[RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Wink Saville]

Add bdl-lib.sh which provides functions to assit in debugging git
shell scripts and tests. The primary public interace are two routines,
bdl and bdl_nsl which print strings. The difference between the two
is that bdl outputs location of the statement optionally followed by
a string to print. For example:

  $ cat -n bdl-exmpl1.sh
       1  #!/usr/bin/env bash
       2  . bdl-lib.sh
       3  bdl "hi"
       4  
       5  # If no parameters just the location is printed
       6  bdl
  $ ./bdl-exmpl1.sh 
  bdl-exmpl1.sh:3: hi
  bdl-exmpl1.sh:6:

bdl_nsl means bdl with no source location being printed and at least
one parameter is required. For example:

  $ cat -n bdl-exmpl2.sh 
       1  #!/usr/bin/env bash
       2  . bdl-lib.sh
       3  bdl_nsl "hi"
       4	
       5  # If no parameters nothing is printed
       6  bdl_nsl
  $ ./bdl-exmpl1.sh 
  hi

These routines can also take two parameters where the first parameter is
the destination for the string. There are two types of destinations,
either a single digit file descriptor 1..9 or a file name. For example:

  $ cat -n bdl-exmpl3.sh
       1  #!/usr/bin/env bash
       2  . bdl-lib.sh
       3  
       4  # Output to STDOUT
       5  bdl_nsl 1 "hi there"
       6  
       7  # Map 5 to STDOUT
       8  exec 5>&1
       9  bdl 5 "yo dude"
      10  
      11  # Output to a file
      12  bdl bdl_out.txt "good bye!"
      13  cat bdl_out.txt
      14  rm bdl_out.txt
  $ ./bdl-exmpl3.sh 
  hi there
  bdl-exmpl3.sh:9: yo dude
  bdl-exmpl3.sh:12: good bye!

If a destination is not provided as a parameter than there are two
variables, bdl_dst and bdl_stdout, that can be used to provide a
defaults. With bdl_dst taking presedence over bdl_stdout and a
destination parameter taking presedence over the variables. For example:

  $ cat -n bdl-exmpl4.sh 
       1  #!/usr/bin/env bash
       2  . bdl-lib.sh
       3  
       4  # Set defaults with bdl_dst taking presedence
       5  bdl_dst=bdl_dst_out.txt
       6  bdl_stdout=5
       7  
       8  bdl_nsl "printed by bdl_nsl to bdl_dst_out.txt"
       9  bdl "printed by bdl to bdl_dst_out.txt"
      10  
      11  # But the parameter the ultimate presedence
      12  bdl bdl_out.txt "good bye to bdl_out.txt"
      13  
      14  cat bdl_dst_out.txt
      15  cat bdl_out.txt
      16  
      17  rm bdl_dst_out.txt
      18  rm bdl_out.txt
      19  
      20  # Now clear bdl_dst and bdl_stdout takes presedence
      21  # but parameters take presedence
      22  bdl_dst=
      23  exec 5>&1
      24  
      25  bdl_nsl "monkeys via 5"
      26  bdl 1 "horses via 1"
      27  bdl bdl_out.txt "orcas to bdl_out.txt"
      28  
      29  cat bdl_out.txt
      30  rm bdl_out.txt
  $ ./bdl-exmpl4.sh 
  printed by bdl_nsl to bdl_dst_out.txt
  bdl-exmpl4.sh:9: printed by bdl to bdl_dst_out.txt
  bdl-exmpl4.sh:26: orcas to bdl_out.txt
  bdl-exmpl4.sh:12: good bye to bdl_out.txt
  monkeys via 5
  bdl-exmpl4.sh:26: horses via 1
  bdl-exmpl4.sh:27: orcas to bdl_out.txt


TODO: More tests and documentation needed.


