File Attributes

Identifying Ownership and Permissions

In the last section, we saw that the output of the ls -l command included a lot extra information besides a listing of file names. The output also listed the owners and permissions for each file and directory.

Each user account on a Linux system (like many operating systems) has a user name and has at least one group membership, and that name and that group membership determine the user and group ownership for all files created under that account.

In order to allow or restrict access to files and directories (for example, to allow other users to read, write to, or run your or others' files), ownership and permissions are set in order to manage that kind of access to those files and directories. There are thus two owners for every file (and directory):

• user owner
• group owner

And there are three permission modes that restrict or expand access to each file (or directory) based on user or group membership:

• (r)ead
• (w)rite
• e(x)ecute (as in a program)

I am emphasizing the rwx in the above list of modes because we will need to remember what these letters stand for when we work with file and directory permissions.

Consider the output of ls -l in some public_html directory that contains a single file called index.html:

-rw-rw-r-- 1 sean sean 11251 Jun 20 14:41 index.html

According to the above output, we can parse the following information about the file:

What's important for us right now are the File permissions row, the Owner name row, and the Group name row.

The Owner and Group names of the index.html file are sean because there is a user account named sean on the system and a group account named sean on the system, and that file exists in the user sean's home directory.

The File permissions row shows:

-rw-rw-r--

Let's ignore the first dash for now. The remaining permissions can be broken down as:

• rw- (read and write only permissions for the Owner)
• rw- (read and write only permissions for the Group)
• r-- (read-only permissions for the World)

We read the output as such (dashes, other than the initial one, signify no permissions):

• User sean is the Owner and has (r)ead and (w)rite permissions on the file but not e(x)ecute permissions (rw-).
• Group sean is the Group owner and has (r)ead and (w)rite permissions on the file but not e(x)ecute permissions (rw-).
• The World can (r)ead the file but cannot (w)rite to the file nor e(x)ecute the file (r--).

The word write is a classical computing term that means, essentially, to edit and save edits of a file. Today we use the term save instead of write, but remember that they are basically equivalent terms.

Since this is an HTML page for a website, the World ownership allows people to view (read) the file but not write (save) to it nor execute (run) it. Any webpage you view on the internet at least has World mode set to read.

Let's take a look at another file. In our /bin directory, we can see a listing for this program (note that I specify the absolute path of the file named bin):

ls -l /bin/zip
-rwxr-xr-x 1 root   root    212K Feb  2  2021  zip*

Since zip is a computer program used to package and compress files, it needs to be e(x)ecutable. That is, users on the system need to be able to run it. But notice that the owner and group names of the file point to the user root. We have already learned that there is a root level in our filesystem. This is the top level directory in our filesystem and is referenced by the forward slash: /. But there is also a root user account. This is the system's superuser. The superuser can run or access anything on the system, and this user also owns most of the system files.

We read the output as such:

• User root is the Owner and has (r)ead, (w)rite, and e(x)ecute (rwx) permissions on the file.
• Group root is the Group owner and has (r)ead and e(x)ecute permissions but not (w)rite permissions (r-x)
• The World has (r)ead and e(x)ecute permissions but not (w)rite (r-x). This permissions allows other users (like you and me) to use the zip program.

The asterisk at the end of the file name (zip*) simply indicates that this file is an executable; i.e., it is a software program that you can run.

Finally, let's take a look at the permissions for a directory. On my system, I run the following command in my home directory, which will show the permissions for my /home/sean directory:

ls -ld

And the output is:

drwx--x--- 51 sean sean 4.0K Jun 23 18:35 ./

This shows that:

This is a little different from the previous examples, but let's parse it:

• Instead of an initial dash, this file has an initial d that identifies this as a directory. Directories in Linux are simply special types of files.
• User sean has read, write, and execute (rwx) permissions.
• Group sean has execute (--x) permissions only.
• The World has no permissions (---).
• ./ signifies the current directory, which happens to be my home directory, since I ran that command at the /home/sean path.

The takeaway from this set of permissions and the ownership is that only the user sean and those in the group sean, which is just the user sean, can access this home directory.

