File Permission

Composition Structure#

-rwxr-xr--

The first character indicates the file type:

• -：File, such as text files, binary files, etc.
• d：Directory.
• l：Symbolic link, Symbolic link (Soft link).
• b：Block device, such as hard disk, CD-ROM, etc.
• c：Character device, such as terminal device, keyboard, etc.
• p：Pipe, inter-process communication.
• s：Socket, network communication, data exchange between processes.

The following characters are grouped into 3 (r=read, w=write, x=execute), corresponding to:

Directory Exec Perm#

Like file execution permission, if you want to operate on a directory, the directory must also have execution permission.

cd /home

ls /home

If there is no execution permission, the above commands will fail.

Umask#

When creating a file or directory, the system uses the value of umask to calculate the file’s permissions.

umask

0002

• The first bit: special mode, not expanded here.
• The second bit: Owner, owner.
• The third bit: Group, group.
• The fourth bit: Others, others.

Full Permissions#

• New file: 666 (default does not require execution permission).
• New directory: 777 (directories all require execution permission).

Permission Calculation#

Permission = Full Permission - umask

• File: 666 - 002 = 664
• Directory: 777 - 002 = 775

umask is a mask, representing the permissions that are not intended to be granted.

chmod#

The command to modify file permissions.

chmod - change file mode bits

Octal Mode#

chmod 755 file

-rwxr-xr-x 1 kuga kuga   16 Aug 23 11:08 file

Symbolic Mode#

u represents user: adds execution permission for the user.

chmod u+x file

g represents group: removes write permission for the group.

chmod g-w file

o represents others: adds read permission for others.

chmod g+r file

a represents the above 3: adds execution permission for all.

chmod a+x file

chown#

Modifies the owner and group of a file.

chown - change file owner and group

Change Owner#

chown user file

Recursively Change Owner#

chown -R user dir

Change Owner and Group#

chown user:group file

Recursively Change Owner and Group#

chown -R user:group dir

Change Group Only#

chown :group file

chgrp#

chgrp - change group ownership

chgrp group file