Add lush feature tests and fix /dev/full test for macOS

Add test suite under testes/lush/ covering backtick commands, argv
parsing, ${} interpolation, and $NAME environment variables. Wire
them into testes/all.lua so they run with the full Lua 5.5 suite.

Skip /dev/full test in files.lua when the device doesn't exist
(macOS has no /dev/full).

testes/all.lua

@@ -199,6 +199,12 @@ dofile('bitwise.lua')
 assert(dofile('verybig.lua', true) == 10); collectgarbage()
 dofile('files.lua')
 
+-- lush shell feature tests
+dofile('lush/commands.lua')
+dofile('lush/argv.lua')
+dofile('lush/interpolation.lua')
+dofile('lush/envvars.lua')
+
 if #msgs > 0 then
   local m = table.concat(msgs, "\n  ")
   warn("#tests not performed:\n  ", m, "\n")

testes/files.lua

@@ -471,13 +471,15 @@ do print("testing flush")
   assert(io.flush())        -- write to device
   assert(f:close())
 
-  local f = io.output("/dev/full")
+  local ok, f = pcall(io.output, "/dev/full")
+  if ok then  -- /dev/full exists (Linux); skip on macOS
     assert(f:write("abcd"))   -- write to buffer
     assert(not f:flush())     -- cannot write to device
     assert(f:write("abcd"))   -- write to buffer
     assert(not io.flush())    -- cannot write to device
     assert(f:close())
   end
+end
 
 
 -- test for multiple arguments in 'lines'

testes/lush/argv.lua

@@ -0,0 +1,53 @@
+-- testes/lush/argv.lua
+-- Tests for argv parsing (issue #04).
+
+print "testing argv parsing"
+
+-- simple whitespace splitting
+do
+  local r = `echo a b c`
+  assert(r.stdout == "a b c\n")
+end
+
+-- single-quoted strings (literal, no escapes)
+do
+  local r = `echo 'hello world'`
+  assert(r.stdout == "hello world\n")
+end
+
+-- double-quoted strings
+do
+  local r = `echo "hello world"`
+  assert(r.stdout == "hello world\n")
+end
+
+-- multiple quoted arguments
+do
+  local r = `echo "a b" "c d"`
+  assert(r.stdout == "a b c d\n")
+end
+
+-- backslash escaping outside quotes
+do
+  local r = `echo hello\ world`
+  assert(r.stdout == "hello world\n")
+end
+
+-- double-quote escapes: \" and \\
+do
+  local r = `echo "hello\"world"`
+  assert(r.stdout == 'hello"world\n')
+end
+
+do
+  local r = `echo "hello\\world"`
+  assert(r.stdout == "hello\\world\n")
+end
+
+-- tab escape in double quotes
+do
+  local r = `echo "a\tb"`
+  assert(r.stdout == "a\tb\n")
+end
+
+print "OK"

testes/lush/commands.lua

@@ -0,0 +1,90 @@
+-- testes/lush/commands.lua
+-- Tests for backtick command execution (issues #02, #03).
+
+print "testing backtick commands"
+
+-- basic command, result is a table with code/stdout/stderr
+do
+  local r = `echo hello`
+  assert(type(r) == "table")
+  assert(type(r.code) == "number")
+  assert(type(r.stdout) == "string")
+  assert(type(r.stderr) == "string")
+end
+
+-- successful command returns exit code 0
+do
+  local r = `true`
+  assert(r.code == 0)
+end
+
+-- failed command returns non-zero exit code
+do
+  local r = `false`
+  assert(r.code == 1)
+end
+
+-- specific exit codes are preserved
+do
+  local r = `sh -c "exit 42"`
+  assert(r.code == 42)
+end
+
+-- command not found returns 127
+do
+  local r = `nonexistent_command_xyz_999`
+  assert(r.code == 127)
+end
+
+-- stdout capture
+do
+  local r = `echo hello`
+  assert(r.stdout == "hello\n")
+end
+
+-- stderr capture
+do
+  local r = `sh -c "echo err >&2"`
+  assert(r.stderr == "err\n")
+  assert(r.stdout == "")
+end
+
+-- stdout and stderr are independent
+do
+  local r = `sh -c "echo out; echo err >&2"`
+  assert(r.stdout == "out\n")
+  assert(r.stderr == "err\n")
+end
+
+-- empty command returns code 0 and empty strings
+do
+  local r = ``
+  assert(r.code == 0)
+  assert(r.stdout == "")
+  assert(r.stderr == "")
+end
+
+-- multi-word output preserved
+do
+  local r = `echo hello world`
+  assert(r.stdout == "hello world\n")
+end
+
+-- backtick as statement (result discarded, no error)
+do
+  `true`
+end
+
+-- backtick result used inline
+do
+  local code = `true`.code
+  assert(code == 0)
+end
+
+-- multiline output captured
+do
+  local r = `printf "a\nb\nc\n"`
+  assert(r.stdout == "a\nb\nc\n")
+end
+
+print "OK"

