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Unix for Mac OS X Users
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Piping output to input


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Unix for Mac OS X Users

with Kevin Skoglund

Video: Piping output to input

Up until now we've been seeing how to direct input and output, but we've been doing it to files and from files. So we have to have these files as sort of the intermediary step between them. What would be great as we could actually take the output from a command and use that as the input into another command, without having to go to a file in between. It's something that you are going to use all the time in Unix. In fact you'll probably use it more often that you will either of those file redirects. To do it, we use something that we called the pipe. I'm just going to type one just so you see what it looks like.
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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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Unix for Mac OS X Users
6h 35m Beginner Apr 29, 2011

Viewers: in countries Watching now:

Unix for Mac OS X Users unlocks the powerful capabilities of Unix that underlie Mac OS X, teaching how to use command-line syntax to perform common tasks such as file management, data entry, and text manipulation. The course teaches Unix from the ground up, starting with the basics of the command line and graduating to powerful, advanced tools like grep, sed, and xargs. The course shows how to enter commands in Terminal to create, move, copy, and delete files and folders; change file ownership and permissions; view and stop command and application processes; find and edit data within files; and use command-line shortcuts to speed up workflow. Exercise files accompany the course.

Topics include:
  • Moving around the file system
  • Creating and reading files
  • Copying, moving, renaming, and deleting files and directories
  • Creating hard links and symbolic links
  • Understanding user identity, file ownership, and sudo
  • Setting file permissions with alpha and octal notation
  • Changing the PATH variable
  • Using the command history
  • Directing input and output
  • Configuring the Unix working environment
  • Searching and replacing using grep and regular expressions
  • Manipulating text with tr, sed, and cut
  • Integrating with the Finder, Spotlight, and AppleScript
Subjects:
Developer Web
Software:
Mac OS X Unix
Author:
Kevin Skoglund

Piping output to input

Up until now we've been seeing how to direct input and output, but we've been doing it to files and from files. So we have to have these files as sort of the intermediary step between them. What would be great as we could actually take the output from a command and use that as the input into another command, without having to go to a file in between. It's something that you are going to use all the time in Unix. In fact you'll probably use it more often that you will either of those file redirects. To do it, we use something that we called the pipe. I'm just going to type one just so you see what it looks like.

We hold down the Shift key and then the Backslash. That's the key that is above the Return key. You get this upright pipe. So that's what I refer to whenever you hear me say pipe and typically we say we pipe something into something, right? We use it as a verb as well. So if we have something like echo "Hello World", we know that would normally output to our Terminal, but if instead we use the pipe, we can redirect that into another Unix command. Don't use a file here. A file is used with those other operators.

Here we can use another command. Let's just do wc as a simple one. So Word Count comes up and it reports to me there is one line, two words, 12 characters. You see what it did? So it took the output from one command, piped it into the other one, so that it received it as input and then that's what it uses. Let's take another example, let's say echo, let's put in our mathematical expression we used before, (3*4) + (11*37), and let's pipe that into bc, right. Before, remember I saved it to a file and then I took that file as input to bc.

Now I'm just saying all right, take this directly and put it into bc, right. It gives me back a result. I piped this output from echo into bc. If we take our fruit file, remember I'm inside my unix_files directory, right here, ls -la. I have this file fruit.txt that we've worked with a few times. Let's cat that fruit file. It's just a list of fruit in an unsorted order and let's pipe that into sort. Notice before I was taking the file and sending it to sort. Now I'm actually outputting it, right. This is generating output and that output is what's being sit in their, okay. Ot's a subtle difference, but it is different and you see we get back this result.

Let's not take that and let's pipe that through unique. So now I get the unique version. Now we've seen a couple of different ways that we can do this, that we can sort them and make them unique. Sort has an option that does it. We can take it from sort into a file and then a file back into unique. I wanted you to see what we're doing here. We're piping it along from one command to another, so we get a string of commands. The first one is catting the file, then we're sorting it, then we're doing unique and at the end of all that, we could actually output it. Let's say unique_sorted_fruit.txt. I already have this file, so it's going to overwrite it. There it goes.

So now all of that went through all those commands and at the end of it then I directed the output to a file. See how that works? It's really, really useful. You could also do something similar to what we did before where we could say well, sort should get its input from fruit.txt, all right. We know what that's going to do, right. That does that and then we could take that and we could pipe that into unique. We know what that does, right, and then we could take that and we can send that out to unique_sorted_fruit, but that's a little less readable. If you just compare the two, I think it's a lot easier to understand what's happening here.

We're reading the file, sending it to the sort, sending it to unique, then it ends up in the file. The other one, you have to jump back and forth to read a little bit. So, both of them accomplish the exact same thing. I just think that the second one is perhaps a little more readable. Another place where this is really useful is we have a long file like our lorem-ipsum file, right. It's very long. We can pipe it into less. There we go. Now we are in less. We can go page by page. Now of course that's the same thing. We could do less lorem-ipsum.txt and that gives us the same results. This takes a file as its argument, but for something like ps aux, now that doesn't take a file as an argument.

That's the processor output that we saw. So we are seeing a list of all our processes piped through less. Guess what? Now we get pagination. We can go through them page by page and we can use forward and back to look at those. All right, there is a very common usage of it is to take something that we're working with and pipe it into less. We will be using this type a lot especially when we get to the chapter on the Unix Power Tools where we look at a lot of the advanced features. We will be able to use this pipe to great effect. So the main thing to remember with pipe though is that unlike the other operators, we're piping it into a command, not into a file, into a command and the output from it should be the output from a command, not from a file.

Even here this is actually output of a command, not a file.

Find answers to the most frequently asked questions about Unix for Mac OS X Users.


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Q: The exercise files for the following movies appear to be broken:
07_02_files
07_03_files
07_04_files
07_05_files
08_03_files

Is there something wrong with them?
These exercises include one or more "dot files", whose file names start with a period. These files are normally hidden from view by the Finder.  So that they would show up in the Finder, the period has been removed from the file names. Additionally, "_example" has been added at the end of the file name to make it clear that the file will not work as-is. To make the dot files usable, either:

1) Open the file in a text editor to view its contents. Note that it may not be possible to double-click the file to open it because there is no file extension (such as .txt).
2) Resave the file under a new name (usually by choosing File > Save As), adding a "." to the beginning of the file name and removing "_example" from the end.

OR

1) Copy and rename the file from the Unix command line using the techniques discussed in this course. Rename the file by adding a "." to the start and removing "_example" from the end. Include the "-i" option to prevent overwriting an existing file unexpectedly.
Example:  cp -i ~/Desktop/Exercise\ Files/Chapter_07/07_02_files/bashrc_example ~/.bashrc
 
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