From dd9ed7190681050676797555e2212884687a8f11 Mon Sep 17 00:00:00 2001
From: scratko <m@scratko.xyz>
Date: Mon, 28 Jul 2025 03:55:01 +0300
Subject: Added README

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+# Simple UNIX Shell
+
+A lightweight command-line shell implemented in C, designed to emulate core
+functionalities of UNIX shells like `sh` or `bash`. This shell supports
+**process management**, **I/O redirection**, **pipelining (|)**, **subshells**,
+**logical operators (&&, ||)**, **interactive input handling including
+autocompletion** and **basic signal handling**. The implementation demonstrates
+system-level programming concepts using **POSIX APIs**.
+
+<img src="shell_1.png" />
+
+<img src="shell_2.png" />
+
+---
+
+## Features
+
+-  **Command Execution**   
+Executes external programs using `fork()` and `execvp()`, with environment lookup
+via `$PATH`.
+
+-  **Foreground and background job execution**  
+Commands can be run in the background by appending `&`.
+
+-  **Redirection**  
+Input/output redirection using `<`, `>`, and `>>`. _Examples:_
+    - `cat < input.txt`
+    - `ls > out.txt`
+    - `echo foo >> log.txt`
+
+-  **Quoting and escape sequences**  
+Basic support for `"quoted strings"` and escape sequences like `\ ` or `\"`.
+
+- **Logical Operators**  
+Implements support for `&&` and `||` operators, enabling conditional command
+execution. These operators can be combined with pipelines and subshells.
+
+- **Subshells**  
+Commands enclosed in parentheses `()` are executed in subshells,
+enabling complex nested logic.
+
+-  **Pipelines**  
+Unix-style pipelines using the pipe (|) operator. Data is passed from one
+process to another via standard input/output.
+_Example: `cat file.txt | grep foo | sort`_
+
+- **Directory Navigation**  
+Supports the built-in `cd` command to change the current working directory via
+`chdir()`.
+
+- **Autocompletion (Tab)**  
+Custom autocompletion implemented using raw terminal input:
+    - If the current token is a command: matches against executables in `$PATH`.
+    - Otherwise: matches file and directory names from the current or specified path.
+
+- **Interactive Line Editing**  
+Terminal is switched to non-canonical mode. Input handling is implemented manually:
+    - Character-by-character input display.
+    - Left/right arrow keys for cursor movement.
+    - `Backspace` support with live line re-rendering.
+    - `Ctrl+D` to terminate the shell.
+    - `Tab` triggers autocompletion.
+
+-  **Signal handling**  
+Handles SIGCHLD to reap zombie processes and prevent defunct children.
+
+---
+
+## Technologies Used
+
+- **Language**: C (C99)
+- **System APIs**: POSIX (`fork`, `execvp`, `waitpid`, `pipe`, `dup2`, `tcsetpgrp`, etc.)
+- **Memory handling**: dynamic memory allocation (`malloc`, `free`)
+- **Terminal control**: `tcgetpgrp`, `tcsetpgrp`, `tcgetattr`, `tcsetattr`
+
+---
+
+## Limitations
+
+While the shell supports many core UNIX behaviors, it has some limitations compared to full-featured shells:
+
+-  No scripting constructs (`if`, `while`, `for`, etc.)
+-  No variable expansion (e.g. `$HOME`, `$PATH`, `$?`)
+-  No command substitution (e.g. `` `command` `` or `$(command)`)
+-  No history (up arrow)
+-  No job control (e.g. fg, bg, jobs).
+-  No advanced signal features (sigaction, sigprocmask, kill, etc.).
+
+You’ll see a prompt where you can enter commands, use pipes, redirection, and
+job control. You can also interact via signals like Ctrl+C and Ctrl+Z.
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