Shell Implementation

Software Systems Assignment 1

Authors

• Preet Harquissandas (CID: 02589338)
• Mikhail Agakov (CID: 02560202)

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1. Overview

s3 is a custom Unix-like shell implemented in C. It supports execution of external programs, built-in commands, redirection, pipelines, batched commands, and optional extensions such as subshells.

This README document describes the features implemented, the architectural decisions made, and any additional enhancements beyond the base requirements.

Summary

Core Requirements

Feature	Status	Description
Basic Execution	Fully Implemented	Fork/Exec/Wait pattern, argument parsing.
Redirection	Fully Implemented	Support for >, >>, and <.
Built-in cd	Fully Implemented	Supports cd, cd <dir>, cd -, cd ~ (home).
Prompt	Fully Implemented	dynamic prompt showing current working directory.
Pipelines	Fully Implemented	Arbitrary length pipelines (cmd1 | cmd2 | cmd3).
Batched Commands	Fully Implemented	Sequential execution using ;.

Proposed Extensions (PEs)

Extension	Status	Description
PE1: Subshells	Fully Implemented	(cmd) executes in a child shell process.
PE2: Nested Subshells	Fully Implemented	Supports deep nesting like ((cmd)).

Extra Features

Extension	Status	Description
Combined Input + Output Redirection	Fully Implemented	Supports instructions like sort < input.txt > output.txt
Complex Recursive Redirection	Fully Implemented	Supports instructions like (ls -l | grep .c) > source_files.txt

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2. Features Implemented

2.1 Basic Command Execution

✔ Parsing of command input using strtok

✔ Execution of external programs using fork() + execvp()

✔ Parent waits using wait() / custom reap()

✔ Support for multiple arguments

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3. Redirection Support

3.1 Output Redirection (> and >>)

✔ Implemented using open() + dup2()

✔ > overwrites file

✔ >> appends to file

3.2 Input Redirection (<)

✔ Implemented using open(O_RDONLY) + dup2()

3.3 Implementation Notes

• Detection of redirection operator using a custom parser that respects parenthesis nesting
• Launch handled through launch_program_with_redirection()
• While the specification only required handling a single redirection operator, we extended this to support multiple operators (see Section 8).

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4. Built-in Command: cd

4.1 Features

✔ Implemented directly in parent process via chdir()

4.2 Prompt Updating

✔ Prompt displays current working directory

✔ Uses getcwd() inside construct_shell_prompt()

Example prompt: [/home/user/projects/s3 s3]$

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5. Pipelining (|)

5.1 Multi-Stage Pipelines

✔ Full support for pipelines of arbitrary length

✔ Implemented using:

• pipe()
• dup2() in each child
• Multiple fork() calls in a loop

5.3 Handling Redirection Inside Pipelines

✔ Redirection allowed on pipeline endpoints

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6. Batched Commands (;)

6.1 Features

✔ Splits the command line into batch units

✔ Each batch unit is processed independently

✔ Supports pipelines or plain commands inside batch elements

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7. Proposed Extensions (PEs)

7.1 PE1: Subshells

✔ Support for grouped commands in parentheses

✔ Each subshell executes in its own process environment

✔ Allows redirection & pipelines involving subshells

7.2 PE2: Nested Subshells

✔ Recursive parsing of nested parentheses

✔ Stack-based matching for ( and )

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8. Extra Features

8.1: Combined Input + Output Redirection

✔ Support for redirecting both standard input and standard output in a single command

✔ Shell parses and records all redirection operators before executing the command

✔ Input and output redirections are applied by configuring the appropriate file descriptors

8.2: Complex Recursive Redirection

✔ Support for applying redirection to entire logical blocks (like subshells) rather than just simple binaries.

✔ Achieved a recursive architecture: the shell detects redirection at the top level, sets up the file descriptors, and then recursively calls the executor for the command string inside.

✔ Compatible with other features such as pipelines and subshell execution

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9. Testing Summary

We used a set of commands covering all features:

Basic Commands

• ls
• wc txt/phrases.txt
• man cat
• grep burn txt/phrases.txt

Redirection

• grep June < calendar.txt
• tr a-z A-Z < phrases.txt

Pipelines

• cat txt/phrases.txt | sort | wc -l
• ps aux | grep python | sort -k 3 -nr | head
• tr a-z A-Z < txt/phrases.txt | grep BURN

Batched Commands

• echo A ; echo B ; ls -l
• mkdir results ; cat txt/phrases.txt | sort > results/sorted_phrases.txt ; echo "Done"

Subshells

• (cd txt ; ls)
• (cat txt/phrases.txt | sort) | head
• Nested: ((echo A))
• (echo a ; (echo b ; (echo c)))

Combined input + output redirection in a single command

• (tr a-z A-Z < txt/phrases.txt) > txt/phrases_upper.txt
• sort < unsorted.txt > sorted.txt
• ((tr a-z A-Z < txt/phrases.txt) | head) > txt/new_file.txt

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10. Compilation & Usage

Compile

gcc *.c -o s3

Run

./s3