Posts on A Random Walk

strace tips for better debugging

Fri, 13 Jun 2025 19:08:50 +0530

Recently, I have been building software without libc to better understand Linux syscalls and internals better. So far, I have built a minimal shell, terminal Snake game, pure ARM64 assembly HTTP server and threads implementation. I have been using strace extensively while debugging.

Useful options and flags

I use a version of the following command:

strace -fintrCDTYyy -o strace.log -v -s128 ./binary

This looks like an alphabet soup of options! Here’s what they do and how they are useful:

-f: Follow child processes/threads. This is especially useful when dealing with spawning processes or threads as otherwise, strace will only trace the parent process.
-v: Print unabbreviated versions of environment, stat, termios and other structs in syscalls. I found this invaluable in conjunction with -s when doing assembly programming to check if the structs were being initialized correctly and if certain arguments were being sent in little/big endian format
-s NUM: Specify the maximum string size to print. Useful for large structs
-o: Save strace output to a log file. It is always better to do this to investigate the output of the original process and strace separately without each cluttering the other
-yy: Print all available information associated with file descriptors. This is great for expanding the file descriptor to either its full path in case of a file or TCP address in case of sockets
-Y: Print command names for PIDs. I found this useful when building the shell to check if the correct program is being executed
-t: Print current timestamp in log
-T: Show time spent in syscalls. Useful for some basic profiling although strace heavily slows down the process.
-r: Print a relative timestamp upon entry to each system call
-n: Print syscall number. Great to quickly find out syscall numbers on new architectures.
-i: Print instruction pointer at the time of syscall. Found this useful when debugging assembly code to check rough location of errors.
-C: Print summary of syscall count, time, errors at the end of regular output

Print stack traces

The -k or --stack-trace prints the stacktrace along with the syscall. This is useful if your program is compiled with with -g. This post is a good read on using strace to show backtraces for a Golang program compiled with GODEBUG.

Bash - Oops I deleted my bin dir challenge

Tue, 20 May 2025 14:44:24 +0530

Oops I deleted my bin dir is a fun set of challenges to investigate a “rogue” process which has been deleting files on the system. It has deleted all programs from the bin directories like ps, htop, lsof. All we have left is a shell prompt…

Bash built-ins

In the challenge, the shell we are using is bash. Bash has many “built-ins” which are commands we can run without having to spawn a process and execute a binary i.e they are part of bash itself.

The most useful one right now is help which lists all the bash built-ins and their arguments. Typing help BUILTIN_NAME will give more details about that built-in command.

List of built-ins


job_spec [&]                                                                   history [-c] [-d offset] [n] or history -anrw [filename] or history -ps[1/218]
 (( expression ))                                                               if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COM>
 . filename [arguments]                                                         jobs [-lnprs] [jobspec ...] or jobs -x command [args]
 :                                                                              kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigs>
 [ arg... ]                                                                     let arg [arg ...]
 [[ expression ]]                                                               local [option] name[=value] ...
 alias [-p] [name[=value] ... ]                                                 logout [n]
 bg [job_spec ...]                                                              mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callb>
 bind [-lpsvPSVX] [-m keymap] [-f filename] [-q name] [-u name] [-r keyseq] [>  popd [-n] [+N | -N]
 break [n]                                                                      printf [-v var] format [arguments]
 builtin [shell-builtin [arg ...]]                                              pushd [-n] [+N | -N | dir]
 caller [expr]                                                                  pwd [-LP]
 case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac                     read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p pro>
 cd [-L|[-P [-e]] [-@]] [dir]                                                   readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C cal>
 command [-pVv] command [arg ...]                                               readonly [-aAf] [name[=value] ...] or readonly -p
 compgen [-abcdefgjksuv] [-o option] [-A action] [-G globpat] [-W wordlist] [>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DEI] [-o option] [-A action] [-G globpat] [>  select NAME [in WORDS ... ;] do COMMANDS; done
 compopt [-o|+o option] [-DEI] [name ...]                                       set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
 continue [n]                                                                   shift [n]
 coproc [NAME] command [redirections]                                           shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]                               source filename [arguments]
 dirs [-clpv] [+N] [-N]                                                         suspend [-f]
 disown [-h] [-ar] [jobspec ... | pid ...]                                      test [expr]
 echo [-neE] [arg ...]                                                          time [-p] pipeline
 enable [-a] [-dnps] [-f filename] [name ...]                                   times
 eval [arg ...]                                                                 trap [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [argument ...]] [redirection ...]                true
 exit [n]                                                                       type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or export -p                                   typeset [-aAfFgiIlnrtux] [-p] name[=value] ...
 false                                                                          ulimit [-SHabcdefiklmnpqrstuvxPT] [limit]
 fc [-e ename] [-lnr] [first] [last] or fc -s [pat=rep] [command]               umask [-p] [-S] [mode]
 fg [job_spec]                                                                  unalias [-a] name [name ...]
 for NAME [in WORDS ... ] ; do COMMANDS; done                                   unset [-f] [-v] [-n] [name ...]
 for (( exp1; exp2; exp3 )); do COMMANDS; done                                  until COMMANDS; do COMMANDS; done
 function name { COMMANDS ; } or name () { COMMANDS ; }                         variables - Names and meanings of some shell variables
 getopts optstring name [arg ...]                                               wait [-fn] [-p var] [id ...]
 hash [-lr] [-p pathname] [-dt] [name ...]                                      while COMMANDS; do COMMANDS; done
 help [-dms] [pattern ...]                                                      { COMMANDS ; }

