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Managing file permissions is a core skill for any Linux administrator or user. In this guide, you’ll learn how to view ownership, adjust user and group assignments, interpret permission bits, and modify permissions using both symbolic and octal notation.

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Table of Contents

  1. Understanding Ownership
  2. Viewing Owner, Group, and Permissions
  3. Changing Owner and Group
  4. File Types and Permission Bits
  5. Permission Evaluation Order
  6. Modifying Permissions with chmod
  7. Using Octal Notation with chmod
  8. Resources & References

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Understanding Ownership

Every file or directory on Linux has:
  • User owner (UID)
  • Group owner (GID)
Only the file’s owner or root can change its permissions or ownership.

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Viewing Owner, Group, and Permissions

Run ls -l to display permissions, owner, group, size, and timestamp: 
$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg
  • -rw-r-----: File type and permission bits
  • aaron   : User owner
  • family   : Group owner

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Changing Owner and Group

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Change Group: chgrp

$ chgrp wheel family_dog.jpg
Example: 
$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chgrp wheel family_dog.jpg

$ ls -l
-rw-r----- 1 aaron wheel 49 Oct 27 14:41 family_dog.jpg
You can only switch to groups you belong to. Use groups to list them:
$ groups
aaron wheel family

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Change Owner: chown

Only root (or via sudo) can change the user owner: 
$ sudo chown jane family_dog.jpg
Example: 
$ ls -l
-rw-r----- 1 aaron wheel 49 Oct 27 14:41 family_dog.jpg

$ sudo chown jane family_dog.jpg

$ ls -l
-rw-r----- 1 jane wheel 49 Oct 27 14:41 family_dog.jpg
You can change both user and group: 
$ sudo chown aaron:family family_dog.jpg

$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

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File Types and Permission Bits

The first character in ls -l indicates the file type:
SymbolType
-Regular file
dDirectory
lSymbolic link
Permission bits follow in three sets (owner, group, others):
BitValueMeaning
r4Read
w2Write
x1Execute (or enter dir)
[!note] For directories:
  • r: list contents
  • w: create/delete files
  • x: change into the directory
The image illustrates file and directory permissions in a Unix-like system, showing "rwx" for owner, group, and others, with a key explaining the meaning of each permission bit.

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Permission Evaluation Order

Linux checks permissions in this order:
  1. Owner
  2. Group
  3. Others
Consider a file owned by aaron:family with permissions -r--rw----: 
$ ls -l
-r--rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg
  • As aaron: owner bits (r--) apply → no write 
    $ echo "Update" >> family_dog.jpg
bash: family_dog.jpg: Permission denied
  • As jane (in family): owner bits skipped, group bits (rw-) apply → can write 
    $ su jane
$ echo "Add this content" >> family_dog.jpg
$ cat family_dog.jpg
Picture of Milo the dog
  • Else: “others” bits determine access.

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Modifying Permissions with chmod

General syntax: 
chmod [who][+|-|=][perms] file
  • who: u (owner), g (group), o (others), a (all)
  • +: add permissions
  • -: remove permissions
  • =: set exact permissions

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Adding Permissions

Grant write to owner: 
$ ls -l
-r--rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod u+w family_dog.jpg

$ ls -l
-rw-rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

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Removing Permissions

Use - to revoke bits. Common patterns:
  • u-w: remove owner write
  • g-rw: remove group read/write
  • o-rwx: remove all for others
The image shows a terminal interface with instructions on removing permissions for users, groups, and others, using options like "u-", "g-", and "o-". Examples include "u-w", "g-rw", and "o-rwx".
Remove read for others: 
$ ls -l
-r--rw-r-- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod o-r family_dog.jpg

$ ls -l
-r--rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

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Setting Exact Permissions

Overwrite existing bits with =: 
$ ls -l
-rw-rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod g=r family_dog.jpg
$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod g=rw family_dog.jpg
$ ls -l
-rw-rw---- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod g= family_dog.jpg
$ ls -l
-rw------- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

$ chmod g=rwx family_dog.jpg
$ ls -l
-rw-rwx--- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

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Combining Multiple Changes

Separate specs with commas: 
$ ls -l
-rw-rwxr-x 1 aaron family 49 Oct 27 14:41 family_dog.jpg

# Owner: add read/write, Group: set to read-only, Others: remove all
$ chmod u+rw,g=r,o= family_dog.jpg

$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg

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Using Octal Notation with chmod

Octal mode is a compact way to set permissions. First, inspect with stat: 
$ stat family_dog.jpg
  File: family_dog.jpg
  Size: 49            Blocks: 8    IO Block: 4096   regular file
Device: fd00h/64768d  Inode: 52946177 Links: 1
Access: (0640/-rw-r-----)  Uid: ( 1000/aaron) Gid: (   10/wheel)
[!note] Each octal digit = read (4) + write (2) + execute (1).
For 0640:
  • Owner 6 = rw-
  • Group 4 = r--
  • Others 0 = ---
The image illustrates octal file permissions, showing the conversion between symbolic, binary, and decimal representations. It includes examples of permissions like "rw-r--r--" and "rwxr-xr-x" with their corresponding binary and decimal values.
Set 0640 directly: 
$ chmod 640 family_dog.jpg

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Resources & References

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