What does "read -p" do in a linux shell script? [closed]

Question

I have a script that I have copied and edited. There are a couple of lines in there that I need explaining if possible please.

These are the lines:

     read -p "please enter the username you wish to create: " username
     if id -u $username >/dev/null 2>&1; then

What does read -p do? What does id -u do? What does >/dev/null 2&1; do? Then further on in the script, it has this line that says this:

     sudo useradd -g $group -s $bash -d $homedir -m $username -p $password

Again please could someone explain all the minus signs in this line please? (-g, -s, -d, -m, -p)

Answer 1

First off, the structure <command> -<option> means that you want to execute <command> using the option corresponding to <option>. A - 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 <command> -<option> [argument]. So, in the useradd example, $group is an argument for the option g.

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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 <command> 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.