Basic shell usage

2.4.4. Basic shell usage#

Open (a terminal)[sec:open_terminal], if you have not an open terminal already. Every time you open a terminal, it will automatically run the default shell program, which is bash on Ubuntu by default. As explained before, the shell interprets your commands and instructs the kernel to start other programs.

As explained about the Unix philosohy, Linux systems contains many small, simple commands that do one thing and do it well. We will start exploring the most basic usage of the shell here with some simple examples, starting a few basic programs.

The examples in this section will also illustrate how programs often support more advanced features and configuration options, allowing to alter their behavior. These configuration options can be specified by adding additional options after the command. Sometimes the programmers have put in so many options that it’s hard to know which program option gives what effect, so in the next section we’ll explore how to look through the built-in manual from within the terminal. Nevertheless, common options of common programs will probably get ingrained into your memory, just by using the terminal often. Just as when you learn a normal language, after some practice you should not need to open the dictionary for every word.

2.4.4.1. Hello terminal!#

As a first example, we will look at the echo command. As its name suggests, it basically repeats whatever arguments you give it, and prints[1] it back to the terminal:

Let’s try it out. Type the following, and don’t forget to press Enter to confirm the command:

$ echo Hello World!

You should see that echo has repeated the words Hello World! in the terminal on a new line after your command. Once the command has completed, you are presented with a new prompt to enter your next command.

Note

The echo command may seem silly at first sight: why would we want to print something in the terminal that we just typed ourselves a second ago? As we shall see, echo is often used in scripts that automate shell operations to inform the user of progress or errors. It can also be used to print the value of variables, just like the “print” commands in other programming languages, or be used to save lines of text in a file. These are all more advanced topics explained later in the manual.

The words after echo are called arguments (characters or numbers following the program name), which act as input for the echo command. In fact, in this case the echo command has received two separate arguments, Hello and World!, since bash treats whitespaces (space, tab, newline) as separators between command and each of its arguments. The echo command prints all of its arguments in order, also separating them with a single space, so we see the same text in the output.

However, the shell ignores any additional whitespace that separates commands and arguments, so if we enter this:

$ echo Hello      World!

the echo command will still just get the same two arguments Hello and World!. Since it prints the arguments one after the other with just a single space, the resulting Hello World! will not have the same whitespaces as you wrote in the command line.

We can demarcate an argument with double quotes to force the shell to pass an argument as-is, even if it includes spaces or other special characters that the shell might otherwise process differently. Try out this:

$ echo "Hello      World!"

Now the echo command receives a single argument, Hello World!, which it prints to the terminal including all the spaces. Note that the double quotes were not part of the argument itself, so it doesn’t appear in the printed line, they only served to tell bash to include the spaces as part of the input.

Exercise 2.20

How many leading spaces will be printed by each of the following commands? Before you test them, predict what you think the output will be.

$     echo Hello World!
$ echo "   Hello World!"
$ echo    "Hello World!"

Exercise 2.21

What do you think will happen if you just run echo without any arguments? Try it out.

Nothing happens
Immediately a new prompt appears
An empty line and then a new prompt appears
An error is shown, echo requires arguments

2.4.4.3. History#

A useful part of any shell is the history. The shell keeps a list of all lines you typed in since the shell was started. This has several uses:

You can later consult this log using the history command, which shows you what you previously entered in the prompt in the shell session.
You can easily repeat earlier commands, or even modify them before executing them again.

Exercise 2.26

Run echo hi and then cal, and then use history to see both commands listed.

{
  "checks": [
    { "type": "commandEvent", "eventType": "builtin.echo",
      "match": { "text": "hi" },
      "desc": "run echo hi" },
    { "type": "commandEvent", "eventType": "coreutil.cal",
      "desc": "run cal" },
    { "type": "stdoutContains", "pattern": "^history$",
      "text": "echo hi",
      "desc": "history should shows echo hi" },
    { "type": "stdoutContains", "pattern": "^history$",
      "text": "cal",
      "desc": "history should shows cal" }
  ]
}

The easiest way of using the history is moving through it using Up and Down. A helpful shortcut is Ctrl-R, which allows you to search in the history. Just start typing the beginning of a command, and it will complete to the last command that matches. Hitting Ctrl-R again cycles through all matches.

Exercise 2.27

Run echo hi and then cal, just to have some command history to look at. Now press Up twice to navigate back to echo hi, but before pressing ENTER change it to echo bye by erasing the last word with Backspace, then run it.

{
  "checks": [
    { "type": "commandEvent", "eventType": "builtin.echo",
      "match": { "text": "hi" },
      "desc": "run echo hi" },
    { "type": "commandEvent", "eventType": "coreutil.cal",
      "desc": "run cal" },
    { "type": "uiEvent", "eventType": "history-navigation",
      "match": { "direction": "up", "command": "echo hi" },
      "desc": "navigate Up to echo hi" },
    { "type": "commandEvent", "eventType": "builtin.echo",
      "match": { "text": "bye" },
      "desc": "change the line to echo bye" }
  ]
}

Finally, you can also repeat specific commands from your history by using the ! shell built-in command, like this:

!! will repeat the last line.
!13 will repeat line 13.
!-5 will repeat the line you entered 5 lines ago.
!mo will repeat the last command you entered that started with the string mo.

Exercise 2.28

Run echo hi and then cal 10 2010 to show the calendar from October 2010. Now use !1 to repeat the echo command from your history. Subsequently, run !cal and confirm this repeats the calendar command with the same date arguments.

{
  "checks": [
    { "type": "commandEvent", "eventType": "builtin.echo",
      "match": { "text": "hi" },
      "desc": "run echo hi" },
    { "type": "commandEvent", "eventType": "coreutil.cal",
      "match": { "month": 10, "year": 2010 },
      "desc": "show calendar of October 2010" },
    { "type": "commandEvent", "eventType": "history.expansion",
      "match": { "expanded": "echo hi" },
      "desc": "use !1 to repeat echo hi from the history" },
    { "type": "commandEvent", "eventType": "history.expansion",
      "match": { "expanded": "cal 10 2010" },
      "desc": "use !cal to repeat calendar of October 2010 from the history" }
  ]
}

Basic shell usage

Contents

2.4.4. Basic shell usage#

2.4.4.1. Hello terminal!#

2.4.4.2. Starting programs, with optional settings#

2.4.4.3. History#