This document discusses using the jq command line tool and Python to filter and format JSON data within Xcode's debugger LLDB. It describes running jq filters on JSON strings extracted from variables, saving the filter and JSON to temporary files for jq to process instead of using stdin. A Python function is defined to run jq from LLDB and return the output.

1. Advanced Debugging With XCode
Extending LLDB
Image by Aijaz Ansari. Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)

2. Aijaz
@_aijaz_

4. (Since 1982)
😱

5. Debug

6. Sample
Project

9. Session
id: String
title: String?
sessionDescription: String?
startTime: Date
endTime: Date
speaker: Speaker?
startDate: String
init?(withId id: String)
Speaker
id: String
name: String
imagePath: String?
bio: String?
twitterHandle: String?
website: String?
sessions: [Session]
init?(withId id: String)
speaker
sessions

10. Demo 1

11. Image by Aijaz Ansari. Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)

12. Let’s Recap

13. # In ~/.lldbinit, available to all LLDB sessions
type summary add --summary-string
"(${var._indexes[0]}, ${var._indexes[1]})" IndexPath
type summary add --summary-string
"${var.title} by ${var.speaker.name}" MyConf.Session
Quick & Dirty Type Summaries

14. Summaries in Python

15. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
sessionFormatter.py

16. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
sessionFormatter.py

17. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
sessionFormatter.py

18. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format ()
sessionFormatter.py

19. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
sessionFormatter.py

20. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
title = valobj.GetChildMemberWithName('title')
name = valobj.GetChildMemberWithName(‘speaker’)
.GetChildMemberWithName('name')
return stripQuotes(title.GetObjectDescription())
+ " by " +
stripQuotes(name.GetObjectDescription())
sessionFormatter.py

21. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
title = valobj.GetChildMemberWithName('title')
name = valobj.GetChildMemberWithName(‘speaker’)
.GetChildMemberWithName('name')
return stripQuotes(title.GetObjectDescription())
+ " by " +
stripQuotes(name.GetObjectDescription())
sessionFormatter.py

22. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
title = valobj.GetChildMemberWithName('title')
name = valobj.GetChildMemberWithName(‘speaker’)
.GetChildMemberWithName('name')
return stripQuotes(title.GetObjectDescription())
+ " by " +
stripQuotes(name.GetObjectDescription())
def __lldb_init_module(debugger, dict):
command = 'type summary add —-python-function
sessionFormatter.format MyConf.Session'
debugger.HandleCommand(command)
sessionFormatter.py

23. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
title = valobj.GetChildMemberWithName('title')
name = valobj.GetChildMemberWithName(‘speaker’)
.GetChildMemberWithName('name')
return stripQuotes(title.GetObjectDescription())
+ " by " +
stripQuotes(name.GetObjectDescription())
def __lldb_init_module(debugger, dict):
command = 'type summary add —-python-function
sessionFormatter.format MyConf.Session'
debugger.HandleCommand(command)
sessionFormatter.py

24. #!/usr/bin/python
import re
regex = re.compile('[^"]*"(.*)"n$')
def stripQuotes(str):
"""Utility method to strip the first and last quote
and anything outside them"""
match = regex.match(str)
if match :
return match.group(1)
return str
def format (valobj,internal_dict):
title = valobj.GetChildMemberWithName('title')
name = valobj.GetChildMemberWithName(‘speaker’)
.GetChildMemberWithName('name')
return stripQuotes(title.GetObjectDescription())
+ " by " +
stripQuotes(name.GetObjectDescription())
def __lldb_init_module(debugger, dict):
command = 'type summary add —-python-function
sessionFormatter.format MyConf.Session'
debugger.HandleCommand(command)
sessionFormatter.py

25. command script import …/sessionFormatter.py
# Add things here, like type summaries, breakpoints
# Even execute Swift or Objective-C code with 'expr'
.lldbinit-MyConf

26. Project-Specific Loading
in main() or application(_:didFinishLaunchingWithOptions:)

27. Any
Questions?

28. { "version": 1,
"speakers": [
{"id": 3,
"img": “chad.jpg",
"name": “Chad Perk",
"twitter": "chad",
"blog": “www.example.com/chad”,
"bio": "Chad is the co-author of…”
},

29. jq

30. http://feelgrafix.com/data_images/out/28/973590-avatar-the-last-airbender.jpg

