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Linux - File System: Fundamental

Linux - File System: Fundamental

File System

File System
- a system used by an operating system to manage files.
- The system controls how data is saved or retrived.
OS stores files and directory in an organized and structred way

File Metadata

$ ls -l
# -rw-rw-r-- 1 jason users 10400 Sep 27 08:52 sales.data

Permissions: -rw-rw-r--
Number of links: 1
Owner name: jason
Group name: users
Number of bytes in the file: 10400
Last modification time: Sep 27 08:52
File name: sales.data

sample

File name

No spaces!
- For some reason, some file names include spaces:
  - encapsulate the entire file name in quotes
    - e.g., ls "my file", ls 'my file'
  - escape spaces using backslash(\)
    - e.g., ls my\ file
Can include
- -: Hyphens
- _: Underscores
CamelCase: capitalize the first letter of each word.

File type

-: Regular file
d: Directory
l: Symbolic link
c:
- Character special file / Character device files
- provide access to devices that transfer data character by character
- Character devices example:
  - audio or graphics cards,
  - input devices like keyboard and mouse.
b:
- Block special file
  - a special type of file that provides an interface to block devices, such as hard drives, SSDs, USB drives, and optical drives.
  - Block devices handle data in fixed-size chunks or “blocks” (usually 512 bytes or more), making them suitable for storage operations.
n: Network file
- refers to a file or resource located on a remote system but accessed over a network as though it were local.
- commonly accessed via protocols like NFS (Network File System), SMB (Server Message Block), or similar technologies that enable file sharing over a network.
p: FIFO
s: Socket

Hidden files

Hidden files begin with a period.
- .file_name
can use ls -a to list all hidden files.

Symbolic Links

Symbolic Links
- a shortcut to another file or directory
- symbol l in the long list

ll /etc/yum.conf
# lrwxrwxrwx. 1 root root 12 Aug  6  2024 /etc/yum.conf -> dnf/dnf.conf

file /etc/yum.conf
# /etc/yum.conf: symbolic link to dnf/dnf.conf

stat /etc/yum.conf
#   File: /etc/yum.conf -> dnf/dnf.conf
#   Size: 12              Blocks: 0          IO Block: 4096   symbolic link
# Device: fd00h/64768d    Inode: 33685685    Links: 1
# Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
# Context: system_u:object_r:etc_t:s0
# Access: 2025-02-11 18:11:09.760730884 -0500
# Modify: 2024-08-06 07:11:09.000000000 -0400
# Change: 2025-01-27 17:00:18.975901543 -0500
#  Birth: 2025-01-27 17:00:18.970901461 -0500

2 Type of link:
- Hard Link, default
- Symbolic Link
inode
- index node
- pointer or number of a file on the hard disk.

Hard Link

Hard Links
- connections made via inode numbers in the Linux file system.
- Every file in a Linux filesystem is associated with an index node (inode), which is a data structure containing metadata about the file.
In a hard link, multiple filenames can point to the same inode number.
- Example: If file A is a hard link to file B, both A and B share the same inode number and are equal in the file system.
Feature:
- Equality:
  - A and B are treated equally by the file system.
- Independence on Deletion:
  - Deleting one does not affect the other.
  - The file’s data and inode persist until all associated hard links are deleted.
- Multiple Path Names:
  - A single file can have multiple valid path names via hard links.
Advantages:
- Protects against accidental deletion:
  - As long as one hard link remains, the file’s data is preserved.
- When Files Are Deleted:
  - The file’s data blocks and inode are released only after the last hard link is removed.
Permission:
- same as regular file -rw-rw-r--

Symbolic Links

Soft Links/Symbolic Links
- special files that act as shortcuts to another file.
- They store the path to the target file rather than pointing to the same inode.
- Example: If file A is a soft link to file B, their inode numbers are different, and A points to a different data block containing the path to B.
Features:
- Shortcut-Like Behavior
  - Similar to shortcuts in Windows, they simply redirect to the target file.
Separate Inodes:
- The inode numbers of the link and the target file are different.
Dependency:
- If the target file (B) is deleted, the soft link (A) remains but points to an invalid location.
Advantages:
- Can link to directories or files on different filesystems.
- Useful for creating flexible paths without duplicating data.
Limitations:
- If the target is deleted or renamed, the soft link becomes broken (invalid).
Permission:
- lrwxrwxrwx

Hard vs soft

Feature	Hard Links	Soft Links
Inode	Share the same `inode` with the target file.	Have a different `inode` from the target file.
Filesystem	Must be on the same filesystem as the target.	Can span across different filesystems.
Deletion	Effect Target file deletion doesn’t affect hard links.	Becomes invalid if the target file is deleted.
Directory	Support Cannot link to directories.	Can link to directories.

