Shell Operating Systems - University Assignment

A practical assignment for understanding operating systems concepts through shell implementation and system programming.

Overview

This project demonstrates core operating systems principles including process management, system calls, and shell implementation. Students work on building a functional shell that interacts with the OS kernel.

Learning Objectives

• Understand process creation and management
• Learn about system calls and their use
• Implement command parsing and execution
• Work with file descriptors and redirection
• Handle process synchronization and signals

---

Available Commands

System Information

authors

Prints the names and logins of the program authors.

• authors - Shows full names and logins
• authors -l - Shows only logins
• authors -n - Shows only names

pid

Prints the PID (Process ID) of the shell process.

ppid

Prints the PPID (Parent Process ID) of the shell.

date

Displays current date and time.

• date - Shows date (DD/MM/YYYY) and time (hh:mm:ss)
• date -d - Shows only the date
• date -t - Shows only the time

infosys

Prints system information obtained via uname system call (system name, machine, node name, version, release).

help

Displays available commands or detailed help for a specific command.

• help - Lists all available commands
• help <command> - Shows help for specific command

---

Directory Management

cd <directory>

Changes the current working directory (uses chdir system call).

• cd - Without arguments, displays current working directory
• cd <path> - Changes to specified directory

cwd

Prints the current working directory.

makedir <directory_name>

Creates a new directory with rwx permissions for owner and rx for others.

makefile <filename>

Creates a new file with read/write permissions for all users.

listdir [-hid] [-long] [-link] [-acc] <directory>

Lists directory contents with optional formatting flags.

• -hid - Include hidden files
• -long - Show long format with ownership and permissions
• -link - Display symbolic link targets
• -acc - Show access times

listfile [-long] [-acc] [-link] <file>

Displays detailed information about specific files.

• -long - Show long listing (hardlinks, inode, user, group, permissions)
• -acc - Show access time
• -link - Show symbolic link target

reclist [-hid] [-long] [-link] [-acc] <directory>

Lists directories recursively with subdirectories shown after parent content.

revlist [-hid] [-long] [-link] [-acc] <directory>

Lists directories recursively with subdirectories shown before parent content.

erase <file_or_directory>

Deletes files and empty directories.

delrec <directory>

Recursively deletes a directory and all its contents.

---

File Operations

open <filename> [cr] [ex] [ro] [wo] [rw] [ap] [tr]

Opens a file and adds it to the shell's open file list.

• cr - Create file if it doesn't exist
• ex - Exclusive access
• ro - Read-only
• wo - Write-only
• rw - Read/Write
• ap - Append mode
• tr - Truncate file
• No arguments - Lists all open files

close <file_descriptor>

Closes a file descriptor and removes it from the open file list.

• No arguments - Lists open files

dup <file_descriptor>

Duplicates a file descriptor using the dup system call.

---

Memory Management

allocate [-malloc|-shared|-createshared|-mmap] [parameters]

Allocates memory blocks using various methods.

• allocate -malloc <size> - Allocate memory via malloc
• allocate -createshared <key> <size> - Create shared memory with key and size
• allocate -shared <key> - Attach to existing shared memory
• allocate -mmap <filename> <permissions> - Memory-map a file (permissions: r/w/x)
• No arguments - Lists all allocated memory blocks

deallocate [-malloc|-shared|-delkey|-mmap] [parameters]

Deallocates memory blocks.

• deallocate -malloc <size> - Free malloc block of size
• deallocate -shared <key> - Detach shared memory
• deallocate -delkey <key> - Delete shared memory from system
• deallocate -mmap <filename> - Unmap file
• deallocate <address> - Free memory at specific address

memfill <address> [size] [character]

Fills memory with a character starting at address for specified bytes.

• Default size: 128 bytes
• Default character: 'a'

memdump <address> [size]

Displays memory contents in hexadecimal and ASCII format.

• Default size: 128 bytes

readfile <filename> <memory_address> [size]

Reads file contents into memory at specified address.

writefile <filename> <memory_address> [size]

Writes memory contents to a file.

read <file_descriptor> <address> [size]

Reads from file descriptor into memory address.

write <file_descriptor> <address> <size>

Writes from memory address to file descriptor.

memory [-funcs|-vars|-pmap|-blocks|-all]

Displays memory-related information.

• -funcs - Shows addresses of 3 program and 3 library functions
• -vars - Shows addresses of local, global, and static variables
• -pmap - Executes pmap/procstat/vmmap to show process memory map
• -blocks - Lists allocated memory blocks
• -all - Shows everything

recurse <n>

Executes a recursive function n times, displaying stack addresses for automatic and static arrays.

---

Process Management

fork

Creates a child process using fork system call and waits for it to complete.

fg <program> [arguments]

Executes a program in the foreground, waits for completion, and displays exit status.

fgpri <priority> <program> [arguments]

Executes a program in the foreground with a specified priority.

back <program> [arguments]

Executes a program in the background.

backpri <priority> <program> [arguments]

Executes a program in the background with a specified priority.

listjobs

Lists all background processes currently running (PID, owner, priority, creation date, command).

deljob [-term|-sig]

Terminates or signals a background process.

exec <program> [arguments]

Executes a program without creating a new process (replaces current shell).

search [-add|-del|-clear|-path]

Manages the executable search list (directories where shell looks for programs).

• search - Lists current directories
• search -add <directory> - Adds directory to search list
• search -del <directory> - Removes directory from search list
• search -clear - Clears entire search list
• search -path - Imports directories from PATH environment variable

---

Environment & Credentials

getuid

Displays real and effective user IDs of the current process.

setuid <uid>

Changes the effective UID of the process.

• setuid -l - Shows current credentials

showvar <variable_names>

Displays environment variable values and memory addresses accessed via environ, getenv(), and main() arguments.

changevar [-a|-e|-p] <variable> <value>

Modifies environment variables.

• -a - Add or modify (direct environment modification)
• -e - Modify if exists (direct environment modification)
• -p - Use putenv() to modify

subsvar [-a|-e] <variable1> <variable2> <value>

Substitutes one environment variable with another.

• -a - Always substitute
• -e - Substitute only if variable exists

environ [-addr|-environ]

Displays process environment.

• No arguments - Shows all environment variables
• -addr - Shows address of environ pointer
• -environ - Shows indexed environ array

---

Shell Control

historic [N] [-N]

Shows command history or executes a previous command.

• historic - Displays all commands with index numbers
• historic <N> - Executes command number N
• historic -<N> - Shows last N commands

quit / exit / bye

Terminates the shell and frees all allocated resources.

---

License

MIT License - See LICENSE file for details