linuxsysprog

Problem Statement

This project is a custom implementation of the ls command, created as a learning exercise in systems programming. The main goals are:

• To understand the underlying mechanisms of how common Unix utilities like ls work by reimplementing core parts of their functionality.
• To gain practical experience with Linux system calls related to file system interaction.

Why create another ls?

The purpose of this project is not to replace the existing ls command, which is a mature and feature-rich utility. Instead, this implementation serves as an educational tool to explore:

• Directory Traversal: How programs read the contents of a directory.
• File Metadata: How to retrieve and interpret information about files and directories (e.g., permissions, ownership, size, timestamps).
• System Calls: Direct interaction with the operating system kernel (specifically using SYS_getdents).
• Output Formatting: Basic C programming techniques for presenting information to the user.

Learning Objectives

Through this project, one can learn about:

• The use of system calls like open, stat, and getdents (via syscall).
• The structure of directory entries (struct linux_dirent).
• Retrieving user and group information (getpwuid, getgrgid).
• Basic C programming, including string manipulation, memory management, and standard I/O.
• The general principles of how command-line tools interact with the file system.

Just wanted to get into a bit of lower level programming

Current Functionality

This ls implementation currently supports the following features:

• List Directory Contents: Lists files and directories in the current directory.
• File Type Indication: Displays a 'd' for directories and a '-' for regular files at the beginning of the permissions string.
• Permissions (Partial): Shows hardcoded permissions (rwxrwxr-x). The actual permission bits are read but not yet fully decoded.
• Number of Hard Links: Displays the number of hard links to the file.
• Owner and Group: Shows the username of the owner and the group name.
• File Size: Displays the size of the file in bytes.
• Last Modification Time: Shows the month, day, and time of the last modification (e.g., "Aug 22 15:04").
• File/Directory Name: Displays the name of the file or directory.

TODO Items

The following items are noted in the source code (ls.c) as planned improvements:

• Sort Output: Currently, the output is not sorted and appears in directory entry order.
• Color Output: Add support for colored output to differentiate file types (similar to ls --color=auto).
• Actual Permissions: Decode and display the actual file permissions (e.g., rwxrw-r--) instead of the current hardcoded string.
• Line Formatting: Improve the overall output formatting for better readability and alignment, potentially mimicking standard ls more closely.

Comparison to Standard ls

This implementation is a very simplified version of the standard ls command found on Linux systems. Key differences include:

• Limited Options: It does not support any command-line options (e.g., -l, -a, -h, -t).
• No Sorting: Output is unsorted.
• No Color: Lacks colored output.
• Simplified Permissions: Permissions are not fully implemented.
• Basic Formatting: Output formatting is rudimentary.
• Error Handling: Error handling is basic.
• Single Directory: Only lists the current directory; does not support specifying other paths.

It serves as a basic demonstration of reading directory entries and file metadata.

Setup and Usage

System Requirements

• Linux-specific: This program is designed for Linux systems due to its use of the SYS_getdents system call and Linux-specific headers. It will likely not compile or run on other operating systems like macOS or Windows without modification.

Building

To compile the program, a Makefile is provided. Simply run:

make

This will produce an executable file named ls in the current directory.

Running

To run the program after building, execute it from your terminal:

./ls

This will list the contents of the current directory, similar to the standard ls command but with the features and limitations described above.