S3 Shell - Software Systems Shell

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Assignment 1: Shell Implementation

Table of Contents

1. Building and Running
2. Basic Commands
3. Commands with Redirection
4. Support for cd
5. Commands with Pipes
6. Batched Commands
7. Proposed Extension 1: Subshells
8. Proposed Extension 2: Nested Subshells
9. Further Enhancements
10. Testing
11. Code Quality

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Building and Running

Compilation

gcc -Wall -Wextra -g s3main.c s3.c jobs.c lexer_fsm.c terminal_control.c -o s3

Running the Shell

./s3

Running Tests

./run_tests.sh

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1. Basic Commands

Status: ✅ Fully Implemented

Key Functions

• launch_program() - Forks and launches basic commands
• child() - Executes execvp() in child process
• parse_command() - Tokenizes command line using custom FSM lexer

2. Commands with Redirection

Status: ✅ Fully Implemented

Key Functions

• launch_program_with_redirection() - Handles commands with redirection
• child_with_output_redirected() - Redirects stdout to file
• child_with_input_redirected() - Redirects stdin from file
• find_redirection_in_line() - Detects redirection operators

Supported > , >>, <

3. Support for cd

Status: ✅ Fully Implemented

Key Functions

• run_cd() - Changes directory using chdir()
• is_cd() - Detects cd commands
• construct_shell_prompt() - Updates prompt with current directory

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4. Commands with Pipes

Status: ✅ Fully Implemented

Key Functions

• launch_pipeline() - Manages multi-stage pipelines
• command_has_pipes() - Detects pipe operators
• split_pipeline() - Tokenizes pipeline stages
• find_redirection_operator() - Handles redirection within pipelines

Features

• Single-stage pipes (cmd1 | cmd2)
• Multi-stage pipes (cmd1 | cmd2 | cmd3 | ...)

5. Batched Commands

Status: ✅ Fully Implemented

Key Functions

• process_input() - Parses and executes batched commands
• trim_whitespace() - Cleans up command tokens

Features

• Sequential execution of multiple commands
• Independent execution (failure of one doesn't stop others)
• Works with pipes, redirection, and subshells
• Proper parenthesis tracking for subshells

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6. Proposed Extension 1: Subshells

Status: ✅ Fully Implemented

Key Functions

• run_subshell() - Forks and executes subshell
• is_subshell() - Detects subshell syntax

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7. Proposed Extension 2: Nested Subshells

Status: ✅ Fully Implemented

Key Functions

• process_input() - Tracks parenthesis depth for nested subshells
• run_subshell() - Recursively handles nested subshells

Features

• Unlimited nesting depth
• Proper parenthesis matching
• Recursive execution model
• Isolation at each level

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8. Further Enhancements

Job Control

Features

• Background job execution with &
• Job listing with jobs command
• Foreground job control with fg [job_id]
• Background job continuation with bg [job_id]
• Job state tracking (RUNNING, STOPPED)
• Process group management
• Signal handling (SIGINT, SIGTSTP, SIGTTOU, SIGTTIN)

Key Functions

• init_jobs() - Initializes job table
• add_job() - Adds job to tracking table
• remove_job_by_pgid() - Removes completed jobs
• find_job_by_id() - Locates job by ID
• print_jobs() - Lists all jobs
• wait_for_job() - Waits for foreground job with WUNTRACED support
• put_job_in_foreground() - Brings job to foreground
• put_job_in_background() - Resumes job in background

Example Commands

[/home/user s3]$ sleep 30 &
[1] 12345
[/home/user s3]$ jobs
[1] RUNNING    sleep 30 &
[/home/user s3]$ fg 1
# Press Ctrl-Z to suspend
[1] STOPPED    sleep 30
[/home/user s3]$ bg 1
[1] sleep 30 &
[/home/user s3]$ jobs
[1] RUNNING    sleep 30 &

Custom FSM-Based Lexer

Features

• Finite State Machine for robust tokenization
• Quote handling (single and double quotes)
• Escape sequence support (\)
• Operator recognition (|, <, >, >>, ;, (, ), &)
• Whitespace handling
• Error detection and reporting

Command History and Terminal Control

Features

• Arrow key navigation (up/down for history, left/right for editing)
• Command history with duplicate prevention
• Raw mode terminal control for character-by-character input
• Backspace support with proper cursor positioning
• In-line editing capabilities

Key Functions

• enable_raw_mode() - Enables raw terminal mode for immediate character input
• disable_raw_mode() - Restores canonical terminal mode
• add_history() - Adds commands to history linked list
• move_cursor_left() / move_cursor_right() - Cursor movement control
• clear_and_redraw() - Redraws prompt and command line

Filename Globbing

Features

• Wildcard expansion for *, ?, and [...] patterns
• Smart detection - only globs when wildcard characters are present
• Multiple argument globbing support
• Seamless integration with all command types

Key Function

• globbing() - Detects wildcard characters and expands filenames using glob()

Example Commands

[/home/user s3]$ ls *.c
jobs.c  lexer_fsm.c  s3.c  s3main.c  terminal_control.c
[/home/user s3]$ echo *.h
jobs.h lexer_fsm.h s3.h terminal_control.h
[/home/user s3]$ cat s3*.c
# Displays contents of all files matching s3*.c

Testing

Automated Test Suite

with full Test coverage may need manual etsting for ctrlZ and ctrlz

# Run full test suite
./run_tests.sh

# Expected output: All tests passed! (23/23)

Code Quality

Code Organization

• s3main.c - Main shell loop and initialization
• s3.c - Core shell functionality
• s3.h - Function declarations and constants
• jobs.c - Job control implementation
• jobs.h - Job control interface
• lexer_fsm.c - FSM-based lexer implementation
• lexer_fsm.h - Lexer interface
• terminal_control.c - Functions for terminal control
• terminal_control.h - Terminal interface

Known Limitations and future plans

• Implemented most shell functionality except from tab completition.