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Directing input from a file

From: Unix for Mac OS X Users

Video: Directing input from a file

Now that we've seen how to change the standard output to go to a file, I'd like to see how to direct input from a file that is instead of using our standard input which would be our keyboard, the things that we type into the command line, we are going to use input from a file. Notice that I'm inside my User directory and I'm inside unix_files. These files are in the Exercise Files as well. The one I'm going to be using to start with is this fruit file. You will remember what's in there. It's just a list of fruit, all right, a bunch of unordered fruit with duplicates in there as well.

Directing input from a file

Now that we've seen how to change the standard output to go to a file, I'd like to see how to direct input from a file that is instead of using our standard input which would be our keyboard, the things that we type into the command line, we are going to use input from a file. Notice that I'm inside my User directory and I'm inside unix_files. These files are in the Exercise Files as well. The one I'm going to be using to start with is this fruit file. You will remember what's in there. It's just a list of fruit, all right, a bunch of unordered fruit with duplicates in there as well.

So what I'm going to do is I'm going to use the sort and we saw that we could do fruit.txt. Essentially what this is doing when we call that is providing as argument fruit.txt which the sort function then uses as its standard input. So it's already doing what we're talking about for us, but I want to show you the way that we can do it all the time, sort of universally for any command that takes input, even if the input is just from the keyboard. We can then use this less than sign. That's the opposite of what we used for output. It's facing the other direction, pointing to the left.

What we are saying is take this fruit.txt file, use that as your input, and send it to the sort command. Use it as if we had actually typed those commands in there. And you will see that it gives us back through the results. Now those results are still coming to our screen. We are not redirecting those right now. We are focused just on directing the input. Let's try another one, wc < lorem_ipsum.txt. So there you see. Now it's using the contents of that and passing it into wc to count it. Notice that it doesn't show the file name anymore, which it did before when we called it the other way, right.

When we call it this way, it actually uses that file name. So this is providing the file as an argument. This is providing that file as input that's being passed to this command. It is a subtle difference. Let me show you one way that maybe a little more obvious. Let's say that we echo, a mathematical expression, (3*4) + (11*37). Okay, so I've just got a simple expression and I'm going to output that to a file. I'll call it calculation.txt. We know how to redirect our output.

Now I'm going to use bc, the calculator that we used in the last chapter. Before we used it from the command line, but instead I'm going to say hey, instead of using what I type, instead take the contents of this file and pass those into bc, and bc happily takes those and returns a result to us. So I think it's a pretty straightforward thing, but there are two important points that I want to make sure you understand. One is that we can both redirect the input and the output. So let's say for example, if I do sort < fruit.txt, the output right now is going to come to my screen.

If instead I want it to go somewhere else, well then I need to direct it as well, sorted_fruit.txt. Okay, see what it's going to do? The input is the fruit.txt; the output is the sorted_fruit.txt. It's the same thing as if we actually had sort and we piped in fruit.txt) > sorted_fruit.txt. All right, that works exactly the same. It has the exact same results. The parentheses aren't necessary though because we always put the input first.

Right, it's always the input, then the output. So I just wanted to see that you can use them together and also that the input always needs to come before the output. I also want to make another point here that the arguments to these always have to be files. Okay, it's very important. We're working with files when we talk about this. You can't for example do sort < cat fruit.txt, right. That's going to read the file? No. We want the file itself, not what we get when we read the file piped into it. It says no such file or directory is cat.

It's expecting a file or a directory to be passed in there. Same thing if we say lorem_ipsum.txt piped into wc as output, right. This won't work either because in this case it expects it to have output and the file on its own doesn't have any output. We have to cat the file to have output and then we need to pipe it into a file. The receiver has to be a file. If I just do wc, not word count, that's actually a file called wc and it happily does it. ls -la and there we see we have a file called wc at the bottom.

Okay, so I just want to make sure that you understand the thing going in using the less than sign needs to be a file and the thing that's coming out of the greater than sign needs to also be a file. All right, so let's go ahead and remove that. The last thing, let's just show you if we use unique case an example. Let's take our sorted_fruit. txt now and let's output it to unique_sorted_fruit.txt. Okay, so that works and it should have sorted it. cat unique_sorted_fruit.txt. So we went through several steps to get that sorted_fruit doing it this way, right.

And we know that there is an option for sort that would let us do all in one step and make it unique. Certainly could use that. But what I am interested in really making point about is the way that we sort of use these files as intermediaries, right. We took a file, we sorted it, we saved it to a file. Then we take that sorted file, we passed that into unique, and we saved that off as another file. It would be great if instead we could take the output from one command and send it straight to another command without a file, right. We can't do that with these operators because these operators are expecting this and this to both be files.

Instead we need another tool to do that. We call that the pipe. That's what we will look at in the next movie.

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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 · 28606 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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