Errata

The explanation of the join command is not clear and the example does not work.

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join --version
join (GNU coreutils) 8.13
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join -j1 file1 file2 will yield the following error:

join -j1 file1 file2
join: file1:3: is not sorted: three

Even if the file1 is sorted first, join will output nothing because the field 1 has no matching entries for both files.

cp command
-f
Force an overwrite of existing files in the destination.

It is not true. Based on man page of cp command it is :
-f, --force
if an existing destination file cannot be opened, remove it
and try again

You can try below to find out :
See the difference between output of these two commands that are run by a
normal user :
cp ~/mypasswd /etc/passwd
cp: cannot create regular file `/etc/passwd': Permission denied

$ cp -f ~/mypasswd /etc/passwd
cp: cannot remove `/etc/passwd': Permission denied

In second case cp tries to open /etc/passwd for writing and since it does
not have permission so -f option tries to remove destination file first and
we see a different error than first case.

rwxr-xr-- 111101100 = 751 is not correct.
Should read 754

The text says "Let’s sort our families by the father’s date of birth, from youngest to oldest." but I think it should be "from oldest to youngest."

Initially, the fdisk command by itself did not work because it was not in the $PATH variable. Once the user returned to her user shell and used su to become root, the $PATH environment variable then contained the /sbin and /usr/sbin directories, so fdisk was found.

Should be something like:

Initially, the fdisk command by itself did not work because it was not in the $PATH variable. If the user used "su" to become root, fdisk command will still not work because $PATH
variable did not change to include the /sbin and /usr/sbin directories. Once the user returned to their user shell and used "su -" to become root, the $PATH environment variable then contained the /sbin and /usr/sbin directories, so fdisk was found.

We can see from this file that the root account has a password, the password was last changed 14,617 days after January 1, 1970, this user can change her password at any time, she does not have to change her password until 99,999 days after January 1, 1970, and if her password is ever set to expire, she will start getting notices 7 days before the actual expiration.

remove " days after January 1, 1970,"
should be something like:

We can see from this file that the root account has a password, the password was last changed 14,617 days after January 1, 1970, this user can change his/her password at any time, he/she does not have to change her password, and if her password is ever set to expire, she will start getting notices 7 days before the actual expiration.

This scan took a little bit longer, but it showed us an additional three open TCP ports that the default scan did not show.

There were only two additional open ports.

The path/filename for the public key is shown as ~/.id_rsa.pub. It should be ~/.ssh/.id_rsa.pub.