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Reading files

From: Unix for Mac OS X Users

Video: Reading files

Now we will learn how to read the contents of a file. Now you may be thinking, why not to use Nano to open and read the contents of a file? Well, you certainly can. In fact I do it all the time. But what we are interested in here are tools which are designed just for reading files. This difference will become important later because we'll learn that to read a file really just means to output the text to your screen and we'll learn to direct that output that places besides our screen and even to other commands. A tool like nano which depends on user interaction would do too much for us and it would get in the way.

Reading files

Now we will learn how to read the contents of a file. Now you may be thinking, why not to use Nano to open and read the contents of a file? Well, you certainly can. In fact I do it all the time. But what we are interested in here are tools which are designed just for reading files. This difference will become important later because we'll learn that to read a file really just means to output the text to your screen and we'll learn to direct that output that places besides our screen and even to other commands. A tool like nano which depends on user interaction would do too much for us and it would get in the way.

Now, we have a couple of options for reading files. The first is cat. cat is the simplest and the one that you would use most often. It has kind of a funny name, because cat is short for concatenate. You see, cat doesn't just read a file. It will concatenate or join several files together. You can think of it as read and output the first file, then immediately read and output the second file, and then the next file and so on. That's its classic and intended use. But if we give cat only one file as an argument, well then it just reads that one file and quits.

There's no second file to ever go on to. So it has a bit of a quirky name, because its name implies using more than one file, while we probably are going to use cat more often on just single files. But we can't use it for both cases. But there's one big problem with using cat when we want to review a long file on the screen. With cat all the output is red and displayed at once. That leaves you scrolling up through the output to find the start of your very long file and then scrolling down again as you read. So, for this case when we have a long file, we have another command that's better called more.

more gives you paginated output. It outputs one page of text and then waits for you to hit the spacebar to move to the next page. Much easier but more also had a problem. more doesn't allow you to go backwards to the pages. You could move through the pages forward, but if you want it to go back, well, then you had to exit out of it and start again from the beginning. That wasn't very convenient, especially if you just wanted to go back one page for just a quick second before you moved on again. So as an improvement on more, we have less. Now, obviously the name less is a play off of more and less offers the ability to scroll backwards.

That also has a little bit memory use. Where more would load the entire document memory, less will just load one page into memory at a time. It's a bit of a Unix joke that since less was an improvement on more, that you can say less is greater than more. Now, on the Mac you can actually say that less is more, because more has been completely replaced by less. You can still type more, but the command that actually will execute is less. The man pages use less. So we have already seen them in action and we have gotten a bit familiar with it. We can use F to go forward, B to go backward, and Q to quit and return to the command prompt.

Let's try them out. So in Terminal, I am in my user directory, and I'll just show you that I have got a couple of new files in there. The first one is called lorem_ipsum.txt and with that is a very long file filled with vague Latin text. Then I also have this file short_file.txt, which is also including the exercise files but it's a very simple file. You couldn't come up with any short file. I just wanted to have something to contrast against the very long lorem_ipsum. So let's start trying these out. So I've got cat and then I can say cat short_file.txt. There you see it.

It just output the contents of the file. Now, as I said, what cat is really for is for concatenating together. So if we had cat short_file.txt and then space followed by newfile.txt. That's just a simple one line file we created in the last movie. There they are. It puts those together. So we've got now the short_file followed by the newfile, one immediately after the other. So that's what concatenate is really designed for. Even though most of the time, you'll probably use it in the first usage just with a single file name. So let's try.

I am going to clear the screen using Command+K. Just cat that lorem_ipsum. Here we go. Ready? Hit Return. Boom! It output all of this text and now look at this scrolled back. Right, I would have to scroll all the way up here and find what I was looking for, not really that useful. So instead what you would want to use is more or less. So on the Mac less is more, so we don't need to bother playing with more at all. We'll just go straight to less and let's try out with lorem_ipsum again and I am going to clear the screen just so we have that old stuff out of there again. And there we are.

Now look at the bottom. It's waiting. It shows me just one page. I am going to hit Space. Now I get the next page. Hit space again. I get the next page. And as I said, F and B allow us to go forward and backwards. So we can toggle between those and then when we are finally done we can hit Q to get out of it. There's one other really useful command in there I want to show you, which is that you can G to go to the beginning or the end. G by itself goes to start of the document. Uppercase G goes to the end of the document, so Shift+G. So let's try it. Let's do Shift+G and we jump to the end.