Challenge 1 - print the current working directory

Solution

The `pwd` built-in shows the *p*resent *w*orking *d*irectory

Challenge 2 - List all files in the current working directory

We can not use ls as it is gone… Which built-in can help us?

HTML toggle headings using Details Summary element and some CSS

Thu, 06 Mar 2025 14:40:15 +0530

Overview of the HTML `details` tag

The details tag creates a toggleable element which we can use to hide sensitive information like spoilers. It has a summary attribute which is visible to the reader and a button which they can click to show then content.

<details>
  <summary>Spoilers!</summary>
  Lot of sensitive information!
</details>

Creating Toggle Headings using `summary` and HTML heading tags

Since summary is an HTML element, we can nest other HTML elements in it. If we want to create something like Notion’s toggle headings (/toggleh1), we can do something like

<details>
  <summary><h3>Spoilers!</h3></summary>
  Spoilers
</details>

This shows the <h3> header but below the taggle i.e not inline. See this fiddle for the minimal example. Searching for this led me to this article which suggested this CSS fix to the problem

details summary > * { 
  display: inline;
}

This CSS snippet inlines all children of the details summary element.

Here’s the fix in action!

Using the `details` element in Hugo markdown

Since version 0.140, Hugo has supported details as a shortcode. We can put arbitrary HTML in the summary e.g

An APL a day! - Caesar Salad

Mon, 24 Feb 2025 02:15:00 +0530

Today’s problem - Caesar Salad

A Caesar cipher, also known as a shift cipher, is one of the simplest encryption techniques. In a Caesar cipher, each letter in the plaintext is replaced by a letter some fixed number of positions away in the alphabet, effectively “shifting” the alphabet.

Write a function that:

takes a single integer left argument representing the size of the shift
takes a character vector right argument representing the plaintext message
returns a result with the same shape as the right argument representing the encrypted message

Notes:

Use ' ',⎕A as the alphabet
You can assume that the plaintext message will contain only characters found in the alphabet.

Hint: The rotate function ⌽ could be helpful in solving this problem.

Examples

      4 caesar 'HELLO WORLDS'
LIPPSD SVPHW
    
      ¯4 caesar 'HELLO WORLDS' ⍝ negative shifts are okay
DAHHKWSKNH O 

      0 caesar 'HELLO WORLDS' ⍝ no shift is okay
HELLO WORLDS

      27 caesar 'HELLO WORLDS'
HELLO WORLDS

      ¯10 caesar '' ⍝ returns an empty vector

Solution

See the solution

{(⍺⌽' ',⎕A)[(' ',⎕A) ⍸⍵]}

Breaking it down:

An APL a day! - Put It In Reverse

Fri, 21 Feb 2025 16:13:02 +0530

Without much ado, today’s problem - Put it in Reverse

The find function X⍷Y identifies the beginnings of occurrences of array X in array Y.

In this problem, you’re asked to return a result that identifies the endings of occurrences of array X in array Y. To keep things simple, X and Y will be at most rank 1, meaning they’ll either be vectors or scalars.

