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In this lesson, we’ll explore how to view and modify file permissions and ownership on a Linux system. You’ll learn to inspect permission bits, change owners and groups, and apply both symbolic and numeric modes with chmod, chown, and chgrp.

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Inspect Current Ownership and Permissions

Run ls -l to display the owner, group, and permission bits for files and directories: 
$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg
  • Owner: aaron
  • Group: family
  • Permissions: -rw-r-----
Only the file owner or the superuser (root) can change these settings.

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Viewing and Changing Group Ownership

Use chgrp to assign a file or directory to a different group you belong to: 
# Syntax: chgrp <group_name> <file_or_directory>
$ chgrp wheel family_dog.jpg
$ ls -l
-rw-r----- 1 aaron wheel 49 Oct 27 14:41 family_dog.jpg
Check your group memberships with: 
$ groups
aaron wheel family
You can only switch a file’s group to one you’re already a member of.

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Changing File Owner with chown

Only root can change file owners. Prefix with sudo if necessary: 
# Syntax: sudo chown <user>[:<group>] <file_or_directory>
$ sudo chown jane family_dog.jpg
$ ls -l
-rw-r----- 1 jane wheel 49 Oct 27 14:41 family_dog.jpg

# Change both owner and group in one go:
$ 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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Understanding Permission Bits

The permissions string (-rwxrwxrwx) breaks down as:
  • First character: file type
    • - = regular file
    • d = directory
    • l = symbolic link
  • Next nine: three triplets for owner, group, and others, each with r (read), w (write), and x (execute).
The image illustrates file and directory permissions in a Unix-like system, showing the permission string "rwxrwxrwx" for owner, group, and others, along with a key explaining the meaning of each permission bit.

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Permission Effects

  • Files
    • r: view contents
    • w: modify contents
    • x: execute (scripts or binaries)
  • Directories
    • r: list entries (ls)
    • w: create/delete entries
    • x: enter directory (cd)

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Modifying Permissions with chmod

Use the symbolic syntax: 
chmod [ugoa][+-=][rwx] <file_or_directory>
ReferenceMeaning
uowner (user)
ggroup
oothers
aall (u, g, o)
+add permissions
-remove permissions
=set exact permissions

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Adding Permissions

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

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

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Removing Permissions

Remove the read bit for others: 
$ chmod o-r family_dog.jpg
$ ls -l
-rw-r----- 1 aaron family 49 Oct 27 14:41 family_dog.jpg
The image shows a guide on removing permissions in a command-line interface, with examples for users, groups, and others. It includes options like u-, g-, and o- with examples such as u-w, g-rw, and o-rwx.

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Setting Exact Permissions

Grant group read-only: 
$ 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

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Combining References

You can comma-separate multiple adjustments: 
$ ls -l
-rw-r--r-- 1 appuser appuser 49 Oct 27 14:41 family.jpg

# Owner read/write, group read, others none
$ chmod u=rw,g=r,o= family.jpg
$ ls -l
-rw-r----- 1 appuser appuser 49 Oct 27 14:41 family.jpg

# Mix add/remove in one command
$ chmod u+rw,g-w family_dog.jpg

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Numeric (Octal) Notation

To inspect the octal value, use stat: 
$ stat family_dog.jpg
  Access: (0640/-rw-r-----)  Uid: ( 1000/aaron)   Gid: (   10/family)
Here owner has rw- (6), group has r-- (4), others have --- (0) → mode 640.
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.
Alternatively, assign values: r=4, w=2, x=1:
  • rwx = 4+2+1 = 7
  • r-x = 4+0+1 = 5
  • r-- = 4+0+0 = 4
The image explains octal permissions in a Unix-like system, showing how the permissions "rw-r-----" translate to the octal value "640". It also provides a key for permission values: read (r) is 4, write (w) is 2, and execute (x) is 1.

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Setting Numeric Permissions

$ chmod 755 family_dog.jpg  # rwxr-xr-x
$ chmod 640 family_dog.jpg  # rw-r-----

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Further Reading

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