You could see it says END down there, and I do just a regular g, lowercase g. It pops you back up to the top of the document. So that's a good and useful to know. There are also a couple of nice options that you can add in. You can do less with a capital M for lorem_ ipsum and it gives you a little bit better prompt. So now you can see at the bottom it actually tells me which lines I'm viewing and how far I am through the document. I get a percentage. There is another one. Let's go back here and do n. That shows line numbers. So it numbers all those lines for me and there's lots more options that you can use with less. The man pages for it are very long.

So you can definitely go in there and see all the different things that you can do with it, but that's will get you started with it.

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This video is part of

Image for Unix for Mac OS X Users
Unix for Mac OS X Users

82 video lessons · 25433 viewers

Kevin Skoglund
Author

 
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  1. 3m 57s
    1. Introduction
      1m 14s
    2. Using the exercise files
      2m 43s
  2. 32m 2s
    1. What is Unix?
      7m 27s
    2. The terminal application
      4m 23s
    3. Logging in and using the command prompt
      5m 19s
    4. Command structure
      5m 22s
    5. Kernel and shells
      5m 25s
    6. Unix manual pages
      4m 6s
  3. 15m 58s
    1. The working directory
      2m 49s
    2. Listing files and directories
      3m 59s
    3. Moving around the filesystem
      4m 58s
    4. Filesystem organization
      4m 12s
  4. 1h 4m
    1. Naming files
      5m 41s
    2. Creating files
      2m 19s
    3. Unix text editors
      6m 39s
    4. Reading files
      5m 35s
    5. Reading portions of files
      3m 27s
    6. Creating directories
      2m 40s
    7. Moving and renaming files and directories
      8m 32s
    8. Copying files and directories
      3m 7s
    9. Deleting files and directories
      3m 38s
    10. Finder aliases in Unix
      4m 10s
    11. Hard links
      5m 30s
    12. Symbolic links
      6m 36s
    13. Searching for files and directories
      6m 32s
  5. 34m 58s
    1. Who am I?
      4m 3s
    2. Unix groups
      1m 52s
    3. File and directory ownership
      6m 41s
    4. File and directory permissions
      4m 27s
    5. Setting permissions using alpha notation
      6m 49s
    6. Setting permissions using octal notation
      3m 49s
    7. The root user
      1m 57s
    8. sudo and sudoers
      5m 20s
  6. 52m 34s
    1. Command basics
      4m 4s
    2. The PATH variable
      4m 13s
    3. System information commands
      3m 40s
    4. Disk information commands
      6m 8s
    5. Viewing processes
      5m 0s
    6. Monitoring processes
      3m 36s
    7. Stopping processes
      3m 19s
    8. Text file helpers
      6m 50s
    9. Utility programs
      7m 28s
    10. Using the command history
      8m 16s
  7. 20m 39s
    1. Standard input and standard output
      1m 24s
    2. Directing output to a file
      4m 13s
    3. Appending to a file
      2m 44s
    4. Directing input from a file
      5m 28s
    5. Piping output to input
      4m 40s
    6. Suppressing output
      2m 10s
  8. 41m 28s
    1. Profile, login, and resource files
      9m 11s
    2. Setting command aliases
      6m 59s
    3. Setting and exporting environment variables
      4m 54s
    4. Setting the PATH variable
      6m 10s
    5. Configuring history with variables
      6m 17s
    6. Customizing the command prompt
      6m 5s
    7. Logout file
      1m 52s
  9. 1h 25m
    1. grep: Searching for matching expressions
      5m 21s
    2. grep: Multiple files, other input
      4m 28s
    3. grep: Coloring matched text
      2m 57s
    4. Introduction to regular expressions
      3m 22s
    5. Regular expressions: Basic syntax
      3m 19s
    6. Using regular expressions with grep
      5m 20s
    7. tr: Translating characters
      8m 17s
    8. tr: Deleting and squeezing characters
      5m 30s
    9. sed: Stream editor
      7m 45s
    10. sed: Regular expressions and back-references
      7m 8s
    11. cut: Cutting select text portions
      7m 42s
    12. diff: Comparing files
      4m 35s
    13. diff: Alternative formats
      4m 30s
    14. xargs: Passing argument lists to commands
      7m 25s
    15. xargs: Usage examples
      7m 59s
  10. 42m 25s
    1. Finder integration
      4m 45s
    2. Clipboard integration
      5m 5s
    3. Screen capture
      3m 42s
    4. Shut down, reboot, and sleep
      3m 34s
    5. Text to speech
      2m 36s
    6. Spotlight integration: Searching metadata
      3m 41s
    7. Spotlight integration: Metadata attributes
      4m 24s
    8. Using AppleScript
      5m 23s
    9. System configurations: Viewing and setting
      5m 51s
    10. System configurations: Examples
      3m 24s
  11. 1m 26s
    1. Conclusion
      1m 26s

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