Write a function that:

takes a scalar or vector left argument
takes a scalar or vector right argument
returns a Boolean result that is the same shape as the right argument where 1’s mark the ends of occurrences of the left argument in the right argument

Examples:

      'abra' findEnd 'abracadabra'
0 0 0 1 0 0 0 0 0 0 1

      'issi' findEnd 'Mississippi'
0 0 0 0 1 0 0 1 0 0 0

      'bb' findEnd 'bbb bbb'
0 1 1 0 0 1 1

      (,42) findEnd 42
0

      42 findEnd 42
1

      (,42) findEnd ,42
1

      'are' 'aquatic' findEnd 'ducks' 'are' 'aquatic' 'avians' 
0 0 1 0

Solution

See the solution

{⌽(⌽⍺)⍷⌽⍵}

We use ⍷ to find beginning of reversed array ⌽⍺ in ⌽⍵
We reverse this to get the ending of ⍺ in ⍵

Taking example 'abra' {⌽(⌽⍺)⍷⌽⍵} 'abracadabra'

An APL a day! - Elimination Sort

Thu, 20 Feb 2025 22:38:33 +0530

I’ve started dabbling with APL. It has been a lot of fun solving the current batch of challenges. Archive. I loved how each challenge focuses on a small set of operators and makes us use them in various (devious!) combinations to solve the problems. I’m going through some of the older challenges to learn about more operators.

The past challenges can be found at APL Quest. Today, we’ll be solving the first problem from the 2023 challenge. Use tryapl.org as an online interactive REPL.

Elimination Sort

An “Elimination Sort” is a somewhat farcical sorting algorithm which starts with the leftmost element and keeps subsequent elements that are at least as large as the previous kept element, discarding all other elements. For example:

EliminationSort 1 3 7 3 5 8 5 8 1 6 1 8 1 10 8 4 3 4 1 4
1 3 7 8 8 8 10

Write a function that:

takes a non-empty numeric vector right argument
returns an “Elimination-sorted” vector of the right argument

Hint: The progressive-maxima idiomatic phrase ⌈, the greater or equal function ≥, and the replicate function / could be helpful in solving this problem.

Re-learning Lua

Sat, 04 Dec 2021 00:00:00 +0530

Lua Basics

S3 Game

Thu, 04 Jun 2020 16:09:34 +0000

Level up your S3 skills by playing this game!

NPM and Node tips to make your dev machine (slightly) safer

Thu, 28 May 2020 10:42:07 +0000

NPM is used as a convenient cross-platform package manager for a lot of developer tools. For many tools, the defacto way to install is npm install -g $TOOL. But installing anything via npm allows it to run untrusted code on your machine.

Here are a few tips to minimize the risk:

1. NEVER run npm as sudo/root

Node’s official documentation recommends not installing global packages as sudo/root. If you have already installed node through nvm ignore this step. If you use a system installed node e.g using Ubuntu’s apt-get, read through this guide for Linux/Mac or npm-g-nosudo which is a shell script for Linux.

If you are lazy (like me), here’s a summary from the linked guide:

mkdir -p "${HOME}/.npm-packages" && npm config set prefix "${HOME}/.npm-packages"

Add this to your .bashrc or .zshrc:

NPM_PACKAGES="${HOME}/.npm-packages"
export PATH="$PATH:$NPM_PACKAGES/bin"
# Preserve MANPATH if you already defined it somewhere in your config.
# Otherwise, fall back to `manpath` so we can inherit from `/etc/manpath`.
export MANPATH="${MANPATH-$(manpath)}:$NPM_PACKAGES/share/man"

2. Install/Use node using `nvm` or `asdf`

Node’s official documentation recommends installing node as an unprivileged user using a node version manager.

Webhook to auto-deploy on git push to Github

Mon, 25 May 2020 17:32:54 +0000

Use git push webhook to run CI or auto-deploy

Bash function to add TILs

Sat, 23 May 2020 12:42:20 +0000

Add TIL to a `til.md` file from the command line

Setting display brightness on Linux from the command line

Tue, 19 May 2020 16:10:26 +0000

Auto-adjust the brightness on display in Linux

Easy VPN using Wireguard

Fri, 14 Dec 2018 11:48:44 +0000

Setup a painless VPN using Wireguard

Git Tagging Tutorial

Thu, 22 Nov 2018 10:44:36 +0000

How to create tags in your git repo from command line

Bash Command Completion - Finding all the cats in your $PATH!

