new file mode 100644
--- /dev/null
+++ b/src/testdir/test_debugger.vim
@@ -0,0 +1,232 @@
+" Tests for the Vim script debug commands
+
+source shared.vim
+source screendump.vim
+
+" Run a Vim debugger command
+" If the expected output argument is supplied, then check for it.
+func RunDbgCmd(buf, cmd, ...)
+  call term_sendkeys(a:buf, a:cmd . "\r")
+  call term_wait(a:buf)
+
+  if a:0 != 0
+    " Verify the expected output
+    let lnum = 20 - len(a:1)
+    for l in a:1
+      call WaitForAssert({-> assert_equal(l, term_getline(a:buf, lnum))})
+      let lnum += 1
+    endfor
+  endif
+endfunc
+
+" Debugger tests
+func Test_Debugger()
+  if !CanRunVimInTerminal()
+    return
+  endif
+
+  " Create a Vim script with some functions
+  call writefile([
+	      \ 'func Foo()',
+	      \ '  let var1 = 1',
+	      \ '  let var2 = Bar(var1) + 9',
+	      \ '  return var2',
+	      \ 'endfunc',
+	      \ 'func Bar(var)',
+	      \ '  let var1 = 2 + a:var',
+	      \ '  let var2 = Bazz(var1) + 4',
+	      \ '  return var2',
+	      \ 'endfunc',
+	      \ 'func Bazz(var)',
+	      \ '  let var1 = 3 + a:var',
+	      \ '  let var3 = "another var"',
+	      \ '  let var3 = "value2"',
+	      \ '  let var3 = "value3"',
+	      \ '  return var1',
+	      \ 'endfunc'], 'Xtest.vim')
+
+  " Start Vim in a terminal
+  let buf = RunVimInTerminal('-S Xtest.vim', {})
+
+  " Start the Vim debugger
+  call RunDbgCmd(buf, ':debug echo Foo()')
+
+  " Create a few stack frames by stepping through functions
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+
+  " check backtrace
+  call RunDbgCmd(buf, 'backtrace', [
+	      \ '  2 function Foo[2]',
+	      \ '  1 Bar[2]',
+	      \ '->0 Bazz',
+	      \ 'line 2: let var3 = "another var"'])
+
+  " Check variables in different stack frames
+  call RunDbgCmd(buf, 'echo var1', ['6'])
+
+  call RunDbgCmd(buf, 'up')
+  call RunDbgCmd(buf, 'back', [
+	      \ '  2 function Foo[2]',
+	      \ '->1 Bar[2]',
+	      \ '  0 Bazz',
+	      \ 'line 2: let var3 = "another var"'])
+  call RunDbgCmd(buf, 'echo var1', ['3'])
+
+  call RunDbgCmd(buf, 'u')
+  call RunDbgCmd(buf, 'bt', [
+	      \ '->2 function Foo[2]',
+	      \ '  1 Bar[2]',
+	      \ '  0 Bazz',
+	      \ 'line 2: let var3 = "another var"'])
+  call RunDbgCmd(buf, 'echo var1', ['1'])
+
+  " Undefined variables
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'frame 2')
+  call RunDbgCmd(buf, 'echo var3', [
+	\ 'Error detected while processing function Foo[2]..Bar[2]..Bazz:',
+	\ 'line    3:',
+	\ 'E121: Undefined variable: var3'])
+
+  " var3 is defined in this level with some other value
+  call RunDbgCmd(buf, 'fr 0')
+  call RunDbgCmd(buf, 'echo var3', ['another var'])
+
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'step', [
+	      \ 'function Foo[2]..Bar',
+	      \ 'line 3: End of function'])
+  call RunDbgCmd(buf, 'up')
+
+  " Undefined var2
+  call RunDbgCmd(buf, 'echo var2', [
+	      \ 'Error detected while processing function Foo[2]..Bar:',
+	      \ 'line    3:',
+	      \ 'E121: Undefined variable: var2'])
+
+  " Var2 is defined with 10
+  call RunDbgCmd(buf, 'down')
+  call RunDbgCmd(buf, 'echo var2', ['10'])
+
+  " Backtrace movements
+  call RunDbgCmd(buf, 'b', [
+	      \ '  1 function Foo[2]',
+	      \ '->0 Bar',
+	      \ 'line 3: End of function'])
+
+  " next command cannot go down, we are on bottom
