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A couple of other handy commands that you can use to view text files are head and tail. As their names suggest, these let you quickly view the beginning (head) of a file or the end (tail) of it. Using the commands is simple: tail mytextfile or head mytextfile By default, both commands will display ten lines of the file. You can override this by using the -n command option followed by the number of lines you want to see. For example, the following will show the last five lines of mytextfile: tail -n5 mytextfile These two commands are very useful when viewing log files that might contain hundreds of lines of text. The most recent information is always at the end, so tail can be used to see what s happened last on your system, as shown in the example in Figure 15-1. Although they re powerful, all of these shell commands don t let you do much more than view text files. If you want to edit files, you ll need to use a text editor such as vi.
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CHAPTER 15 WORKING WITH TEXT FILES
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Figure 15-1. The tail command can be very useful for viewing the last few lines of a log file.
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STANDARD INPUT AND OUTPUT
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If you ve read any of the Ubuntu man pages, you might have seen references to standard input and standard output. Like many things in Ubuntu, this sounds complicated but is merely a long-winded way of referring to something that is relatively simple. Standard input is simply the device that Ubuntu normally takes input from. In other words, on the majority of desktop PCs when you re using the command-line shell, standard input refers to the keyboard. However, it s important to note that it could also feasibly refer to the mouse or any other device on your system capable of providing input; even some software can take the role of providing standard input. Standard output is similar. It refers to the device to which output from a command is usually sent. In the majority of cases at the command line, this refers to the monitor screen, although it could feasibly be any kind of output device, such as your PC s sound card and speakers. The man page for the cat command says that it will concatenate files and print on the standard output. In other words, for the majority of desktop Ubuntu installations, it will combine (concatenate) any number of files together and print the results on screen. If you specify just one file, it will display that single file on your screen. In addition to hardware devices, input can also come from a file containing commands, and output can also be sent to a file instead of the screen, or even sent directly to another command. This is just one reason why the command-line shell is so flexible and powerful.
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CHAPTER 15 WORKING WITH TEXT FILES
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Using a Command-Line Text Editor
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A variety of text editors can be used within the shell, but three stand out as being ubiquitous: ed, vi, and Emacs. The first in that list, ed, is by far the simplest. That doesn t necessarily mean that it s simple to use or lacks powerful features, but it just doesn t match the astonishing power of both vi and Emacs. To call vi and Emacs simple text editors is to do them a disservice, because both are extremely powerful interactive environments. In particular, Emacs is considered practically an operating system in itself, and some users of Linux treat it as their shell, executing commands and performing everyday tasks, such as reading and sending e-mail from within it. There are entire books written solely about Emacs and vi.
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Tip A fourth shell-based text editor found on many Linux systems is nano. This offers many word
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processor-like features that can be helpful if you ve come to Linux from a Windows background.
The downside of all the power within Emacs and vi is that both packages can be difficult to learn to use. They re considered idiosyncratic by even their most ardent fans. Both involve the user learning certain unfamiliar concepts, as well as keyboard shortcuts and commands. Although there are debates about which text editor is better and which is best, it s generally agreed that vi offers substantial text-editing power but isn t too all-encompassing. It s also installed by default on Ubuntu. On Ubuntu, Emacs must be installed as an optional extra. Both text editors are normally available on virtually every installation of Linux or Unix. We ll concentrate on using vi here. It s important to understand that there isn t just one program called vi. There are many versions. The original vi program, supplied with Unix, is rarely used nowadays. The most common version of vi is a clone called vim, for vi improved, and this is the version supplied with Ubuntu. However, there are other versions, such as Elvis. Most work in a virtually identical way.
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