Wink Saville (1):
  bdl-lib.sh: add bash debug logger

 bdl-exmpl.sh       |  46 ++++++++++++
 bdl-lib.sh         | 215 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 t/t0014-bdl-lib.sh | 115 ++++++++++++++++++++++++++++
 t/test-lib.sh      |   4 +
 4 files changed, 380 insertions(+)
 create mode 100755 bdl-exmpl.sh
 create mode 100644 bdl-lib.sh
 create mode 100755 t/t0014-bdl-lib.sh

-- 
2.16.3

[RFC PATCH 1/1] bdl-lib.sh: add bash debug logger

[Wink Saville]

Add bdl-lib to assist the programmer in debugging scripts and tests.
---
 bdl-exmpl.sh       |  46 ++++++++++++
 bdl-lib.sh         | 215 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 t/t0014-bdl-lib.sh | 115 ++++++++++++++++++++++++++++
 t/test-lib.sh      |   4 +
 4 files changed, 380 insertions(+)
 create mode 100755 bdl-exmpl.sh
 create mode 100644 bdl-lib.sh
 create mode 100755 t/t0014-bdl-lib.sh

diff --git a/bdl-exmpl.sh b/bdl-exmpl.sh
new file mode 100755
index 000000000..a47d82bca
--- /dev/null
+++ b/bdl-exmpl.sh
@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Examples using bdl-lib.sh
+
+# Source bdl-lib.sh
+. bdl-lib.sh
+
+# These both output to the default bdl_stdout=1
+bdl
+bdl "hi"
+bdl 1 "hi"
+
+# Output to a file as parameter
+echo -n >bdl_out.txt
+bdl bdl_out.txt "hi to bdl_out.txt"
+cat bdl_out.txt
+
+# Output to a file via bdl_dst
+echo -n >bdl_out.txt
+bdl_dst=bdl_out.txt
+bdl "hi to bdl_out.txt"
+cat bdl_out.txt
+bdl_dst=
+
+# Output to FD 5 connected to 1 via bdl_stdout
+bdl_stdout=5
+exec 5>&1
+bdl
+bdl "hi"
+bdl 5 "hi"
+exec 1>&5
+bdl_stdout=1
+
+# No printing to stdout
+echo -n >bdl_out.txt
+bdl_stdout=1
+bdl_dst=
+bdl 0 "not printed"
+bdl_stdout=0
+bdl
+bdl bdl_out.txt "printed to bdl_out.txt"
+bdl "not printed"
+bdl_stdout=1
+cat bdl_out.txt
+
+# This prints a "0" since there is only one parameter
+bdl 0
diff --git a/bdl-lib.sh b/bdl-lib.sh
new file mode 100644
index 000000000..cecf726bb
--- /dev/null
+++ b/bdl-lib.sh
@@ -0,0 +1,215 @@
+# ##################################################################
+# Bash Debug logger scriplet, source into your script to use.
+#
+# Write debug info with file name and line number.
+#
+# If the number of parameters == 0 then just the file
+# name and line number are printed to the destination.
+#
+# If the number of parameters == 1 then the file
+# name and line number are printed followed by a space
+# and then the parameter.
+# 
+# If number of parameters > 1 then the first parameter
+# is the destination and the other parameters are written
+# to the destination.
+#
+# The destination can be the first parameter to bdl
+# or bdl_stdout or bdl_dst. If the destination is empty
+# then the data is written to bdl_stdout unless bdl_stdout
+# is empty or 0 then no data is written. Also, if the
