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Missing Semester 07 - Debugging and Profiling

On logging, ANSI escape codes, debuggers, profilers, and resource monitoring.

Written by Eva Dee on (about a 4 minute read).

Debugging permalink

1. prinf debugging a.k.a. console.log() permalink

2. Logging permalink

Which is better because:

  • you can log to files, sockets, or even remote servers instead of standard output
  • you can add severity levels (+ color coding)
  • logging more information

🤔 How do you output colors in the terminal? ANSI escape codes are standardized commands used to manipulate the behavior and appearance of the text in a terminal or terminal emulator.

ANSI escape sequence is a sequence of ASCII characters, the first two of which are the ASCII "Escape" and the left-bracket character "[". The character or characters following the escape and left-bracket characters specify an alphanumeric code that controls a keyboard or display function.

The most basic terminals have a set of 8 different colors:

  • Black: \u001b[30m

  • Red: \u001b[31m

  • Green: \u001b[32m

  • Yellow: \u001b[33m

  • Blue: \u001b[34m

  • Magenta: \u001b[35m

  • Cyan: \u001b[36m

  • White: \u001b[37m

  • Reset: \u001b[0m


  • print "\u001b[30m A \u001b[31m B \u001b[32m C \u001b[33m D \u001b[0m"

This will print letters A (in black), B (pink), C (green), D (yellow).

3. Third party logs permalink

  • var/log/system.log

Or more commonly, the system log (/var/log/system.log):

  • log show

🤔 You can also see system logs in:

Finder > Applications > Utilities > Console.

  • log show --last 10s

Show logs for the past 10 seconds.

  • logger "hello logs"

Add stuff to the system log (using a shell program called logger).

  • log show --last 1m | grep hello

Find that log (the one you just added)

🤔 More log files on macOS permalink

  • System Log Folder: /var/log
  • System Log: /var/log/system.log
  • Mac Analytics Data: /var/log/DiagnosticMessages
  • System Application Logs: /Library/Logs
  • System Reports: /Library/Logs/DiagnosticReports
  • User Application Logs: ~/Library/Logs (in other words, /Users/NAME/Library/Logs)
  • User Reports: ~/Library/Logs/DiagnosticReports (in other words, /Users/NAME/Library/Logs/DiagnosticReports)

4. Debuggers permalink

Debuggers are programs that let you interact with the execution of a program and allow you to:

  • Halt execution of the program when it reaches a certain line.
  • Step through the program one instruction at a time.
  • Inspect values of variables after the program crashed.
  • Conditionally halt the execution when a given condition is met.

5. Static analysis permalink

Static analysis programs take source code as input and analyze it using coding rules to reason about its correctness (linters), e.g. shellcheck for shell scripts, prettier for HTML, CSS, JS.

Profiling permalink

1. Timing permalink

Similarly to the debugging case, in many scenarios, it can be enough to just print the time it took your code between two points.

  • real time: elapsed time from start to finish of the program, including the time taken by other processes and time taken while blocked
  • user time: Amount of time spent in the CPU running user code
  • system time: Amount of time spent in the CPU running kernel code


  • time curl &> /dev/null

Will output how long in real, user, and system time it takes to curl that specific url.

2. Profilers permalink

Most of the time, when people refer to profilers, they actually mean CPU profilers.

  • tracing profilers: tracing profilers keep a record of every function call your program makes
  • sampling profilers: probe your program periodically (commonly every millisecond) and record the program’s stack

🤔 Logging vs Tracing vs Monitoring

  • We use logging to represent state transformations within an application. When things go wrong, we need logs to establish what change in the state caused the error.
  • A trace represents a single user’s journey through an entire stack of an application. It is often used for optimization purposes. For example, you would use it to establish little used part of a stack or bottlenecks within specific parts of the stack.
  • Instrumenting an application and monitoring the results represents the use of a system. It is most often used for diagnostic purposes. For example, we would use monitoring systems to alert developers when the system is not operating “normally”.

🤔 Also this Twitter thread.

There's also line profiler, memory profiler, event profiling.

3. Resource monitoring permalink

  • htop

A process viewer.

  • du -h [path]

List the sizes of a directory and any subdirectories in human-readable form (i.e. auto-selecting the appropriate unit for each size)

  • lsof

Lists file information about files opened by processes.

  • lsof path/to/file

Find the processes that have a given file open.

  • lsof -i :port

Find the process that opened a local internet port.