Thu, 15 Nov 2018 09:39:37 +0000

How to search for programs on the terminal

Command-line productivity tips : Getting help in the terminal

Sun, 11 Nov 2018 17:30:30 +0000

What to do when you are stuck in the command-line

Git: How to keep your fork updated with remote repository

Fri, 09 Nov 2018 12:50:54 +0000

How to keep your Git fork updated with changes from remote

Ensuring that a shell script runs exactly once

Tue, 06 Nov 2018 18:53:23 +0000

How to ensure that only one instance of a shell script runs at a time

Surviving the Linux OOM Killer

Thu, 04 Oct 2018 15:48:41 +0000

How OOM Killer frees memory

Algorithms: Gray Binary Code

Sat, 18 Aug 2018 17:52:23 +0000

Gray Binary Code - A different way of ordering numbers

100 Days Of Code

Mon, 13 Aug 2018 13:40:27 +0000

My 100 Days Of Code Challenge

Algorithms: Generating Combinations

Sun, 12 Aug 2018 20:20:11 +0000

Generate combinations of n numbers taken r at a time

Awk - A useful little language

Sat, 26 May 2018 13:58:58 +0000

Speed up text processing tasks with Awk

JVM Primer Part 2 - Debugging memory issues

Wed, 16 May 2018 11:53:42 +0000

JVM Primer - Debugging memory issues

A Primer on JVM Memory Management and Troubleshooting - 1

Tue, 15 May 2018 14:14:49 +0000

This series is a summary of Oracle’s JVM troubleshooting course which gives an overview on JVM memory management, Hotspot VM’s garbage collection options, various memory errors and how to troubleshoot them.

In this post (part 1), we will have a look at how JVM manages memory and its different garbage collectors.

You can find Part 2 here

JVM Memory Management Overview

The JVM provides automatic memory management to free the programmer from manually managing memory. New objects are allocated on heap memory. A root set consists of pointers to external memory, static variables, threads, JNI references and internal JVM structures. Objects directly reachable from the root set must be kept in heap. Objects reachable from any of the reachable objects must also be in heap. This group of objects are the only ones which can be used by a program. The unreachable objects (garbage) are removed using a process called garbage collection (GC). Reachable objects are compacted i.e moved to contiguous space in heap. This is important as otherwise, the heap will become fragmented.

Snake case to camel case and back using regular expressions and Python

Wed, 09 May 2018 00:10:00 +0530

Convert back and forth between snake and camel case using Python and the re module

Cargo refused to build my project - A Rust debugging story!

Wed, 09 May 2018 00:00:00 +0530

An afternoon of debugging!

Disk Space Debugging Checklist

Fri, 04 May 2018 00:00:00 +0530

Checklist for debugging disk issues on Linux servers

Markdown (and some Hugo) Tutorial

Sun, 20 Aug 2017 00:00:00 +0000

Markdown Basics

You can write regular markdown here and Jekyll will automatically convert it to a nice webpage. I strongly encourage you to take 5 minutes to learn how to write in markdown - it’ll teach you how to transform regular text into bold/italics/headings/tables/etc.

Here is some bold text

Here is a secondary heading

Here’s a useless table:

Number	Next number	Previous number
Five	Six	Four
Ten	Eleven	Nine
Seven	Eight	Six
Two	Three	One

How about a yummy crepe?

Code and Syntax highlighting

The following are two code samples using syntax highlighting.

Hello World

Mon, 03 Mar 2014 22:38:33 +0530

Hello World!

Posts on A Random Walk

strace tips for better debugging

Useful options and flags

Print stack traces

Bash - Oops I deleted my bin dir challenge

Bash built-ins

List of built-ins

Challenge 1 - print the current working directory

Solution

Challenge 2 - List all files in the current working directory

HTML toggle headings using Details Summary element and some CSS

Overview of the HTML details tag

Creating Toggle Headings using summary and HTML heading tags

Using the details element in Hugo markdown

An APL a day! - Caesar Salad

Examples

Solution

See the solution

An APL a day! - Put It In Reverse

Examples:

Solution

An APL a day! - Elimination Sort

Elimination Sort

Re-learning Lua

S3 Game

NPM and Node tips to make your dev machine (slightly) safer

1. NEVER run npm as sudo/root

If you are lazy (like me), here’s a summary from the linked guide:

2. Install/Use node using nvm or asdf

Webhook to auto-deploy on git push to Github

Bash function to add TILs

Setting display brightness on Linux from the command line

Easy VPN using Wireguard

Git Tagging Tutorial

Bash Command Completion - Finding all the cats in your $PATH!

Command-line productivity tips : Getting help in the terminal

Git: How to keep your fork updated with remote repository

Ensuring that a shell script runs exactly once

Surviving the Linux OOM Killer

Algorithms: Gray Binary Code

100 Days Of Code

Algorithms: Generating Combinations

Awk - A useful little language

JVM Primer Part 2 - Debugging memory issues

A Primer on JVM Memory Management and Troubleshooting - 1

JVM Memory Management Overview

Snake case to camel case and back using regular expressions and Python

Cargo refused to build my project - A Rust debugging story!

Disk Space Debugging Checklist

Markdown (and some Hugo) Tutorial

Markdown Basics

Here is a secondary heading

Code and Syntax highlighting

Hello World

Overview of the HTML `details` tag

Creating Toggle Headings using `summary` and HTML heading tags

Using the `details` element in Hugo markdown

2. Install/Use node using `nvm` or `asdf`