Use Cases
- Hard Links:
  - Best for maintaining multiple valid references to critical files within the same filesystem.
- Soft Links:
  - Ideal for creating shortcuts or referencing files across different filesystems.

Command

List inode

CMD	DESC
`ls -i`	list inode numbers and identify linked files

Hard link

CMD	DESC
`ln file hlink`	Create a hard link with different file name
`ln target_file`	Create a hard link with the same file name

Soft link

CMD	DESC
`ln -s file slink`	Create a soft link with different file name
`ln -s target_file`	Create a soft link with the same file name

Lab: Hard Link

Create hard link

mkdir -p /root/lndir/hardlink
mkdir -p /root/lndir/hardlink/testdir
touch /root/lndir/hardlink/onefile

# create hard link for a file
ln /root/lndir/hardlink/onefile /root/lndir/oneln

# confirm
ll -i /root/lndir/oneln /root/lndir/hardlink/onefile
# 20538441 -rw-r--r--. 2 root root 0 Feb 12 20:21 /root/lndir/hardlink/onefile
# 20538441 -rw-r--r--. 2 root root 0 Feb 12 20:21 /root/lndir/oneln

# try to create hard link for a directory
ln /root/lndir/hardlink/testdir /root/lndir/onetestln
# ln: /root/lndir/hardlink/testdir: hard link not allowed for directory

Test file content

echo "this is oneln" > /root/lndir/oneln
cat /root/lndir/oneln
# this is oneln
cat /root/lndir/hardlink/onefile
# this is oneln

echo "update: onefile" >> /root/lndir/hardlink/onefile
cat /root/lndir/oneln
# this is oneln
# update: onefile
cat /root/lndir/hardlink/onefile
# this is oneln
# update: onefile

Create new hard link pointing to file

ln /root/lndir/hardlink/onefile /root/lndir/oneln2
ln /root/lndir/hardlink/onefile /root/lndir/oneln3
ln /root/lndir/hardlink/onefile /root/lndir/oneln4

ll -i /root/lndir
# total 16
# 20538440 drwxr-xr-x. 3 root root 36 Feb 12 20:21 hardlink
# 20538441 -rw-r--r--. 5 root root 30 Feb 12 20:32 oneln
# 20538441 -rw-r--r--. 5 root root 30 Feb 12 20:32 oneln2
# 20538441 -rw-r--r--. 5 root root 30 Feb 12 20:32 oneln3
# 20538441 -rw-r--r--. 5 root root 30 Feb 12 20:32 oneln4

Delete file

rm /root/lndir/hardlink/onefile
# rm: remove regular file '/root/lndir/hardlink/onefile'? y

ll -i /root/lndir
# total 16
# 20538440 drwxr-xr-x. 3 root root 21 Feb 12 20:37 hardlink
# 20538441 -rw-r--r--. 4 root root 30 Feb 12 20:32 oneln
# 20538441 -rw-r--r--. 4 root root 30 Feb 12 20:32 oneln2
# 20538441 -rw-r--r--. 4 root root 30 Feb 12 20:32 oneln3
# 20538441 -rw-r--r--. 4 root root 30 Feb 12 20:32 oneln4

cat /root/lndir/oneln
# this is oneln
# update: onefile

Lab: Soft Link

Create soft link

mkdir -p /root/lndir/softlink
mkdir -p /root/lndir/softlink/softdir
touch /root/lndir/softlink/softdir/softfile
echo "this is softfile" >> /root/lndir/softlink/softfile

# create soft link
ln -s /root/lndir/softlink/softdir /root/lndir/softdirln
ln -s /root/lndir/softlink/softfile /root/lndir/softfileln

ll -i /root/lndir/
# total 0
# 20538440 drwxr-xr-x. 3 root root 21 Feb 12 20:37 hardlink
# 2039722 lrwxrwxrwx. 1 root root 28 Feb 12 21:00 softdirln -> /root/lndir/softlink/softdir
# 2039723 lrwxrwxrwx. 1 root root 29 Feb 12 21:00 softfileln -> /root/lndir/softlink/softfile
# 20538441 drwxr-xr-x. 3 root root 37 Feb 12 20:59 softlink

cat /root/lndir/softfileln
# this is softfile

cd /root/lndir/softdirln
pwd
# /root/lndir/softdirln
ll
# total 0
# -rw-r--r--. 1 root root 0 Feb 12 20:59 softfile

