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Files in Linux

A file is a logical collection of data stored on disk, represented by a pathname and identified by an inode. Everything in Linux is treated as a file, including devices and processes.

Features of files in linux

  • Everything is a file: Devices, directories, and processes are treated as files.
  • Case-sensitive: File.txt and file.txt are different.
  • Hierarchical structure: Files are organized in a tree-like directory system.
  • Permissions: Each file has read, write, and execute permissions for user, group, and others.
  • Ownership: Files are owned by a user and a group.
  • Hidden files: Files starting with . are hidden.
  • File types: Includes regular files, directories, symbolic links, device files, etc.
  • No file extensions required: File type is determined by content, not extension.
  • Symbolic and hard links: Files can be linked to others for easy access.
  • Metadata: Each file stores info like size, creation/modification time, and permissions.
  • Inodes: Each file has an inode storing metadata (except name).

Types of Files

File Type Symbol Description / Purpose
Regular File - Contains data, text, or program instructions. Most common file type.
Directory d A folder that holds other files and directories.
Symbolic Link l A reference or shortcut to another file or directory.
Character Device c Represents devices that handle data character by character (e.g., keyboard, serial port).
Block Device b Represents devices that handle data in blocks (e.g., hard drives, USB drives).
Socket s Used for inter-process communication, often in networking.
Named Pipe (FIFO) p Enables communication between processes; data flows in a first-in-first-out manner.

File Permissions

Check the permissions of File

ls -l <file-path>

stat -c %A <file-path>

stat -c %a <file-path>

namei -m <file-path>

Permissions chart

Symbolic Binary Octal Meaning
--- 000 0 No permissions
--x 001 1 Execute only
-w- 010 2 Write only
-wx 011 3 Write and execute
r-- 100 4 Read only
r-x 101 5 Read and execute
rw- 110 6 Read and write
rwx 111 7 Read, write, execute all

Changing permissions of file

chmod <Options> <file-path>

Ways to write options

  1. in octal form 777, 644
  2. u+x form
    1. In place of u specify u for file owner, g for group, o for others and a for all
    2. in place of + put + to add permissions or - to remove permissions
    3. in place of x put r for read, w for write and x for execute

Examples

u+x to add execute permissions for file owner

g-w to remove write permissions for user belonging to the file group

a+rx to add read and execute permissions to all users

File Ownership

File Owner

Know the file owner

ls -l <file-path>

stat -c "%U" <file-path>

namei -o <file-path>

Change the file owner

chown newowner <file-path>

File Group

Know file group

ls =l <file-path>

stat -c "%G" <file-path>

namei -o <file-path>

Change file group

chgrp GROUP <file-path>

To change file owner and group

chown USER:GROUP <file-path>

Change file owner or group of directories and inherit to sub files and directories

chown -R newowner <path-of-directory>

chgrp -R newgroup <path-of-diretcory>

chown -R owner:group <path-of-directory>

File Metadata

Metadata Type Description Command to View In Inode?
Inode number Unique file identifier stat, ls -i ✓ Yes
File type Regular file, directory, link, etc. stat, file, ls -l ✓ Yes
File size Size in bytes stat, ls -l, du ✓ Yes
Hard link count Number of hard links stat, ls -l ✓ Yes
Owner (UID) User ID of owner stat, ls -l, id ✓ Yes
Group (GID) Group ID stat, ls -l ✓ Yes
Permissions rwx permissions for owner/group/others stat, ls -l ✓ Yes
Special permissions setuid, setgid, sticky bit stat, ls -l ✓ Yes
Access time (atime) Last access timestamp stat ✓ Yes
Modify time (mtime) Last content modification stat, ls -l ✓ Yes
Change time (ctime) Last metadata change stat ✓ Yes
Birth time Creation time (if supported) stat ✓ Yes
Device ID Storage device identifier stat ✓ Yes
Block size I/O block size stat ✓ Yes
Blocks allocated Disk blocks used stat, du ✓ Yes
Filename The actual file name ls, find ✗ No
Directory path Full path to file pwd, realpath ✗ No
Extended attributes ACLs, SELinux contexts, custom attrs getfacl, getfattr ✗ No
File attributes chattr flags (immutable, append-only, etc.) lsattr ✗ No
MIME type File format/content type file --mime-type ✗ No
File content Actual data inside the file cat, less, head ✗ No

File Attributes

Flag Name Effect View Modify
i Immutable Cannot modify, delete, rename, or link lsattr file sudo chattr +i file / sudo chattr -i file
a Append-only Only append; no overwrite or delete lsattr file sudo chattr +a file / sudo chattr -a file
A No atime update Reading file won’t update atime lsattr file sudo chattr +A file / sudo chattr -A file
d No dump Excluded from dump backups lsattr file chattr +d file / chattr -d file
S Synchronous updates Writes are flushed immediately lsattr file sudo chattr +S file / sudo chattr -S file
T Top directory Hint for placement under dir trees lsattr dir sudo chattr +T dir / sudo chattr -T dir
D Synchronous dir updates Dir updates written synchronously lsattr dir sudo chattr +D dir / sudo chattr -D dir
C NOCOW (btrfs) Disable copy-on-write for file/dir lsattr file sudo chattr +C file / sudo chattr -C file
j Data journaling (ext) Journal data as well as metadata lsattr file sudo chattr +j file / sudo chattr -j file
c Compressed (FS-dependent) File stored compressed on-disk lsattr file sudo chattr +c file / sudo chattr -c file
u Undelete (legacy) Try to keep data for undeletion lsattr file sudo chattr +u file / sudo chattr -u file
s Secure deletion (legacy) Attempt to zero on delete lsattr file sudo chattr +s file / sudo chattr -s file