+  call RunDbgCmd(buf, 'down', ['frame is zero'])
+  call RunDbgCmd(buf, 'up')
+
+  " next command cannot go up, we are on top
+  call RunDbgCmd(buf, 'up', ['frame at highest level: 1'])
+  call RunDbgCmd(buf, 'where', [
+	      \ '->1 function Foo[2]',
+	      \ '  0 Bar',
+	      \ 'line 3: End of function'])
+
+  " fil is not frame or finish, it is file
+  call RunDbgCmd(buf, 'fil', ['"[No Name]" --No lines in buffer--'])
+
+  " relative backtrace movement
+  call RunDbgCmd(buf, 'fr -1')
+  call RunDbgCmd(buf, 'frame', [
+	      \ '  1 function Foo[2]',
+	      \ '->0 Bar',
+	      \ 'line 3: End of function'])
+
+  call RunDbgCmd(buf, 'fr +1')
+  call RunDbgCmd(buf, 'fram', [
+	      \ '->1 function Foo[2]',
+	      \ '  0 Bar',
+	      \ 'line 3: End of function'])
+
+  " go beyond limits does not crash
+  call RunDbgCmd(buf, 'fr 100', ['frame at highest level: 1'])
+  call RunDbgCmd(buf, 'fra', [
+	      \ '->1 function Foo[2]',
+	      \ '  0 Bar',
+	      \ 'line 3: End of function'])
+
+  call RunDbgCmd(buf, 'frame -40', ['frame is zero'])
+  call RunDbgCmd(buf, 'fram', [
+	      \ '  1 function Foo[2]',
+	      \ '->0 Bar',
+	      \ 'line 3: End of function'])
+
+  " final result 19
+  call RunDbgCmd(buf, 'cont', ['19'])
+
+  " breakpoints tests
+
+  " Start a debug session, so that reading the last line from the terminal
+  " works properly.
+  call RunDbgCmd(buf, ':debug echo Foo()')
+
+  " No breakpoints
+  call RunDbgCmd(buf, 'breakl', ['No breakpoints defined'])
+
+  " Place some breakpoints
+  call RunDbgCmd(buf, 'breaka func Bar')
+  call RunDbgCmd(buf, 'breaklis', ['  1  func Bar  line 1'])
+  call RunDbgCmd(buf, 'breakadd func 3 Bazz')
+  call RunDbgCmd(buf, 'breaklist', ['  1  func Bar  line 1',
+	      \ '  2  func Bazz  line 3'])
+
+  " Check whether the breakpoints are hit
+  call RunDbgCmd(buf, 'cont', [
+	      \ 'Breakpoint in "Bar" line 1',
+	      \ 'function Foo[2]..Bar',
+	      \ 'line 1: let var1 = 2 + a:var'])
+  call RunDbgCmd(buf, 'cont', [
+	      \ 'Breakpoint in "Bazz" line 3',
+	      \ 'function Foo[2]..Bar[2]..Bazz',
+	      \ 'line 3: let var3 = "value2"'])
+
+  " Delete the breakpoints
+  call RunDbgCmd(buf, 'breakd 1')
+  call RunDbgCmd(buf, 'breakli', ['  2  func Bazz  line 3'])
+  call RunDbgCmd(buf, 'breakdel func 3 Bazz')
+  call RunDbgCmd(buf, 'breakl', ['No breakpoints defined'])
+
+  call RunDbgCmd(buf, 'cont')
+
+  " Make sure the breakpoints are removed
+  call RunDbgCmd(buf, ':echo Foo()', ['19'])
+
+  " Delete a non-existing breakpoint
+  call RunDbgCmd(buf, ':breakdel 2', ['E161: Breakpoint not found: 2'])
+
+  " Expression breakpoint
+  call RunDbgCmd(buf, ':breakadd func 2 Bazz')
+  call RunDbgCmd(buf, ':echo Bazz(1)')
+  call RunDbgCmd(buf, 'step')
+  call RunDbgCmd(buf, 'breaka expr var3')
+  call RunDbgCmd(buf, 'breakl', ['  4  expr var3'])
+  call RunDbgCmd(buf, 'cont', ['Breakpoint in "Bazz" line 4',
+	      \ 'Oldval = "''another var''"',
+	      \ 'Newval = "''value2''"',
+	      \ 'function Bazz',
+	      \ 'line 4: let var3 = "value3"'])
+
+  call RunDbgCmd(buf, 'breakdel *')
+  call RunDbgCmd(buf, 'breakl', ['No breakpoints defined'])
+
+  " finish the current function
+  call RunDbgCmd(buf, 'finish', [
+	      \ 'function Bazz',
+	      \ 'line 5: End of function'])
+  call RunDbgCmd(buf, 'cont')
+
+  call StopVimInTerminal(buf)
+
+  call delete('Xtest.vim')
+endfunc