+# destination is 0 no data is written.
+#
+# Examples:
+#$ cat -n bdl-exmpl.sh 
+#     1	#!/usr/bin/env bash
+#     2	# Examples using bdl-lib.sh
+#     3	
+#     4	# Source bdl-lib.sh
+#     5	. bdl-lib.sh
+#     6	
+#     7	# These both output to the default bdl_stdout=1
+#     8	bdl
+#     9	bdl "hi"
+#    10	bdl 1 "hi"
+#    11	
+#    12	# Output to a file as parameter
+#    13	echo -n >bdl_out.txt
+#    14	bdl bdl_out.txt "hi to bdl_out.txt"
+#    15	cat bdl_out.txt
+#    16	
+#    17	# Output to a file via bdl_dst
+#    18	echo -n >bdl_out.txt
+#    19	bdl_dst=bdl_out.txt
+#    20	bdl "hi to bdl_out.txt"
+#    21	cat bdl_out.txt
+#    22	bdl_dst=
+#    23	
+#    24	# Output to FD 5 connected to 1 via bdl_stdout
+#    25	bdl_stdout=5
+#    26	exec 5>&1
+#    27	bdl
+#    28	bdl "hi"
+#    29	bdl 5 "hi"
+#    30	exec 1>&5
+#    31	bdl_stdout=1
+#    32	
+#    33	# No printing to stdout
+#    34	echo -n >bdl_out.txt
+#    35	bdl_stdout=1
+#    36	bdl_dst=
+#    37	bdl 0 "not printed"
+#    38	bdl_stdout=0
+#    39	bdl
+#    40	bdl bdl_out.txt "printed to bdl_out.txt"
+#    41	bdl "not printed"
+#    42	bdl_stdout=1
+#    43	cat bdl_out.txt
+#    44	
+#    45	# This prints a "0" since there is only one parameter
+#    46	bdl 0
+#
+#
+# This is the output of bdl-exmpl.sh
+#
+#  $ /bin/bash ./bdl-exmpl.sh
+#  bdl-exmpl.sh:8:
+#  bdl-exmpl.sh:9: hi
+#  bdl-exmpl.sh:10: hi
+#  bdl-exmpl.sh:14: hi to bdl_out.txt
+#  bdl-exmpl.sh:20: hi to bdl_out.txt
+#  bdl-exmpl.sh:27:
+#  bdl-exmpl.sh:28: hi
+#  bdl-exmpl.sh:29: hi
+#  bdl-exmpl.sh:40: printed to bdl_out.txt
+#  bdl-exmpl.sh:46: 0
+# ##################################################################
+
+BDL_LOADED=t
+
+# Prompt waitng for a return, q will exit
+bdl_pause () {
+	read -p "Line ${BASH_LINENO}: $@" bdl_pause_v_
+	[[ "$bdl_pause_v_" == "q" ]] && exit 1
+}
+
+# Initialize bdl variables if user didn't
+[[ "$bdl_dst" == "" ]] && bdl_dst=
+[[ "$bdl_stdout" == "" ]] && bdl_stdout=1
+[[ "$bdl_call_depth" == "" ]] && bdl_call_depth=0
+[[ "$bdl_call_stack_view" == "" ]] && bdl_call_stack_view=f
+
+# Initialize priviate bdl variables
+_bdl_call_lineno_offset_array=()
+_bdl_call_lineno_offset_array_idx=0
+_bdl_call_save=()
+_bdl_call_save_idx=0
+
+# Push bdl state and initialize call meta data.
+#
+# $1 is value for bdl_call_depth
+# $2 Optional text of a script with where bdl calls
+#    will be found and used to compute lineno info.
+bdl_push () {
+	# Push the bdl data
+	_bdl_call_save[$_bdl_call_save_idx]="bdl_dst=$bdl_dst; \
+bdl_stdout=$bdl_stdout; \
+bdl_call_depth=$bdl_call_depth; \
+bdl_call_stack_view=$bdl_call_stack_view; \
+_bdl_call_lineno_offset_array=(${_bdl_call_lineno_offset_array[*]}); \
+_bdl_call_lineno_offset_array_idx=$_bdl_call_lineno_offset_array_idx"