Delete dir

ll  /root/lndir/
# total 0
# drwxr-xr-x. 3 root root 21 Feb 12 20:37 hardlink
# lrwxrwxrwx. 1 root root 28 Feb 12 21:00 softdirln -> /root/lndir/softlink/softdir
# lrwxrwxrwx. 1 root root 29 Feb 12 21:00 softfileln -> /root/lndir/softlink/softfile

cat /root/lndir/softfileln
# cat: /root/lndir/softfileln: No such file or directory

cd /root/lndir/softdirln
# -bash: cd: /root/lndir/softdirln: No such file or directory

`file`: Display the file type

Command	Desc
`file filename`	Display type of a file
`file *`	Display each file’s type in current directory

Lab: Display File Type

file /root/anaconda-ks.cfg
# /root/anaconda-ks.cfg: ASCII text

stat /root/anaconda-ks.cfg
#   File: /root/anaconda-ks.cfg
#   Size: 983             Blocks: 8          IO Block: 4096   regular file
# Device: fd00h/64768d    Inode: 16777347    Links: 1
# Access: (0600/-rw-------)  Uid: (    0/    root)   Gid: (    0/    root)
# Context: system_u:object_r:admin_home_t:s0
# Access: 2025-02-11 19:29:16.921673803 -0500
# Modify: 2025-01-27 02:32:44.652168357 -0500
# Change: 2025-01-27 02:32:44.652168357 -0500
#  Birth: 2025-01-27 02:32:44.571167174 -0500

file /root/Desktop/
# /root/Desktop/: directory

stat /root/Desktop/
#   File: /root/Desktop/
#   Size: 6               Blocks: 0          IO Block: 4096   directory
# Device: fd00h/64768d    Inode: 17645509    Links: 2
# Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
# Context: unconfined_u:object_r:admin_home_t:s0
# Access: 2025-02-11 17:59:12.994301344 -0500
# Modify: 2025-02-11 17:24:49.403729585 -0500
# Change: 2025-02-11 17:24:49.403729585 -0500
#  Birth: 2025-02-11 17:24:49.403729585 -0500

Device File

a major number points to the device driver
a minor number points to a unique device or partition that the device driver controls.
c: character devcie file

ll /dev/console
# crw--w----. 1 root tty 5, 1 Feb 10 22:28 /dev/console

file /dev/console
# /dev/console: character special (5/1)

stat /dev/console
#   File: /dev/console
#   Size: 0               Blocks: 0          IO Block: 4096   character special file
# Device: 5h/5d   Inode: 12          Links: 1     Device type: 5,1
# Access: (0620/crw--w----)  Uid: (    0/    root)   Gid: (    5/     tty)
# Context: system_u:object_r:console_device_t:s0
# Access: 2025-02-10 22:28:25.029000147 -0500
# Modify: 2025-02-10 22:28:25.029000147 -0500
# Change: 2025-02-10 22:28:25.029000147 -0500
#  Birth: 2025-02-10 22:28:21.209000000 -0500

b: block devcie file

ll /dev/nvme0n1
# brw-rw----. 1 root disk 259, 0 Feb 10 22:28 /dev/nvme0n1

file /dev/nvme0n1
# /dev/nvme0n1: block special (259/0)

stat /dev/nvme0n1
#   File: /dev/nvme0n1
#   Size: 0               Blocks: 0          IO Block: 4096   block special file
# Device: 5h/5d   Inode: 261         Links: 1     Device type: 103,0
# Access: (0660/brw-rw----)  Uid: (    0/    root)   Gid: (    6/    disk)
# Context: system_u:object_r:fixed_disk_device_t:s0
# Access: 2025-02-11 17:28:54.759067590 -0500
# Modify: 2025-02-10 22:28:25.062000149 -0500
# Change: 2025-02-10 22:28:25.062000149 -0500
#  Birth: 2025-02-10 22:28:23.114000067 -0500

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Note_Tech

Linux - File System: Fundamental

File System

File Metadata

File name

File type

Hidden files

Symbolic Links

Hard Link

Symbolic Links

Hard vs soft

Command

Lab: Hard Link

Lab: Soft Link

file: Display the file type

Lab: Display File Type

Device File

`file`: Display the file type