Editing Files

Overview

Teaching: 5 min
Exercises: 5 min

Questions

• How can I edit files?

Objectives

• Learn how to edit files in Linux/Unix.

Editing files

Let’s pick up where we left off, in our data-shell directory and use ls -F to see what it contains:

$ ls -F

creatures/  data/  molecules/  north-pacific-gyre/  notes.txt  pizza.cfg  solar.pdf  thesis/  writing/

Last time, we created the thesis directory, but there is nothing in it yet:

$ ls -F thesis

Create a text file

Let’s change our working directory to thesis using cd, then run a text editor called Nano to create a file called draft.txt:

$ cd thesis
$ nano draft.txt

Which Editor?

When we say, ‘nano is a text editor’ we really do mean ‘text’: it can only work with plain character data, not tables, images, or any other human-friendly media. We use it in examples because it is one of the least complex text editors. However, because of this trait, it may not be powerful enough or flexible enough for the work you need to do.

On Linux/Unix systems (such including the Unix underlying Mac OS), many programmers use Emacs or Vim (both of which require a bit more time to learn), or a graphical editor such as Gedit.

No matter what editor you use, it will search within and save files to your current working directory as its default location.

Let’s type in a few lines of text. Once we’re happy with our text, in nano we can press Ctrl+O (press the Ctrl or Control key and, while holding it down, press the O key) to write our data to disk (we’ll be asked what file we want to save this to: press Return to accept the suggested default of draft.txt).

Once our file is saved, we can use Ctrl+X to quit the editor and return to the shell.

The Control, Ctrl, or ^ Key

The Control key is often labeled the Ctrl key on the keyboard. There are various ways in which using the Control key may be described. For example, you may see an instruction to press the Ctrl key and, while holding it down, press the X key, described as any of:

• Control-X
• Control+X
• Ctrl-X
• Ctrl+X
• ^X
• C-x

In nano, along the bottom of the screen you’ll see ^G Get Help ^O WriteOut. This means that you can use Control-G to get help and Control-O to save your file.

nano doesn’t leave any output on the screen after it exits, but ls now shows that we have created a file called draft.txt:

$ ls

draft.txt

Creating Files in Different Ways

We have seen how to create text files using the nano (or ay other) editor, and the touch command.

Programs and scripts can also create files. Most data files are created in this way, including probably all of the data files you will use in this class. More about that later.

Finally, let’s see one more way to create a file containing content.

$ echo "Testing" > test.txt

1. What did the echo command do (try again but just type echo "Testing")?

Solution

1. The echo command literally echoes the text that follows. When nothing else is specified, the text is sent to stdout i.e., “standard output” - the terminal window.

2. The symbol > redirects the output, in this case to a new file we named test.txt

3. You can inspect the contents of the file test.txt by using an editor. Alternatively, you can print the contents of the file to the terminal window using the cat command: cat test.txt.

4. Note - only use cat in this way if the file is small and contains only text characters (no special characters, binary data, etc.). Otherwise, you may be in for some surprises, or at least a long wait!

Let’s try something else. Type this at the command line:

$ printf "Testing again\nHere is another line.\n" >> test.txt

1. What happened this time?

Solution

1. The printf command is for “formatted print” and works very much like the same command in C or Python (among other languages).

2. The symbol >> appends the output to the end of the file test.txt. If we had used >, the file would have been overwritten.

3. Within the quoted text, the \n is called an escape sequence. The backslash \ indicates the following character should be interpreted as having a special meaning: \n means a new line. To print the backslash itself in a formatted print, use \\.

If your terminal gets “messed up”…

If you accidentally cat a binary file, not only may your screen fill with gibberish, but your session many get “messed up” (e.g., the text won’t clear from your screen, your cursor disappears, scrolling behaves incorrectly, or the formatting may have changed). This happens because some of the binary, when dumped to stdout, may have special meanings that change formatting or other terminal behavior.

You can always clear your screen with the command clear. This command clears your visible window, but also all prior screen information, i.e., you will not be able to scroll up and see previous content.

If there are more serious problems with your terminal session, uss the reset command. This will almost always restore your session to working order.

What’s In A Name?

You may have noticed that all of the files we are working with are named ‘something dot something’, and in this part of the lesson, we always used the extension .txt. This is just a convention: we can call a file mythesis or almost anything else we want. However, people use two-part names most of the time to help them (and their programs) tell different kinds of files apart. The second part of such a name is called the filename extension, and indicates what type of data the file holds: .txt signals a plain text file, .pdf indicates a PDF document, .cfg is a configuration file full of parameters for some program or other, .png is a PNG image fime, and so on.

This is just a convention, albeit an important one. Files contain bytes: it’s up to us and our programs to interpret those bytes according to the rules for plain text files, PDF documents, configuration files, images, and so on.

Naming a PNG image of a whale as whale.mp3 doesn’t somehow magically turn it into a recording of whalesong, though it might cause the operating system to try to open it with a music player when someone double-clicks it.

Another very handy command is file. This command will tell you the format and likely purpose of the file:

$ file test.txt

The file command can be used with wildcards to list the types of multiple files

Key Points

• Depending on the type of work you do, you may need a more powerful text editor than Nano.

• Most files’ names are something.extension. The extension isn’t required, and doesn’t guarantee anything, but is normally used to indicate the type of data in the file.