+	_bdl_call_save_idx=$((_bdl_call_save_idx+1))
+
+	# Set meta data to fudge line numbers when bdl is used in tests.
+	bdl_call_depth=$1
+	shift
+	_bdl_call_lineno_offset_array_idx=0
+	_bdl_call_lineno_offset_array=()
+
+	if test "$1" != ""
+	then
+		# Read the script and find lines that begin with "bdl "
+		# and compute their offsets and saving them in an array
+		# that bdl will use to compute compute the lineno.
+		IFS=$'\n' read -d '' -r -a test_run_script_array <<< "$@"
+		for i in "${!test_run_script_array[@]}"; do
+			ln=${test_run_script_array[$i]}
+			tln="$(sed -e 's/^[[:space:]]*//' <<<$ln)"
+			if [[ "$tln" =~ ^bdl\  ]]
+			then
+				_bdl_call_lineno_offset_array+=$((i+1))
+			fi
+		done
+	fi
+}
+
+# Pop a previously save state.
+bdl_pop () {
+	_bdl_call_save_idx=$((_bdl_call_save_idx-1))
+	eval "${_bdl_call_save[$_bdl_call_save_idx]}"
+}
+
+# Write debug info with no source or line number
+bdl_nsl () {
+	if (( $# > 1 )); then
+		bdl_nsl_v_=$1
+		shift
+	else
+		bdl_nsl_v_=$bdl_dst
+	fi
+	[[ "$bdl_nsl_v_" == "" ]] && bdl_nsl_v_=$bdl_stdout
+	if [[ "$bdl_nsl_v_" != "" && "$@" != "" ]]; then
+		if [[ $bdl_nsl_v_ =~ ^[0-9] ]]; then
+			# There's probably a better way, but this "works":
+			case $bdl_nsl_v_ in
+				1) echo "$@" 1>&1 ;;
+				2) echo "$@" 1>&2 ;;
+				3) echo "$@" 1>&3 ;;
+				4) echo "$@" 1>&4 ;;
+				5) echo "$@" 1>&5 ;;
+				6) echo "$@" 1>&6 ;;
+				7) echo "$@" 1>&7 ;;
+				8) echo "$@" 1>&8 ;;
+				9) echo "$@" 1>&9 ;;
+				*) : ;; # 0 and all other characters are nop's
+			esac
+		else
+			echo "$@" >> $bdl_nsl_v_
+		fi
+	fi
+	return 0
+}
+
+# Write debug info with file name and line number.
+bdl () {
+	#View the call stack
+	if test "$bdl_call_stack_view" != "f"
+	then
+		for i in "${!BASH_SOURCE[@]}"; do
+			(( $i == 0 )) && ln=${LINENO} || ln=${BASH_LINENO[${i}-1]}
+			bdl_nsl "[$i] ${BASH_SOURCE[$i]##*/}:${FUNCNAME[$i]}:${ln}"
+		done
+	fi
+
+	# The ${@:+ } only adds a space if $@ isn't empty.
+	# This is done because We allow the call to bdl to
+	# have no parameters and bdl then just prints the
+       	# file name and line number which can be useful
+	# to know a line was processed but there is no need
+	# to print any other data.
+	bdl_ln=${BASH_LINENO[${bdl_call_depth}]}
+	if (( ${_bdl_call_lineno_offset_array_idx} < ${#_bdl_call_lineno_offset_array[@]} ))
+	then
+		bdl_offset=${_bdl_call_lineno_offset_array[$_bdl_call_lineno_offset_array_idx]}
+		bdl_ln=$((bdl_ln+bdl_offset))
+		_bdl_call_lineno_offset_array_idx=$((_bdl_call_lineno_offset_array_idx+1))
+	fi
+	if (( $# <= 1 )); then
+		bdl_nsl $bdl_dst "${BASH_SOURCE[${bdl_call_depth}+1]##*/}:${bdl_ln}:${@:+ }$@"
+	else
+		v_=$1
+		shift
+		bdl_nsl $v_ "${BASH_SOURCE[${bdl_call_depth}+1]##*/}:${bdl_ln}:${@:+ }$@"
+	fi
+	return 0
+}