31. $ cat avatar.json
[{"name":"Aang","sex":"M","born":-12,"died":153,"bending":["Air","Water","Earth","Fire","Energy"],"identity":
{"nationality":"Southern Air Temple","ethnicity":"Air Nomad"},"spouse":"Katara","children":[{"sex":"M","name":"Bumi"},
{"sex":"F","name":"Kya"},{"sex":"M","name":"Tenzin"}]},{"name":"Katara","sex":"F","born":85,"died":null,"bending":
["Water","Blood"],"identity":{"nationality":"Southern Water Tribe","ethnicity":"Water Tribe"},"spouse":"Aang","children":
[{"sex":"M","name":"Bumi"},{"sex":"F","name":"Kya"},{"sex":"M","name":"Tenzin"}]},{"name":"Sokka","sex":"M","born":84,"died":
164,"bending":[],"identity":{"nationality":"Southern Water Tribe","ethnicity":"Water Tribe"},"spouse":null,"children":[]},
{"name":"Toph Beifong","sex":"F","born":88,"died":null,"bending":["Earth","Metal"],"identity":{"nationality":"Gaoling, Earth
Kingdom","ethnicity":"Earth Kingdom"},"spouse":null,"children":[{"sex":"F","name":"Lin Beifong"},{"sex":"F","name":"Suyin
Beifong"}]},{"name":"Iroh","sex":"M","born":null,"died":null,"bending":["Fire","Energy"],"identity":{"nationality":"Fire Nation
Capital, Fire Nation","ethnicity":"Fire Nation"},"spouse":null,"children":[{"sex":"M","name":"Lu Ten"}]},
{"name":"Zuko","sex":"M","born":83,"died":null,"bending":["Fire","Energy"],"identity":{"nationality":"Fire Nation Capital, Fire
Nation","ethnicity":"Fire Nation"},"spouse":null,"children":[{"sex":"F","name":"Izumi"}]},
{"name":"Kya","sex":"F","born":null,"died":null,"bending":["Water"],"identity":{"nationality":"Southern Water
Tribe","ethnicity":"Water Tribe, Air Nomad"},"spouse":null,"children":[]},
{"name":"Bumi","sex":"M","born":null,"died":null,"bending":["Air"],"identity":{"nationality":"United Republic","ethnicity":"Water
Tribe, Air Nomad"},"spouse":null,"children":[]},{"name":"Tenzin","sex":"M","born":null,"died":null,"bending":["Air"],"identity":
{"nationality":"Republic City, United Republic","ethnicity":"Water Tribe, Air Nomad"},"spouse":null,"children":
[{"sex":"F","name":"Jinora"},{"sex":"F","name":"Ikki"},{"sex":"M","name":"Meelo"},{"sex":"M","name":"Rohan"}]},{"name":"Lin
Beifong","sex":"F","born":120,"died":null,"bending":["Earth","Metal"],"identity":{"nationality":"Republic City, United
Republic","ethnicity":"Earth Kingdom"},"spouse":null,"children":[]},{"name":"Suyin Beifong","sex":"F","born":
126,"died":null,"bending":["Earth","Metal"],"identity":{"nationality":"Republic City, United Republic","ethnicity":"Earth
Kingdom"},"spouse":null,"children":[{"sex":"M","name":"Bataar Jr."},{"sex":"F","name":"Opal"},{"sex":"M","name":"Wei"},
{"sex":"M","name":"Wing"},{"sex":"M","name":"Huan"}]}]
$

32. # Pretty print the jq
$ jq '.' avatar.json
command input filefilter

33. # Pretty print the jq
$ jq '.' avatar.json
...
{
"sex": "M",
"name": "Huan"
}
]
}
]
$

34. # Pretty print the jq
$ jq '.' avatar.json
...
{
"sex": "M",
"name": "Huan"
}
]
}
]
$
# The ']' on the last line tells me this is an array

35. # Pretty print the jq
$ jq '.' avatar.json
...
{
"sex": "M",
"name": "Huan"
}
]
}
]
$
# The ']' on the last line tells me this is an array
# The '}' on the 2nd-last line tells me this is an array of objects

36. # List the keys of each object
$ jq ' .[] | keys ' avatar.json
foreach item in array print the keys of each object
pipe the output of one filter into the input of the next

37. # List the keys of each object
$ jq ' .[] | keys ' avatar.json
[
"bending",
"born",
"children",
"died",
"identity",
"name",
"sex",
"spouse"
]
[
"bending",
"born",…
$

38. # The name of each character
$ jq ' .[] | .name ' avatar.json
foreach item in array extract the ‘name’ field

39. # The name of each character
$ jq ‘.[] | .name’ avatar.json
"Aang"
"Katara"
"Sokka"
"Toph Beifong"
"Iroh"
"Zuko"
"Kya"
"Bumi"
"Tenzin"
"Lin Beifong"
$

40. # The name of each female character
$ jq ' .[] | select(.sex == "F") | .name ' avatar.json
foreach item in array
extract the ‘name’ field
where sex is F

41. # The name of each female character
$ jq ' .[] | select(.sex == "F") | .name ' avatar.json
"Katara"
"Toph Beifong"
"Kya"
"Lin Beifong"
"Suyin Beifong”
$

42. Demo 2

43. Image by Aijaz Ansari. Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)

44. (lldb) jq ‘.speakers[]|keys’ jsonString
jq filter (program) String
1. Gather input
2. Run the jq filter on the string, saving the output
3. Print the output