We might ask why the directory has an e(x)ecutable bit set for the owner and the group if a directory is not an executable file. That is, it's not a program or software. This is so that the owner and the group can access that directory using, for example, the cd (change directory) command. If the directory was not executable, like it's not for the World (---), then it would not be accessible with the cd command, or any other command. In this case, the World (users who are not me) cannot access my home directory.

Changing File Permissions and Ownership

Changing File Permissions

All the files and directories on a Linux system have default ownership and permissions set. This includes new files that we might create as we use our systems. There will be times when we will want to change the defaults, for example, the kinds of defaults described above. There are several commands available to do that, and here I'll introduce you to the two most common ones.

1. The chmod command is used to change file (and directory) permissions (or file mode bits).
2. The chown command is used to change a file's (and directory's) owner and group.

The chmod command changes the -rwxrwxrwx part of a file's attributes that we see with the ls -l command. Each one of those bits (the r, the w, and the x) are assigned the following octal values:

There are three octal values for the three set of permissions represented by -rwxrwxrwx. If I bracket the sets (for demonstration purposes only), they look like this:

-[rwx][rwx][rwx]

The first set describes the permissions for the owner. The second set describes the permissions for the group. The third set describes the permissions for the World.

We use the chmod command and the octal values to change a file or directory's permissions. For each set, we add up the octal values. For example, to make a file read (4), write (2), and executable (1) for the owner only, and zero out the permissions for the group and World, we use the chmod command like so:

chmod 700 paper.txt

We use 7 because 4+2+1=7, and we use two zeroes in the second two places since we're removing permissions for group and World.

If we want to make the file read, write, and executable by the owner, the group, and the world, then we repeat this for each set:

chmod 777 paper.txt

More commonly, we might want to restrict ownership. Here we enable rw- for the owner, and r-- for the group and the World:

chmod 644 paper.txt

Because 4+2=6 for owner, and 4 is read only for group and World, respectively.

Changing File Ownership

In order to change the ownership of a file, we use the chown command followed by the name of the owner. We can optionally change the owner of the group by adding a colon (no spaces) and the name of the group.

We can see what groups we belong to with the groups command. On one system that I have an account on, I am a member of two groups: a group sean (same as my user name on this system), and a group sudo, which signifies that I'm an administrator on this system (more on sudo later in the semester).

groups
sean sudo 

We can only change the user and group ownership of a file or directory if we have administrative privileges (sudo administrative access), or if we share group membership. This means that, unless we have sudo (admin) privileges, we often might change the group name for a file or directory than the user owner. Later in the semester, you will have to do this kind of work (change user and group names) of files and directories. In the meantime, let's see some examples:

Imagine that my Linux user account belongs to the group sisFaculty, and that there are other users on the Linux system (my colleagues at work) who are also members of this group. If I want to make a directory or file accessible to them, then I can change the group name of a file I own to sisFaculty. Let's call that file testFile.txt. To change only the group name for the file:

chown :sisFaculty testFile.txt

I can generally only change the user owner of a file if I have admin access on a system. In such a case, I might have to use the sudo command (you do not have access to the sudo command on our shared server, but you will have it later on your virtual machines). In this case, I don't need the colon. To change the owner only, say from the user sean to the user tmk:

sudo chown tmk testFile.txt

To change both user owner and group name, we simply specify both names and separate those names by a colon, where the syntax is chown USER:GROUP testFile.txt

sudo chown tmk:sisFaculty testFile.txt

After using the chown command to change either the owner or group, we should double check the file or directory's permissions using the chmod command. Here I make it so that the user owner and the group sisFaculty has (r)ead and (w)rite access to the file. I use sudo because, as the user sean, I'm changing the file permissions for a file that I do not own:

sudo chmod 660 testFile.txt

Conclusion

In this section, we learned:

• how to identify file/directory ownership and permissions
• and how to change file/directory ownership and permissions.

Specifically, we looked at two ways to change the attributes of a file. This includes changing the ownership of a file with the chown command, and setting the read, write, and execute permissions of a file with the chmod command.

The commands we used to change these attributes include:

• chmod : for changing file permissions (or file mode bits)
• chown : for changing file ownership

We also used the following commands:

• ls : list directory contents
  • ls -ld : long list directories themselves, not their contents
• groups : print the groups a user is in
• sudo : execute a command as another user