diff --git a/t/t0014-bdl-lib.sh b/t/t0014-bdl-lib.sh
new file mode 100755
index 000000000..a0546189a
--- /dev/null
+++ b/t/t0014-bdl-lib.sh
@@ -0,0 +1,115 @@
+#!/bin/sh
+
+test_description='test bash debug logger'
+
+# Only execute if shell is bash
+if test "$BASH_VERSION" != ""
+then
+
+. ../bdl-lib.sh
+bdl_dst=output
+
+fi
+
+. ./test-lib.sh
+
+test_expect_success 'do nothing' '
+	printf "" >output &&
+	printf "" >expected &&
+	test_cmp expected output
+'
+
+# Only execute if shell is bash
+if test "$BASH_VERSION" != ""
+then
+
+test_expect_success 'bdl_nsl only prints nothing' '
+	printf "" >output &&
+	bdl_nsl &&
+	printf "" >expected &&
+	test_cmp expected output
+'
+
+test_expect_success 'bdl_nsl with string prints string only' '
+	printf "" >output &&
+	bdl_nsl "test 1" &&
+	printf "test 1\n" >expected &&
+	test_cmp expected output
+'
+
+test_lineno=$LINENO
+test_expect_success 'bdl only print source and linenumber' '
+	printf "" >output &&
+	bdl &&
+	printf "t0014-bdl-lib.sh:$((test_lineno+3)):\n" >expected &&
+	test_cmp expected output
+'
+
+test_lineno=$LINENO
+test_expect_success 'bdl with string prints source and linenumber and string' '
+	printf "" >output &&
+	bdl "test 1" &&
+	printf "t0014-bdl-lib.sh:$((test_lineno+3)): test 1\n" >expected &&
+	test_cmp expected output
+'
+
+test_expect_success 'bdl 0 "nothing printed"' '
+	printf "" >output &&
+	printf "" >expected &&
+	bdl 0 "nothing printed" &&
+	test_cmp expected output
+'
+
+# Save current bdl_dst and restore when test completes
+#bdl_dst_save=$bdl_dst
+#bdl_stdout_save=$bdl_stdout
+bdl_push
+test_expect_success 'bdl bdl_dst empty bdl_stdout=0 nothing printed' '
+	bdl_dst= &&
+	bdl_stdout=0 &&
+	printf "" >output &&
+	printf "" >expected &&
+	bdl "nothing printed" &&
+	test_cmp expected output
+'
+#bdl_dst=$bdl_dst_save
+#bdl_stdout=$bdl_stdout_save
+bdl_pop
+
+# Testing subroutine calls from a test verify bdl_push/pop works
+# for direct calls and nested calls
+subsub_lineno=$LINENO
+subsub () {
+	bdl_push 0
+	bdl "subsub line"
+	bdl_pop
+	return "0"
+}
+
+sub_lineno=$LINENO
+sub () {
+	bdl_push 0
+	bdl "sub line"
+	subsub
+	bdl_pop
+	return "0"
+}
+
+test_expect_success 'test calls subsub' '
+	printf "" >output &&
+	subsub &&
+	printf "t0014-bdl-lib.sh:$((subsub_lineno+3)): subsub line\n" >expected &&
+	test_cmp expected output
+'
+
+test_expect_success 'test sub calls subsub' '
+	printf "" >output &&
+	sub &&
+	printf "t0014-bdl-lib.sh:$((sub_lineno+3)): sub line
+t0014-bdl-lib.sh:$((subsub_lineno+3)): subsub line\n" >expected &&
+	test_cmp expected output
+'
+
+fi
+
+test_done
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 7740d511d..85142e462 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -680,11 +680,15 @@ test_run_ () {
 		trace=$trace_tmp
 	fi
 
