if not ok then
-      io.stderr:write(reason .. "\n")
+      io.stderr:write(reason, "\n")
     end
   end
 else
-  local result, reason = runCommand(table.unpack(args))
+  local result, reason = runCommand(...)
   if not result then
-    io.stderr:write(reason .. "\n")
+    io.stderr:write(reason, "\n")
     return 1
   end
 end
diff --git a/src/main/resources/assets/opencomputers/loot/openos/boot/00_base.lua b/src/main/resources/assets/opencomputers/loot/openos/boot/00_base.lua
index 50fe336b7..76c4c04ae 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/boot/00_base.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/boot/00_base.lua
@@ -32,6 +32,7 @@ end
 function print(...)
   local args = table.pack(...)
   local stdout = io.stdout
+  local old_mode, old_size = stdout:setvbuf()
   stdout:setvbuf("line")
   local pre = ""
   for i = 1, args.n do
@@ -39,6 +40,6 @@ function print(...)
     pre = "\t"
   end
   stdout:write("\n")
-  stdout:setvbuf("no")
+  stdout:setvbuf(old_mode, old_size)
   stdout:flush()
 end
diff --git a/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua b/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
index 8d1929bbc..9ae2e27be 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/boot/01_process.lua
@@ -26,18 +26,22 @@ local kernel_load = _G.load
 local intercept_load
 intercept_load = function(source, label, mode, env)
   if env then
-    local loader = setmetatable(
-    {
-      env = env,
-      load = intercept_load,
-    },
-    {__call = function(tbl, _source, _label, _mode, _env)
-      return tbl.load(_source, _label, _mode, _env or tbl.env)
-    end})
-    if env.load and (type(env.load) ~= "table" or env.load.load ~= intercept_load) then
-      loader.load = env.load
+    local prev_load = env.load or intercept_load
+    local env_mt = getmetatable(env) or {}
+    local env_mt_index = env_mt.__index
+    env_mt.__index = function(tbl, key)
+      if key == "load" then
+        return function(_source, _label, _mode, _env)
+          return prev_load(_source, _label, _mode, _env or env)
+        end
+      elseif type(env_mt_index) == "table" then
+        return env_mt_index[key]
+      elseif env_mt_index then
+        return env_mt_index(tbl, key)
+      end
+      return nil
     end
-    env.load = loader
+    setmetatable(env, env_mt)
   end
   return kernel_load(source, label, mode, env or process.info().env)
 end
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/filesystem.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/filesystem.lua
index 2e4ab54a0..2346e7ae7 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/filesystem.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/filesystem.lua
@@ -136,7 +136,7 @@ function filesystem.concat(...)
 end
 
 function filesystem.get(path)
-  local node, rest = findNode(path)
+  local node = findNode(path)
   if node.fs then
     local proxy = node.fs
     path = ""
@@ -155,7 +155,7 @@ end
 
 function filesystem.realPath(path)
   checkArg(1, path, "string")
-  local node, rest, vnode, vrest = findNode(path, false, true)
+  local node, rest = findNode(path, false, true)
   if not node then return nil, rest end
   local parts = {rest or nil}
   repeat
@@ -199,7 +199,7 @@ function filesystem.link(target, linkpath)
     return nil, "not a directory"
   end
 
-  local node, rest, vnode, vrest = findNode(linkpath_real, true)
+  local _, _, vnode, _ = findNode(linkpath_real, true)
   vnode.links[filesystem.name(linkpath)] = target
   return true
 end
@@ -235,7 +235,7 @@ function filesystem.mount(fs, path)
   if fstab[real] then
     return nil, "another filesystem is already mounted here"
   end
-  for path,node in pairs(fstab) do
+  for _,node in pairs(fstab) do
     if node.fs.address == fs.address then
       fsnode = node
       break
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/sh.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/sh.lua
index 7ff6e33d4..a17cf0ddd 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/sh.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/sh.lua
@@ -133,7 +133,7 @@ function sh.expand(value)
   end)
   :gsub("%${(.*)}", function(key)
     if sh.internal.isIdentifier(key) then
-      return sh.internal.expandKey(key)
+      return os.getenv(key) or ''
     end
     io.stderr:write("${" .. key .. "}: bad substitution\n")
     os.exit(1)
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/text.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/text.lua
index 1a0d8fd0e..764ba408f 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/text.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/text.lua
@@ -42,10 +42,10 @@ function text.wrap(value, width, maxWidth)
 end
 
