Understanding File Permissions on Mac

File permissions define who and what can read, write to, and execute a given file on your Mac. Maintaining well-defined file permissions will both improve your overall system security and ensure that automated processes can execute within a given workflow when called.

Today, we’ll take a look at two ways of representing file permissions, what they mean, and how to use Chmod to change permissions on a given file.

How does file ownership work on Mac?

Files on macOS have permissions defined for three subsets of system users. There are permissions granted to the single user who owns the file, the group of users the owner is associated with, and everyone else.

• User: The owner of a given file – the file’s creator by default.
• Group: A group of users who will share common permissions – “staff” is the default group on macOS.
• Everyone Else (Others): All other users and groups in the system.

What are the different permission levels on Mac files?

| Permissions Description | Octal Value     | File Permissions Set     |
| :---------------------- | :-------------: | -----------------------: |
| No access               |       0         |           ---            |
| Execute                 |       1         |           --x            |
| Write                   |       2         |           -w-            |
| Write and execute       |       3         |           -wx            |
| Read                    |       4         |           r--            |
| Read and execute        |       5         |           r-x            |
| Read and write          |       6         |           rw-            |
| Read, write and execute |       7         |           rwx            |

The values r, w, and x are pretty straightforward. They represent read, write and execute permissions respectively. The octal values are less straightforward but useful for setting distinct permissions for all user types in a single command, which we’ll look at below.

The octal values represent each possible combination of permissions as a single integer ranging from 0 to 7. Combinations of permissions have an octal value that is equal to the sum of the octal values of each individual permission being granted.

For example, write and execute (with an octal value of 3) is the sum of the values that represent execute (1) and write (2). As another example, read and write (with an octal value of 6) is the sum of read (4) and write (2) privileges.

How to Change File Permissions on Mac with Chmod

On Mac, you can use the utility chmod to change the permissions on a given file. For example, to make a file executable for all users, you can run either of the following commands:

1. chmod ugo+x example.sh

Above, we have identified each user type we want to set the execute permission for (every type in this case), used the + symbol to indicate the adding of new privileges, and passed x to indicate the execute permission. We could also use the - symbol to indicate the removal of privileges. Furthermore, because we are setting the same permission to all user types, we can shorten the above to the more commonly used chmod +x example.sh.

2. chmod 111 example.sh

Here, we are using the octal value of the execute permission (1) to explicitly set the permission levels for each user type in the following order – user, group, everyone else.

Setting distinct permission levels on a file with a single command

Because we always set the permissions of the user, group, and everyone else in that order, we can use the octal values from the table above to set distinct permissions for a given file’s owner, group, and everyone else in a single command, like so:

chmod 610 example.sh

Above, we gave the user a 6 (read and write permissions), the group a 1 (execute permission), and everyone else 0 (no access).

TL;DR

File permissions on Mac determine which users can read, write to, and execute a given file. This is important because when managed well, file permissions can enhance system security, but when done badly it can have the inverse effect. Above, we outlined the various types of permissions, the various user types to which permissions can be assigned, and how to go about setting these permission types.