+	test "$BDL_LOADED" = "t" && bdl_push 4 "$@"
+
 	setup_malloc_check
 	test_eval_ "$1"
 	eval_ret=$?
 	teardown_malloc_check
 
+	test "$BDL_LOADED" = "t" && bdl_pop
+
 	if test -z "$immediate" || test $eval_ret = 0 ||
 	   test -n "$expecting_failure" && test "$test_cleanup" != ":"
 	then
-- 
2.16.3

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Johannes Schindelin]

Hi Wink,

On Tue, 27 Mar 2018, Wink Saville wrote:

> Add bdl-lib.sh which provides functions to assit in debugging git
> shell scripts and tests.

Interesting idea. It is Bash-only, though... (and it is not a secret that
I want to discourage using Unix shell scripts in Git's production code,
they are a decent way to prototype things, but they fall short of being
robust and portable in practice, and don't get me started on speed
issues).

So rather than spending time on making it easier to debug shell scripts, I
would love to see us going into the direction of a consistent C source
code. I still believe that we can get there, and that the benefits are
worth the (huge) effort.

Ciao,
Johannes

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Wink Saville]

On Fri, Mar 30, 2018 at 3:34 AM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> Hi Wink,
>
> On Tue, 27 Mar 2018, Wink Saville wrote:
>
>> Add bdl-lib.sh which provides functions to assit in debugging git
>> shell scripts and tests.
>
> Interesting idea. It is Bash-only, though... (and it is not a secret that
> I want to discourage using Unix shell scripts in Git's production code,
> they are a decent way to prototype things, but they fall short of being
> robust and portable in practice, and don't get me started on speed
> issues).
>
> So rather than spending time on making it easier to debug shell scripts, I
> would love to see us going into the direction of a consistent C source
> code. I still believe that we can get there, and that the benefits are
> worth the (huge) effort.
>
> Ciao,
> Johannes

I totally agree the code base should use primarily one language!

But that's not the case now and "bdl" gave me insight into the workings
of rebase--interactive and I found little guidance on how to debug
or learn the code base. So I thought I'd see if there was interest
in what I'd done.

If I were to make it non-bash specific would there be any interest?

-- Wink

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Johannes Schindelin]

Hi Wink,

On Fri, 30 Mar 2018, Wink Saville wrote:

> On Fri, Mar 30, 2018 at 3:34 AM, Johannes Schindelin
> <Johannes.Schindelin@gmx.de> wrote:
> >
> > On Tue, 27 Mar 2018, Wink Saville wrote:
> >
> >> Add bdl-lib.sh which provides functions to assit in debugging git
> >> shell scripts and tests.
> >
> > Interesting idea. It is Bash-only, though... (and it is not a secret
> > that I want to discourage using Unix shell scripts in Git's production
> > code, they are a decent way to prototype things, but they fall short
> > of being robust and portable in practice, and don't get me started on
> > speed issues).
> >
> > So rather than spending time on making it easier to debug shell
> > scripts, I would love to see us going into the direction of a
> > consistent C source code. I still believe that we can get there, and
> > that the benefits are worth the (huge) effort.
> 
> I totally agree the code base should use primarily one language!

Great!

> But that's not the case now and "bdl" gave me insight into the workings
> of rebase--interactive and I found little guidance on how to debug
> or learn the code base. So I thought I'd see if there was interest
> in what I'd done.

And realistically, requiring Bash is not all that much of a problem when
targeting developers.

> If I were to make it non-bash specific would there be any interest?

After thinking about this more, I am a lot less opposed to including this
in Git's source code. However, as it is not necessary for Git's
functionality, it should probably go into contrib/, and I would much
rather have a more descriptive name such as
contrib/bash-debugging-library/...

Ciao,
Johannes

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Wink Saville]

On Thu, Apr 5, 2018 at 6:37 AM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> After thinking about this more, I am a lot less opposed to including this
> in Git's source code. However, as it is not necessary for Git's
> functionality, it should probably go into contrib/, and I would much
> rather have a more descriptive name such as
> contrib/bash-debugging-library/...
>
> Ciao,
> Johannes

I'll move it, thanks for the feed back and considering it for inclusion.

-- Wink

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Johannes Schindelin]

Hi Wink,

On Thu, 5 Apr 2018, Wink Saville wrote:

> On Thu, Apr 5, 2018 at 6:37 AM, Johannes Schindelin
> <Johannes.Schindelin@gmx.de> wrote:
> > After thinking about this more, I am a lot less opposed to including this
> > in Git's source code. However, as it is not necessary for Git's
> > functionality, it should probably go into contrib/, and I would much
> > rather have a more descriptive name such as
> > contrib/bash-debugging-library/...
> 
> I'll move it, thanks for the feed back and considering it for inclusion.

Note: I have nothing to do with including it. That is the sole discretion
of Junio (who is offline for a week or two, if I understood correctly).

Ciao,
Johannes

Re: [RFC PATCH 0/1] bdl-lib.sh: add bash debug logger

[Wink Saville]

On Thu, Apr 5, 2018 at 12:32 PM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> Hi Wink,

> Note: I have nothing to do with including it. That is the sole discretion
> of Junio (who is offline for a week or two, if I understood correctly).
>
> Ciao,
> Johannes

txs, I understand.