45. #!/usr/bin/python
import commands
import lldb
import shlex

46. #!/usr/bin/python
import commands
import lldb
import shlex
# The actual python function that is bound to the lldb command.
def jq_command(debugger, command, result, dict):

47. def jq_command(debugger, command, result, dict):

48. def jq_command(debugger, command, result, dict):
target = debugger.GetSelectedTarget()

49. def jq_command(debugger, command, result, dict):
target = debugger.GetSelectedTarget()
process = target.GetProcess()

50. def jq_command(debugger, command, result, dict):
target = debugger.GetSelectedTarget()
process = target.GetProcess()
thread = process.GetSelectedThread()

51. def jq_command(debugger, command, result, dict):
target = debugger.GetSelectedTarget()
process = target.GetProcess()
thread = process.GetSelectedThread()
frame = thread.GetSelectedFrame()

52. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()

53. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)

54. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)

55. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]

56. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable

57. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())

58. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"

59. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry
about escaping special characters.
jq_filter_file = "/tmp/jq_filter"

60. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>"
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry
about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq
will be invoked on the file, not using stdin
jq_json_file = “/tmp/jq_json"

61. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()

62. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()
f = open(jq_filter_file, 'w')
f.write(jq_filter)
f.close()

63. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()
f = open(jq_filter_file, 'w')
f.write(jq_filter)
f.close()
# invoke jq and capture the output
output = commands.getoutput("%s -f %s %s" % (
jq_exe, jq_filter_file, jq_json_file) )

64. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()
f = open(jq_filter_file, 'w')
f.write(jq_filter)
f.close()
# invoke jq and capture the output
output = commands.getoutput("%s -f %s %s" % (
jq_exe, jq_filter_file, jq_json_file) )
print >>result, output

65. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()
f = open(jq_filter_file, 'w')
f.write(jq_filter)
f.close()
# invoke jq and capture the output
output = commands.getoutput("%s -f %s %s" % (
jq_exe, jq_filter_file, jq_json_file) )
print >>result, output
Gather Input
Run Command
Print Output

66. def jq_command(debugger, command, result, dict):
frame = debugger.GetSelectedTarget().GetProcess().GetSelectedThread().GetSelectedFrame()
# The command is called like "jq <filter> <stringVar>”
command_args = shlex.split(command)
jq_filter = command_args[0]
val = frame.var(command_args[1]) # access the variable
val_string = eval(val.GetObjectDescription())
# path to the jq executable.
jq_exe = "/Users/aijaz/local/bin/jq"
# We save the filter to a file so that we don’t have to worry about escaping special characters.
jq_filter_file = "/tmp/jq_filter"
# the value of the NSString variable is saved in this file jq will be invoked on the file, not using stdin
jq_json_file = "/tmp/jq_json"
# write the json file and jq filter to temp files
f = open(jq_json_file, 'w')
f.write(val_string)
f.close()
f = open(jq_filter_file, 'w')
f.write(jq_filter)
f.close()
# invoke jq and capture the output
output = commands.getoutput("%s -f %s %s" % (
jq_exe, jq_filter_file, jq_json_file) )
print >>result, output
def __lldb_init_module(debugger, dict):
# Add any commands contained in this module to LLDB
command = 'command script add -f jq.jq_command jq'
debugger.HandleCommand(command)

67. What have
we learned?

68. Validate your
hypothesis

69. (if you’re lucky enough to have a hypothesis.)

70. Observe

71. Analyze

72. Extend

73. Any
Questions?

74. https://github.com/aijaz/lldbPythonScripts
http://aijaz.net/2017/01/11/lldb-python/index.html
https://github.com/facebook/chisel

75. LLDB: https://lldb.llvm.org/
A blog post from January, where I write about most of the same stuff:
Extending LLDB: http://aijaz.net/2017/01/11/lldb-python/index.html
JQ
jq: https://stedolan.github.io/jq/
A Facebook library that adds a lot of cool extensions to LLDB
Facebook Chisel: https://github.com/facebook/chisel
WWDC Sessions:
Debugging Tips and Tricks https://developer.apple.com/videos/play/wwdc2016/417/
What’s new in LLDB: https://developer.apple.com/videos/play/wwdc2015/402/
Introduction to LLDB and the Swift REPL https://developer.apple.com/videos/play/wwdc2014/409/
Advanced Swift Debugging in LLDB https://developer.apple.com/videos/play/wwdc2014/410/
Debugging in Xcode 6 https://developer.apple.com/videos/play/wwdc2014/413/
Debugging with Xcode https://developer.apple.com/videos/play/wwdc2013/407/
* Advanced Debugging with LLDB https://developer.apple.com/videos/play/wwdc2013/413/
* Debugging in Xcode https://developer.apple.com/videos/play/wwdc2012/412/
* Debugging with Xcode https://developer.apple.com/videos/play/wwdc2012/415/
*: Contains material related to this talk

76. Thank You!
https://www.theodysseyonline.com/overthinking-spongebob
@_aijaz_