2.2 Using the Shell
When you install Linux, you should create at least one regular user to be your personal account. For this chapter, you should log in as the regular user.
2.2.1 The Shell Window
After logging in, open a shell window (often referred to as a terminal). The easiest way to do so from a GUI like Gnome or KDE is to open a terminal application, which starts a shell inside a new window. Once you’ve opened a shell, it should display a prompt at the top that usually ends with a dollar sign ($). On Ubuntu, that prompt should look like name@host:path$, and on Fedora, it’s [name@host path]$, where name is your username, host is the name of your machine, and path is your current working directory (see Section 2.4.1). If you’re familiar with Windows, the shell window will look something like a DOS command prompt; in macOS the Terminal application is essentially the same as a Linux shell window.
This book contains many commands that you will type at a shell prompt. They all begin with a single $ to denote the shell prompt. For example, type this command (just the part in bold, not the $) and press ENTER:
$ echo Hello there.
Many shell commands in this book start with #. You should run these commands as the superuser (root), so they require extra caution. The best practice when running them is to use sudo in order to provide some protection and a log that you can look up later for possible errors. You’ll see how to do this in Section 2.20.
Now enter this command:
$ cat /etc/passwd
This command displays the contents of the /etc/passwd system information file and then returns your shell prompt. Don’t worry about what this file does right now; you’ll learn all about it in Chapter 7.
Commands usually begin with a program to run and may be followed by arguments that tell the program what to operate on and how to do so. Here, the program is cat, and there is one argument, /etc/passwd. Many arguments are options that modify the default behavior of a program and typically begin with a dash (-). You’ll see this shortly in the discussion of ls. There are some exceptions that don’t follow this normal command structure, however, such as shell built-ins and the temporary use of environment variables.
2.2.2 cat
The cat program is one of the easiest in Unix to understand; it simply outputs the contents of one or more files or another source of input. The general syntax of a cat command is as follows:
$ cat file1 file2 ...
When you run this command, cat prints the contents of file1, file2, and any other files that you specify as arguments (denoted by ... in the preceding example), and then exits. The program is called cat because it performs concatenation when it prints the contents of more than one file. There are many ways to run cat; let’s use it to explore Unix I/O.
2.2.3 Standard Input and Standard Output
Unix processes use I/O streams to read and write data. Processes read data from input streams and write data to output streams. Streams are very flexible. For example, the source of an input stream can be a file, a device, a terminal window, or even the output stream from another process.
To see an input stream at work, enter cat (with no arguments) and press ENTER. This time, you won’t get any immediate output, and you won’t get your shell prompt back because cat is still running. Now type anything and press ENTER at the end of each line. When used like this, the cat command repeats any line that you type. Once you’re sufficiently bored, press CTRL-D on an empty line to terminate cat and return to the shell prompt.
The reason cat adopts an interactive behavior here has to do with streams. When you don’t specify an input filename, cat reads from the standard input stream provided by the Linux kernel rather than a stream connected to a file. In this case, the standard input is connected to the terminal where you run cat.
Pressing CTRL-D on an empty line stops the current standard input entry from the terminal with an EOF (end-of-file) message (and often terminates a program). Don’t confuse this with CTRL-C, which usually terminates a program regardless of its input or output.
Standard output is similar. The kernel gives each process a standard output stream where it can write its output. The cat command always writes its output to the standard output. When you ran cat in the terminal, the standard output was connected to that terminal, so that’s where you saw the output.
Standard input and output are often abbreviated as stdin and stdout. Many commands operate as cat does; if you don’t specify an input file, the command reads from stdin. Output is a little different. Some programs (like cat) send output only to stdout, but others have the option to send output directly to files.
There is a third standard I/O stream, called standard error. You’ll see it in Section 2.14.1.
One of the best features of standard streams is that you can easily manipulate them to read and write to places other than the terminal, as you’ll learn in Section 2.14. In particular, you’ll learn how to connect streams to files and other processes.