Linux Filesystem

What is it? 

• The Linux Filesystem is the way that data is stored to the harddrive
• Also contains information regarding processes and connected devices
• Root directory is /. User directories are located in /home/<username>

What is it? 

• Often called Virtual Filesystem because it is an abstraction layer for FS like EXT4, FAT32, etc. 
• Why is it done this way?
  • Kernel can support various implementation
  • Easy interoperation between various file system types

What is it? 

• Often called Virtual Filesystem because it is an abstraction layer for FS like EXT4, FAT32, etc. 
• Why is it done this way?
  • Kernel can support various implementation
  • Easy interoperation between various file system types

Hierarchy

Hierarchy

File System Implementations

Filesystem Layout

Filesystem Layout

• Block - basic unit of a filesystem 
  • Boot Block
  • Superblock
  • i-node table
  • Data block
• Partition - Where filesystems are contained
• Disk - Contains multiple partitions

Filesystem Layout

• Boot block
  • First block in the file system
  • Used to help the OS boot
• Superblock
  • Follows the boot block
  • Stores metadata for the file system 
    • Number of blocks 
    • Size of Blocks 
    • Size of i-node table
    • type of file system 
    • etc

Filesystem Layout

• I-node table
  • List of index nodes 
  • Contains metadata about a file
• Data block - well... it's exactly as it sounds

How to View Filesystem Metadata

I-nodes (Inodes)

• Used to represent files and directories
• Contains metadata about the file such as size, physical location, owner, group, etc.
• Files are assigned an I-node number on creation
  • Unique identifier to help with indexing
• The number of i-nodes on a system is fixed

Directories

• Used to represent files and directories
• Contains metadata about the file such as size, physical location, owner, group, etc.
• Files are assigned an I-node number on creation
  • Unique identifier to help with indexing
• The number of i-nodes on a system is fixed

A Very Deep Dive

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

Explore your Linux FS

See what you can discover about your user directory. Based on what we found. 

Processes

What are they?

• Processes are running applications. 
• They are the fundamental concept for working with programs

/proc

• Otherwise known as the procfs
• In-memory file system containing details about processes
• Contains information like memory, stack, addresses, referenced shared libraries, etc. 

/proc exploration

• Run the command ls /proc
• Gain familiarity by navigating around the /proc path
• What do you think is in /proc/cmdline?
• See /proc/cpuinfo and /proc/meminfo
• See man 5 proc for more info

/proc/<pid>

• Contains all the details about a running application
  • cmdline - the command line used to run the process
  • cwd - the current working directory
  • environ - details about the working environment
  • exe - a symbolic link to the actual binary
  • fd - A subdirectory containing any file descriptors used by the program
  • maps - a list of anything mapped into the process
  • status - Displays information  about the process
  • task - A subdirectory containing threads of the process

What's also useful about it?

• Can read/write kernel settings via the shell from procfs as if they were on disk
• Can also use a standard API to configure the procfs as well
  • setrlimit 
  • getrlimit 
  • prlimit