A Quick Overview of Unix-Style Permissions

This is a copy of an overview I wrote for people that were coming across an laravel-based file permission GitHub issue in the BookStack project. Adding it here as an easier location to reference.

There are other, more extensive & technical breakdowns but this is intended to give a developer a 10 minute run-down of the basics.

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Files and folders have 3 main permissions: read, write & execute. Files and folders are also assigned an owner and group.

Reading Permissions

You can run ls -alh in the terminal to show the files and folders with their permissions:

# ls -alh
drwxrwxr-x  5 dan      dan 4.0K May 28 13:58 .
drwxrwxr-x 18 dan      dan 4.0K Jun  9 10:38 ..
-rw-r--r--  1 dan      dan 5.6K Dec 10 18:19 book_default_cover.png
drwxr-xr-x  2 dan      dan 4.0K Jun  9 10:40 dist
-rw-rwxr--  1 www-data dan  11K Oct 26  2016 favicon.ico
-rw-r--r--  1 dan      dan  412 Aug  9  2017 .htaccess

The permissions are on the left, in the first column. The starting d is shown if it’s a directory. Then there are three sets of rwx.

• The first set of rwx is the permissions for the owner.
• The second set of rwx is the permissions for the group.
• The third set of rwx is the permissions for everyone else.

Each of these characters represents read, write or execute. A hyphen (-) is shown instead if the permission is not granted. Note that execute permissions are required on folders to enter them.

The owner assigned to a file/folder can be seen in the third column. The group can be seen in the fourth. In the example above the file favicon.ico is assigned to the group dan and is owned by www-data. The owner www-data has permission to read and write the file. The group dan has permission to read, write, or execute the file. Everyone else can only read the file.

Octal Format

Permissions may also be shown as numbers in an octal format. In the octal format each permission has a number:

• Read = 4
• Write = 2
• Execute = 1
• No permission = 0

These numbers are summed together into a single digit. For example, Having all permissions will be shown as a 7 or only having Read+Execute permissions will be shown as a 5. These totals are often used in a set of 3 to represent the permissions for the group, owner & everyone else.

In the example command output above, the permissions for favicon.ico could be shown as 674. The .htaccess file permissions could be shown as 644. All permissions granted to everyone would show as 777.

Setting Permissions

There are two main commands for controlling permissions:

• chmod (Change mode), Used to set permissions.
• chown (Change ownership), Used to change the owner and group.

For both of these commands using -R will set permissions recursively upon all child files and directories.

chmod usage

# Format:
chmod [OPTIONS] PERMISSONS FILES...

# Example:
# Grant the owner and group 'read+write+execute' permissions
# Give everyone else 'read and execute' permissions
# In the './storage' directory and all files+folders within.
chmod -R 775 ./storage 

chown usage

# Format:
chown [OPTIONS] USER:GROUP

# Example:
# In the './storage' directory and all files+folders within
# Set the owner to be 'dan' and set the group to be 'www-data'
chown -R dan:www-data ./storage 

Common use

For things such as file uploads, you’d generally want these to be both readable and writable by the webserver. The user and group your web server runs as will depend on your system and config. On ubuntu it’s common for apache and nginx to run as www-data, both as the owner and group. In this case, If i wanted to give the webserver permission to upload and serve files within the ./storage directory I might do the following:

# Recursively set myself as the owner and the web server as the assigned group
chown -R dan:www-data ./storage

# Recursively allow myself and the webserver to Read, Write and Execute files & folders
# While allowing everyone else to only Read or Execute
chmod -R 775 ./storage