More about the command prompt

Gaël Donval <g.donval@bath.ac.uk>

From now on, you will spend a lot of your time in the terminal, at the command prompt, typing command lines. As you saw in Fig. 14, the command prompt (where you can type command lines) can be roughly decomposed thusly:

[<username>@<hostname> <working directory>]$

The <username> should be yours, <hostname> should be either balena-01 or balena-02 for the time being and your working directory should be ~ which is a shortcut notation for your home directory.

Useful conventions

Representing the full prompt each time we want you to type something in is not really convenient. Instead, we will abide by the usual convention and reduce the full prompt to $: as you may have figured out now, $ denotes the start of the area where you can input commands:

$ <command>

Here <command> is a placeholder representing a command to type, $ just indicates that the following command needs to be typed at the prompt; $ itself shouldn’t be typed. Once the command is typed, you need to press Enter to actually execute it.

Sometimes, we also want to give you the computer’s answer to a command, in which case the answer will be given in the same block but without $:

$ <command>
<answer>

For instance try the following (remember to press Enter at the end of each command):

$ echo "Hello World!"
Hello World!
$ echo "One Two One Two"
One Two One Two

The command echo just displays back what you typed after it. Notice how you get your prompt back after each of these commands and can start to type other things right away.

Help, I am stuck!

Now, as you become more and more proficient with the command line, you will have to type in more complex command. Try the following ones:

$ watch -t echo "Hello World!"
$ echo "Hello World!" | less

Their exact meaning does not matter yet. However if you typed any one of these commands correctly, you should now be stuck: the command prompt does not appear anymore no matter what you do or how long you wait! You will soon understand why…

In the meanwhile you need to learn how to get the prompt back when you are stuck: depending on the program, pressing q or Ctrl-c (i.e. keep the control key pressed and press the key c) should be enough to get the command prompt back. There is no harm done in trying both if you are unsure.

Understanding what you type

We understand that this is all new to you and we expect that you will need some time to get familiar with it. However there is no point in blindly typing things in.

Instead, we urge you to go slowly in order to ensure you understand what you are doing (more on that in What does command X do?). For instance, let us say we asked you to try to input the command, whoami giving the following snippet:

$ whoami
ggd21

As described before, we expect you to type whoami and press Enter. That specific reply is my user name. Do you expect to get the same answer? Try it!

Most returned values we will give you in this documentation are like this: they are meant as relevant examples rather than specific answers.

What does command X do?

There are 4 main ways to know what a command is about:

  • $ man <X> should display a manual page (if not, try another method). It should contain different sections such as NAME where the command is briefly described; a SYNOPSIS where the different ways to invoke that command are given and a DESCRIPTION containing a longer description and the list of possible options and flags. Press q to quit.

  • $ info <X> provides very similar information but the description is usually better than with man. Again press q to quit.

  • $ <X> --help calls for the help page of program <X> by passing it the --help flag. Those help pages are usually shorter than the other ones: they are really useful once you are familiar with a command but are unsure about a specific option.

  • Search for the command name on the internet. Be careful with this method though as the version of the program you can find on the internet might be different.

Now look for the definition and usage of the following commands: ls, pwd, cd, whoami, cp, mv, rm, mkdir, touch, echo, date and nano. We suggest that you write down that list with the corresponding definitions for future reference: notice the similarly between the command name and what it is used for. Square-bracketed elements are usually optional, anything starting with a dash (-) or a double-dash (--) is usually called a flag and in this particular context pipes (|) often represent alternatives.

You don’t need to go through all the options for all those commands now, however if you see a command like ls -s you should feel comfortable with consulting the man page (and any other source of information you need) to understand how the flag -s is altering ls’s behaviour!

Note

Don’t hesitate to experiment with ls as much as you want to: that command is perfectly safe and it will give you a feeling of how flags are used to tweak a program’s behaviour!

What’s the command to do Y?

The manual pages are great resources to know how to use a command you already know about for a given application but they are poor resources to discover new commands. Let us suppose you didn’t go through the list of commands in the previous section and still don’t know how to copy a folder. The easiest (and recommended) way to find an answer is to look for it on the internet. Try either of these:

how to copy directory command line linux

how to copy directory bash

You are going to be flooded with answers to that question that hundreds of clueless people already asked in various places: searching for information should be your very first reaction; asking for answers should be your last recourse.

The bash I am mentioning in the second search is the specific name of the command prompt language you will use on Balena. You may also see references to the Bash shell which is just a fancy name for the command prompt that happens to be used on Balena.

Hint

Appending bash as a keyword is a great way to get relevant answers.

Warning

You must always understand what you type, especially when it’s coming from the internet: always look at the manual page if unsure and read the full explanations. Most commands need adapting and some can damage you data.

What if something goes wrong?

Learning a new interface can be overwhelming but please keep in mind that however magical it looks, the computer is going to do exactly what you told it to do. If something went wrong somewhere, it most certainly mean that you misused something in the first place. Fortunately though the computer is going to tell you when and (often) where something went wrong. For instance if you mistyped something and wrote a command that doesn’t exist, you will see something like this:

$ idontexist
-bash: idontexist: command not found

If you try to point to an non-existing folder, you’ll get something like:

$ ls /i_m_not_there_either
ls: cannot access /i_m_not_there_either: No such file or directory

If you try to use an invalid option, you will get:

$ cp -z
cp: invalid option -- 'z'
Try `cp --help' for more information.

Pay attention to these: it is much more helpful for us to hear something like “I don’t understand, it says that program <X> does not exist, what should I do?” rather than “there’s an error” or even worse “it’s not working”. A specific error message usually leads to a quick solution: “it’s not working” covers everything from minor numerical errors to your computer catching fire.

How to edit a file?

Use nano (you should have looked for that command in What does command X do? already).

How to look at file content?

You could use an editor like nano for this task but what is called a pager would be more suitable. We recommend that you use less for that purpose:

$ less <filename>

You must press q to quit that program. You can search for terms by pressing /<search term> and press Enter. Then press n to go to the next occurrence of <search term> and Shift-n to go back to the previous one.

Press Shift-g to reach the end of the file. If the calculation is still running and you need to see the latest updates of <filename>, press Shift-f.

Note

the name less is a pun in reference to an older pager called more. You may safely replace all occurrence of more you may find in documentation by less. You can also dump the entirety of a short file to the screen using the command cat.