jaywcjlove/reference Quick Reference Guide

APT is the package manager for Debian and its derivatives. It automates the downloading, configuration, and installation of software in both binary and source formats. While originally designed as a frontend for dpkg to handle .deb files, versions like APT-RPM allow it to manage RPM packages on supported systems.

Note: Most apt commands must be executed with sudo privileges.

Julia is a high-performance, open-source, multi-platform programming language designed for scientific computing. Key features include:

• JIT Compilation: Uses an LLVM-based Just-In-Time compiler for C/FORTRAN-like performance.
• Dynamic Typing: Supports dynamic typing with a multiple dispatch mechanism designed for parallel and distributed computing.
• Built-in Package Manager: Includes a native system for managing dependencies.
• Mathematical Support: Built-in mathematical functions (e.g., Gamma function) and out-of-the-box complex number support.
• Metaprogramming: Supports code generation via Lisp-like macros.

The .NET CLI is a cross-platform toolchain provided with the .NET SDK used to develop, build, run, test, and publish .NET applications.

Commands follow the structure: dotnet <command> [arguments] [options]

Commonly used short options include:

• -c|--configuration
• -f|--framework
• -o|--output
• -r|--runtime

To check your environment, use:

$ dotnet --info
$ dotnet --version
$ dotnet --list-sdks
$ dotnet --list-runtimes

Because accessing a non-existent attribute in Nix causes an error, it is a best practice to implement a 'safe get' pattern for nested structures. This involves checking if an attribute exists using builtins.hasAttr and if the current level is an attribute set using builtins.isAttrs before recursing.

let
  safeGet = attrs: path: default: 
    let
      keys = builtins.split "\." path;
      getValue = obj: keyList: 
        if keyList == [] then obj
        else if !builtins.isAttrs obj then default
        else if !builtins.hasAttr (builtins.head keyList) obj then default
        else getValue (obj.${builtins.head keyList}) (builtins.tail keyList);
    in
      getValue attrs keys;

  config = { database = { host = "localhost"; }; };
in
  { dbHost = safeGet config "database.host" "default-host"; }

iptables manages traffic using three primary chains, each controlling a different type of connection flow:

• INPUT: Controls behavior for incoming connections destined for the local system.
• OUTPUT: Controls behavior for outgoing connections originating from the local system.
• FORWARD: Controls connections passing through the system (e.g., routing or NATing), where the destination is not the local machine itself.

A Makefile defines rules for building targets. When you run make, it follows a specific workflow:

1. Reads all Makefiles and those included via include.
2. Initializes variables.
3. Infers implicit rules and analyzes all rules.
4. Creates a dependency chain for all target files.
5. Determines which targets need to be regenerated based on dependencies.
6. Executes the generation commands.

Basic Rule Syntax:

TARGET: PREREQUISITES
	COMMAND

• target: The file to be generated or a label (like clean).
• prerequisites: A list of files required to build the target.
• command: The shell command to execute. CRITICAL: Commands must start with a TAB character.

a.txt: b.txt c.txt
	cat b.txt c.txt > a.txt

React Native uses a Flexbox-based layout system. Unlike web CSS, all containers are display: flex by default.

Key properties:

• flexDirection: Determines the direction of children. Values: row, row-reverse, column, column-reverse. (Default is column).
• justifyContent: Determines how children are aligned along the primary axis. Values: flex-start, flex-end, center, space-around, space-between, space-evenly.
• flex: A number that defines how much space a component should occupy relative to its siblings.

// Row layout with distributed spacing
<View style={{ flexDirection: 'row', justifyContent: 'space-between' }}>
  <View style={{ flex: 1 }} />
  <View style={{ flex: 1 }} />
</View>