 function text.wrappedLines(value, width, maxWidth)
-  local line, nl
+  local line
   return function()
     if value then
-      line, value, nl = text.wrap(value, width, maxWidth)
+      line, value = text.wrap(value, width, maxWidth)
       return line
     end
   end
diff --git a/src/main/resources/assets/opencomputers/loot/openos/lib/tty.lua b/src/main/resources/assets/opencomputers/loot/openos/lib/tty.lua
index 5afbe46b7..8d708282b 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/lib/tty.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/lib/tty.lua
@@ -42,42 +42,7 @@ local function read_history(handler, cursor, change)
   end
 end
 
-local function tab_handler(handler, cursor)
-  local hints = handler.hint
-  if not hints then return end
-  local main_kb = tty.keyboard()
-  -- tty may not have a keyboard
-  -- in which case, we shouldn't be handling tab events
-  if not main_kb then
-    return
-  end
-  if not handler.cache then
-    handler.cache = type(hints) == "table" and hints or hints(cursor.data, cursor.index + 1) or {}
-    handler.cache.i = -1
-  end
-
-  local cache = handler.cache
-  
-  if #cache == 1 and cache.i == 0 then
-    -- there was only one solution, and the user is asking for the next
-    handler.cache = hints(cache[1], cursor.index + 1)
-    if not handler.cache then return end
-    handler.cache.i = -1
-    cache = handler.cache
-  end
-
-  local change = kb.isShiftDown(main_kb) and -1 or 1
-  cache.i = (cache.i + change) % math.max(#cache, 1)
-  local next = cache[cache.i + 1]
-  if next then
-    local tail = unicode.len(cursor.data) - cursor.index
-    cursor:clear()
-    cursor:update(next)
-    cursor:move(-tail)
-  end
-end
-
-local function key_down_handler(handler, cursor, char, code)
+function tty.key_down_handler(handler, cursor, char, code)
   local c = false
   local backup_cache = handler.cache
   handler.cache = nil
@@ -86,7 +51,7 @@ local function key_down_handler(handler, cursor, char, code)
     return --close
   elseif code == keys.tab then
     handler.cache = backup_cache
-    tab_handler(handler, cursor)
+    tty.on_tab(handler, cursor)
   elseif code == keys.enter or code == keys.numpadenter then
     cursor:move(math.huge)
     cursor:draw("\n")
@@ -307,11 +272,6 @@ function tty.read(handler, cursor)
   checkArg(2, cursor, "table", "nil")
 
   handler.index = 0
-  handler.touch = handler.touch or "touch_handler"
-  handler.drag = handler.drag or "touch_handler"
-  handler.clipboard = handler.clipboard or "clipboard_handler"
-  handler.key_down = handler.key_down or key_down_handler
-
   cursor = cursor or tty.internal.build_vertical_reader()
 
   while true do
@@ -329,11 +289,8 @@ function tty.read(handler, cursor)
       tty.drawText("^C\n")
       return false
     elseif address == main_kb or address == main_sc then
-      local handler_method = handler[name]
+      local handler_method = handler[name] or tty[name .. "_handler"]
       if handler_method then
-        if type(handler_method) == "string" then -- special hack to delay loading tty stuff
-          handler_method = tty[handler_method]
-        end
         -- nil to end (close)
         -- false to ignore
         -- true-thy updates cursor
diff --git a/src/main/resources/assets/opencomputers/loot/openos/opt/core/full_tty.lua b/src/main/resources/assets/opencomputers/loot/openos/opt/core/full_tty.lua
index 7dcd499f1..67ab6b9e1 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/opt/core/full_tty.lua
+++ b/src/main/resources/assets/opencomputers/loot/openos/opt/core/full_tty.lua
@@ -1,7 +1,8 @@
 local tty = require("tty")
 local unicode = require("unicode")
+local kb = require("keyboard")
 
