Building off the previous post, we now explore the dual topics of users and permissions.
Linux is inherently a multi-user operating system. This can be quite confusing for desktop users of Windows or MacOS, and completely foreign to smartphone/table users.
Using Windows as an example, it’s common for someone to have a username that matches their real name. Once they open the computer and type their password, that’s about as far as they get. Often, the default user in Windows is an Administrator, so they are unrestricted and never consider rights and privileges associated with their day-to-day work.
Linux and other UNIX-like operating systems have the central concept of a
user. There are effectively unlimited users available on a Linux system. Similarly, Linux has the concept of a
group, which can contain one or more users. A group cannot be a member of another group, but a user can be a member of many groups. Fairly intuitive.
Linux keeps track of these users by means of a
uid, short for “user id”. Similarly, it keeps track of groups by a
gid short for “group id”. These are simple integers that are unique to each user and group.
To see this in action, open a Terminal and type the following:
devil@ubuntuVM:~$ id uid=1000(devil) gid=1000(devil) groups=1000(devil),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),131(lxd),132(sambashare)
id command will display the
gid of the active user, as well as the groups that user is a part of, plus their
You’ll notice some familiar looking words in there. The first result
uid=1000(devil) simply tells me that I am user id 1000, and my username is devil. Similarly, I have a group named devil, and its
gid is 1000. Ubuntu assigns new users their own dedicated group, so you’ll often see this.
Did you notice the
27(sudo) entry? Here is our first exposure to permissions. The default configuration of
sudo(we learned this in the last lesson) will only allow users in the
sudo group to run commands with superuser privilege.
The other groups listed here perform a similar function, allowing any user in that group to perform some privileged function.
Since users and groups are effectively unlimited, you can craft the permissions to satisfy access concerns for nearly any random arrangement of people.
In the previous lesson, you may have noticed a bunch of extra text in columns to the left of the directory listing. I ignored that on purpose, but I will explain it now.
Let’s use a single example to keep it simple:
root@ubuntuVM:/# ls -l /home total 4 drwxr-xr-x 15 devil devil 4096 Jun 17 22:51 devil
This output tells me that within the
/home directory, there is a sub-directory called
devil. The full path to this directory is
The first column basically looks like nonsense, but it has a meaning. It reads
drwxr-xr-x, which is a graphical representation of its permissions.
In Linux, a file or directory has three sets of permissions for three sets of users. The permissions are
execute. They are often abbreviated as
x. You’ll notice that the
devil directory starts with
drwx, which simply means that
devil is a directory (
d) with the read, write, and execute permissions set (
rwx) on the first set of users.
write are fairly obvious. What is
execute? In the case of a script, it will not run when called directly unless the
execute permission is enabled. A directory will not display its contents unless its
execute permission is set. Think of
execute as a permission that allows you to “do stuff” with it.
What are the sets of users? They are as follows:
When you list a file permission using
ls -l, it will display the permissions in a string in the order above. Set permissions are displayed with
x. If a permission is unset, it will display
Going back to my example above, we can interpret the permissions for
- user = read, write, execute
- group = read, execute
- other = read, execute
OK, seems good enough. Who are
group? They are displayed in the next column. In this case,
/home/devil is owned by
group=devil. All non-devil users and non-devil groups are given the permissions defined in
Linux allows you to change a file’s permissions at any time using the
chmod accepts two forms for permissions. One is intuitive, one a bit less so.
The intuitive form users the familiar abbreviations from before. Let’s create a new file using
touch, then modify its permissions. What do you think this does?
devil@ubuntuVM:~$ touch testfile devil@ubuntuVM:~$ chmod u=rwx,g=rw,o=r testfile
If you said “read-write-execute for the user, read-write for the group, read-only for all others” then you’re getting it!
If not, just take another look until it clicks.
The unintuitive form of
chmod is known as “numeric mode”, using numbers to represent permissions instead of letter abbreviations. Linux nerds love to use powers of 2, and they have done so here.
Consider the following value-number pairs:
- read = (2^2) = 4
- write = (2^1) = 2
- execute = (2^0) = 1
The neat thing about powers of two is that each pair or triplet adds to a unique value. Using these powers of two, we can express up to 8 combinations (2^3) of permissions with just an integer.
- 0 (no permissions)
- 1 (execute)
- 2 (write)
- 3 (write-execute) = 2 (write) + 1 (execute)
- 4 (read)
- 5 (read-execute) = 5 (read) + 1 (execute)
- 6 (read-write) = 4 (read) + 2 (write)
- 7 (read-write-execute) = 4 (read) + 2 (write) + 1 (execute)
Not obviously useful, but consider this example. Instead of writing out the long form
chmod u=rwx,g=rwx,o=rx, I could simply write
Take a look at the
u=rwx and think about how that is equivalent to
7 using the numeric math above.
OK, now we know how to create a file (
touch) and modify its permissions (
chmod). How do we change ownership? We use
Let’s create a file, change its permission, then change its ownership.
root@ubuntuVM:~# touch test root@ubuntuVM:~# ls -l test -rw-r--r-- 1 root root 0 Jun 17 23:24 test root@ubuntuVM:~# chmod 664 test root@ubuntuVM:~# ls -l test -rw-rw-r-- 1 root root 0 Jun 17 23:24 test root@ubuntuVM:~# chown devil:root test root@ubuntuVM:~# ls -l test -rw-rw-r-- 1 devil root 0 Jun 17 23:24 test
A Few Housekeeping Commands
Since we’re working with files already, let’s learn how to move, copy, rename, and delete.
root@ubuntuVM:~# cp test test2 root@ubuntuVM:~# ls -l total 4 -rw-rw-r-- 1 devil root 0 Jun 17 23:24 test -rw-r--r-- 1 root root 0 Jun 17 23:27 test2
test2 is created with different permissions from
test? All new files will be created with owner and group equal to the current user. In this case,
Now let’s rename test2 to test69 (lol) using
root@ubuntuVM:~# mv test2 test69 root@ubuntuVM:~# ls -l total 4 -rw-rw-r-- 1 devil root 0 Jun 17 23:24 test -rw-r--r-- 1 root root 0 Jun 17 23:27 test69
Then finally, we clean up after ourselves by removing the files with
root@ubuntuVM:~# rm test test69 root@ubuntuVM:~# ls -l total 0
In the next lesson, I will cover how to start, stop, enable and disable services. A service is a program that executes in the background without user interaction. This is particularly useful for running servers and automatic tasks like updates, backups, and cleanup.
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