testes/lush/envvars.lua

@@ -0,0 +1,69 @@
+-- testes/lush/envvars.lua
+-- Tests for environment variable access with $NAME syntax (issue #05).
+
+print "testing environment variables"
+
+-- $NAME reads an existing environment variable
+do
+  local p = $PATH
+  assert(type(p) == "string")
+  assert(#p > 0)
+end
+
+-- $NAME returns nil for unset variable
+do
+  local v = $_LUSH_TEST_UNSET_VAR
+  assert(v == nil)
+end
+
+-- $NAME = expr sets an environment variable
+do
+  $_LUSH_TEST_A = "hello"
+  assert($_LUSH_TEST_A == "hello")
+end
+
+-- $NAME = expr with number coerces to string
+do
+  $_LUSH_TEST_B = 42
+  assert($_LUSH_TEST_B == "42")
+end
+
+-- $NAME = nil unsets the variable
+do
+  $_LUSH_TEST_C = "temp"
+  assert($_LUSH_TEST_C == "temp")
+  $_LUSH_TEST_C = nil
+  assert($_LUSH_TEST_C == nil)
+end
+
+-- env vars are visible to child processes
+do
+  $_LUSH_TEST_D = "from_lush"
+  local r = `sh -c "echo $_LUSH_TEST_D"`
+  -- the $_LUSH_TEST_D here is NOT lush interpolation (no {}),
+  -- it's a literal string passed to sh which expands it
+  assert(r.stdout == "from_lush\n", r.stdout)
+end
+
+-- overwriting an env var
+do
+  $_LUSH_TEST_E = "first"
+  $_LUSH_TEST_E = "second"
+  assert($_LUSH_TEST_E == "second")
+end
+
+-- env var used in interpolation
+do
+  $_LUSH_TEST_F = "works"
+  local r = `echo ${$_LUSH_TEST_F}`
+  assert(r.stdout == "works\n")
+end
+
+-- clean up
+$_LUSH_TEST_A = nil
+$_LUSH_TEST_B = nil
+$_LUSH_TEST_D = nil
+$_LUSH_TEST_E = nil
+$_LUSH_TEST_F = nil
+
+print "OK"

testes/lush/interpolation.lua

@@ -0,0 +1,55 @@
+-- testes/lush/interpolation.lua
+-- Tests for ${expr} interpolation in backtick commands (issue #02).
+
+print "testing string interpolation"
+
+-- single interpolation
+do
+  local name = "world"
+  local r = `echo ${name}`
+  assert(r.stdout == "world\n")
+end
+
+-- multiple interpolations
+do
+  local a = "hello"
+  local b = "world"
+  local r = `echo ${a} ${b}`
+  assert(r.stdout == "hello world\n")
+end
+
+-- interpolation with number
+do
+  local n = 42
+  local r = `echo ${n}`
+  assert(r.stdout == "42\n")
+end
+
+-- interpolation with expression
+do
+  local x = 3
+  local r = `echo ${x + 1}`
+  assert(r.stdout == "4\n")
+end
+
+-- interpolation with string concatenation
+do
+  local prefix = "hello"
+  local r = `echo ${prefix .. " world"}`
+  assert(r.stdout == "hello world\n")
+end
+
+-- interpolation surrounded by literal text
+do
+  local mid = "b"
+  local r = `echo a${mid}c`
+  assert(r.stdout == "abc\n")
+end
+
+-- literal $ (not followed by {) is kept
+do
+  local r = `echo $`
+  assert(r.stdout == "$\n")
+end
+
+print "OK"