-function tty.touch_handler(handler, cursor, gx, gy)
+function tty.touch_handler(_, cursor, gx, gy)
   if cursor.data == "" then
     return false
   end
@@ -33,8 +34,9 @@ function tty.touch_handler(handler, cursor, gx, gy)
   end
   return false -- no further cursor update
 end
+tty.drag_handler = tty.touch_handler
 
-function tty.clipboard_handler(handler, cursor, char, code)
+function tty.clipboard_handler(handler, _, char, _)
   handler.cache = nil
   local first_line, end_index = char:find("\13?\10")
   if first_line then
@@ -49,3 +51,38 @@ function tty.clipboard_handler(handler, cursor, char, code)
   return char
 end
 
+function tty.on_tab(handler, cursor)
+  local hints = handler.hint
+  if not hints then return end
+  local main_kb = tty.keyboard()
+  -- tty may not have a keyboard
+  -- in which case, we shouldn't be handling tab events
+  if not main_kb then
+    return
+  end
+  if not handler.cache then
+    handler.cache = type(hints) == "table" and hints or hints(cursor.data, cursor.index + 1) or {}
+    handler.cache.i = -1
+  end
+
+  local cache = handler.cache
+  
+  if #cache == 1 and cache.i == 0 then
+    -- there was only one solution, and the user is asking for the next
+    handler.cache = hints(cache[1], cursor.index + 1)
+    if not handler.cache then return end
+    handler.cache.i = -1
+    cache = handler.cache
+  end
+
+  local change = kb.isShiftDown(main_kb) and -1 or 1
+  cache.i = (cache.i + change) % math.max(#cache, 1)
+  local next = cache[cache.i + 1]
+  if next then
+    local tail = unicode.len(cursor.data) - cursor.index
+    cursor:clear()
+    cursor:update(next)
+    cursor:move(-tail)
+  end
+end
+
diff --git a/src/main/resources/assets/opencomputers/loot/openos/usr/man/sh b/src/main/resources/assets/opencomputers/loot/openos/usr/man/sh
index 9341521e6..bc6607c9f 100644
--- a/src/main/resources/assets/opencomputers/loot/openos/usr/man/sh
+++ b/src/main/resources/assets/opencomputers/loot/openos/usr/man/sh
@@ -13,17 +13,20 @@ DESCRIPTION
 
   Single quotes also suppress variable expansion. Per default, expressions like `$NAME` and `${NAME}` are expanded using environment variables (also accessible via the `os.getenv` method).
 
-  Basic globbing is supported, i.e. '*' and '?' are expanded approriately. For example:
+  Globbing is supported, i.e. '*' and '?' are expanded approriately. For example:
     ls b?n/
   will list all files in `/bin/` (and, if it exists `/ban` and so on).
     cp /bin/* /usr/bin/
   will copy all files from `/bin` to `/usr/bin`.
 
-  The shell provides basic redirects and piping:
+  The shell provides redirects and piping:
     cat f > f2
   copies the contents of file `f` to `f2`, for example.
     echo 'this  is  a  "test"' >> f2
   will append the string 'this is a "test"' to the file `f2`.
+    2>/dev/null ./some_program_with_errors
+  will redirect all stderr to /dev/null [i.e. supress errors].
+  This example also demonstrates redirects can go at the front
 
   Redirects can be combined:
     cat < f >> f2