unix Permissions Understanding Permissions

Example

Let's say there is a file we would like to execute, a bash script named add.sh, for example. Typing ./add.sh however, yields a permission error. Getting the permissions is a simple process.

To determine the permissions a file has, type:

ls -l filename, or, in our case, ls -l ./add.sh

This prints the following to the console:

-r--r--r-- 1 username groupname 0 Jan 4 12:00 add.sh

Let's stop and understand what this means. There are three different types for permissions: owner, group, others. Distinct permissions apply to each permission type.

There are also three permission actions, which more broadly also describe what exactly a user can do to a file. These are: (read: r, write: w, execute: x).

So, back up to that string of dashes and r's. Each permission group has three potential abilities. The groups are listed in the order owner-group-others and the actions as read-write-execute.

But wait, that means there's an extra character at the beginning of the string. This is actually the file descriptor character. We can see there is a - there, but other characters exist for things like directories(d), sockets(s), symbolic link(l) etc.

This leaves us with essentially this information: a file where owner, group, and others have read permissions. No other permissions granted.

Let's alter this to allow the owner to also write and execute the file. Note: Depending on the permissions, it may be necessary to prepend sudo to this command.

chmod 744 add.sh
ls -l add.sh

Prints out

-rwxr--r-- 1 username groupname 0 Month time add.sh

Now, the owner of the file can execute the file by typing

./add.sh