Linux Utilities & Shells in C

A collection of low-level Linux utilities and progressively featured command-line shells written in C. This project explores Unix internals, system calls, and process management from the ground up.

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Project Components

1. Basic Utilities

Implemented using raw system calls — open(), read(), write(), getcwd(), rename(), and unlink() — rather than higher-level library abstractions.

Utility	Description	Key System Calls
pwd	Prints the current working directory	getcwd()
echo	Writes arguments to stdout with spacing	write()
cp	Copies a source file to a destination using a 1 KB buffer	open(), read(), write()
mv	Moves/renames a file; falls back to copy+delete if rename() fails across filesystems	rename(), unlink()

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2. The Shell Series

A progression of shell implementations, each layer adding new capabilities.

🔹 FemtoShell

The most minimal shell — a basic REPL loop supporting only echo and exit. No child processes, no external commands. A proof-of-concept for the shell execution model.

Supported built-ins: echo, exit

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🔹 PicoShell

A fully functional interactive shell. Parses user input dynamically, runs external programs as child processes, and handles several built-in commands.

Supported built-ins: echo, pwd, cd, exit

Key features:

• Uses fork() + execvp() + waitpid() to spawn and reap child processes
• Tokenizes input with strtok() and builds argv arrays dynamically using realloc() and strdup()
• Tracks and returns the last command's exit status
• Flushes stdout before forking to prevent buffer duplication in the child

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🔹 NanoShell

Extends PicoShell with support for shell variables and environment management.

Supported built-ins: echo, pwd, cd, export, exit

Key features:

• Shell variables — assign variables with name=value syntax (no spaces); stored in a dynamic ShellVar array using realloc
• Variable substitution — expands $VAR tokens in any argument before execution, checking shell variables first, then the environment via getenv()
• export — promotes a shell variable into the process environment with setenv(), making it available to child processes
• 65 KB input buffer — supports long and complex command lines

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🔹 MicroShell

The most complete shell in the series. Builds on NanoShell with full I/O redirection support.

Supported built-ins: echo, pwd, cd, export, exit

Key features:

• Everything in NanoShell, plus:
• I/O redirection — supports < (stdin), > (stdout), and 2> (stderr) operators
• Redirection is applied in child processes via dup2() before execvp(), so the command's file descriptors are transparently replaced
• Redirection operators are stripped from the argument list before execution, so argv passed to the command stays clean
• Built-in commands validate redirect targets (e.g., checks that > files can be opened) without executing the redirection itself

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Learning Goals

This project was built to develop hands-on familiarity with:

• File I/O — manual buffer management and file descriptor lifecycle
• Process management — how a shell spawns, executes, and reaps child processes
• Memory management — dynamic allocation with realloc and strdup
• Shell internals — variable storage, substitution, and environment propagation
• I/O redirection — using dup2() to rewire standard file descriptors in child processes
• System calls — direct interaction with the Linux kernel, bypassing libc wrappers where possible
• Error handling — checking return values of every system call and exiting with meaningful codes

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How to Compile & Run

Each component has its own source file. Compile and run any utility with:

gcc microshell.c -o microshell
./microshell

Replace microshell.c with the desired source file (e.g., pwd.c, cp.c, nanoshell.c).

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🗂 Project Structure

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├── pwd.c           # Print working directory
├── echo.c          # Echo arguments to stdout
├── cp.c            # File copy utility (1 KB buffered)
├── mv.c            # File move/rename utility
├── femtoshell.c    # Minimal shell: echo + exit only
├── picoshell.c     # Functional shell: fork/exec, built-ins, dynamic parsing
├── nanoshell.c     # Shell variables, $VAR substitution, export
└── microshell.c    # I/O redirection (<, >, 2>) via dup2

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