First off, the structure - means that you want to execute - after a command means that the following letter is an option. Most commands have several options you can use. Options are usually defined using either a single letter or a couple of words separated by -.
Side Note: For options that are a couple of words rather than a single letter, often it will use two minus signs -- instead of one, signifying that it is a "long named" option.
So, using the read -p example, this means you want to execute read using the p option, which stands for prompt.
Now, sometimes an option will require an argument. In your examples, the options to useradd have arguments. Arguments are usually defined like -. So, in the useradd example, $group is an argument for the option g.
Now for the commands themselves:
read is a bash built-in (not a POSIX shell command) that reads from standard input.
- The
-p option makes it read as a prompt, meaning it doesn't add a trailing newline before trying to read input.
if checks the return status of the test command (in this case id -u $username >/dev/null 2>&1)
- If the return status is 0, the
then part is executed
id prints user groups and ids
- The
-u option "prints only the effective user ID".
- >/dev/null 2>&1 redirects standard input and standard error to
/dev/null, meaning they do not get printed to the terminal.
useradd creates a new user
-g sets the initial group for the user-s sets the name of the user's login shell-d sets the name of the user's login directory-m says to create the user's home directory if it does not exist.-p defines the user's encrypted password.
For future reference, you can look up commands in the linux manual pages by doing man on the command line. These manual pages tell you what a command does, as well as explaining all of its options.
Bash built-ins like read are all on a single man page that is not the easiest thing to use. For those I find googling them easier. Usually http://ss64.com/ will come up in the results, which contains the info from the bash built-ins man page, but separated into different pages by command. I find this much easier to use.
