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xargs: Passing argument lists to commands

From: Unix for Mac OS X Users

Video: xargs: Passing argument lists to commands

In this movie we'll learn to use xargs to pass argument lists to commands. xargs is kind of a funny name. It's short for execute as arguments. What xargs does is it parses an input stream into items and then it loops through each item in that list and passes it to a command. I think it's easier to understand if we actually see it in action. So you remember that we had the wc command before for word count. So if we do word count on our lorem_ipsum.txt file that's inside our Unix files directory, you see that it comes back and it tells us the number of lines, words, and characters that are in lorem_ipsum.

xargs: Passing argument lists to commands

In this movie we'll learn to use xargs to pass argument lists to commands. xargs is kind of a funny name. It's short for execute as arguments. What xargs does is it parses an input stream into items and then it loops through each item in that list and passes it to a command. I think it's easier to understand if we actually see it in action. So you remember that we had the wc command before for word count. So if we do word count on our lorem_ipsum.txt file that's inside our Unix files directory, you see that it comes back and it tells us the number of lines, words, and characters that are in lorem_ipsum.

Now, let's try a variation on this. Let's say if we want to echo lorem_ipsum .txt and we want to pipe that into wc. Now what it's doing is it's saying not the file lorem_ipsum.txt, but this string, this actual text lorem_ipsum.txt, how many characters does that have? There is 16. So that's what it's doing. It's not looking at the file. It's looking at the string. If we instead want to pass that string to wc as an argument, then we use xargs. That's what it does.

So now we get the exact same result as we did before. It's the exact same thing. It took the string and the input and it passed it to the command wc. The man pages refer to this as the utility commands. So it passes it to its utility command. xargs gives us a nice way to see what it's actually doing using the -t option. So now we can actually see the command right before it runs. It will output what its doing. So it's running wc lorem_ipsum.txt. Now, if we don't supply a utility command at all, let me just show you what it does.

It actually just echoes. it calls bin echo and then the file. So as a default setting, it will just echo that argument back to you. Now, that explains what xargs does. It passes its input as an argument to a utility command. But the power of xargs is in looping through a list of arguments. Let's imagine that instead of just this that now instead we are going to have lorem_ipsum.txt us_presidents.csv. And let's go ahead and put that again with the word count at the end. Now you can see that it passes both of these as arguments to word count.

So it's the same as if we had typed we lorem_ipsum.txt us_presidents.csv. But sometimes you don't want xargs to run its command with all of the arguments. What we want is for it to loop. And so we can do that by using the -n option to limit how many arguments it passes. So we'll just say 1 to start with. We can also take that space away to make it a little more clear that the 1 goes with the n, and now it will take one argument, pass it to wc, and then it will loop and it will take the next argument and pass it to wc. So n is the number of arguments that we use on each loop through.

So now that you see the difference between what it did before. Now it's actually calling wc two times, not once, but twice. And to see an example of this using higher numbers of n, let's say that we had echo 1 2 3 4, and let's pipe that into xargs, and then -n2, and we don't have to specify anything. We're just going to echo those back. Now you see what it does. 1 and 2 get passed in as an argument, then 3 and 4 get passed in as an argument. -t will make that really clear. It's passing in the first two, then it goes through again. A second loop and passes in the next two.

If there had been an uneven number, well, then that last one would just get passed in by itself. So for example, let's say that we have ls to list our directory. We can pipe that into xargs. That will be all of the file names in our directory, and we'll do -n 3 and echo, and now you see we get three file names and then we get a new line. We get three more file names, because it's just echoing each of those file names broken up. xargs also has a -L option which limits what it takes in. Let me show you that. If I just had echo 1 2 3 4 and I pipe that into xargs with the capital L 2 option, you don't really notice a difference, because it's all just one line.

But let's instead use a multiline document and it will become a lot clearer. Let's say we have head of lorem_ipsum.txt, pipe it into xargs, and we will use -L 2, and I won't specify the echo. That's the default. What we're seeing is the first two lines of the file echoing back to us. If I instead use -n, you'll see that we get the first two words of each line. Now, the n and L options are mutually exclusive, so you don't want to use both. And they can be a little bit tricky, because xargs can find its arguments by splitting on either spaces or on lines, and it depends a little bit on the format that the input was sent in and whether or not you've specified the n or L the option.

It can be a little tricky, so you may have to play with it a bit to get it to do exactly what you want. Let me show you another example. Let's say we have cat, our file fruit.txt, and we'll pipe that into xargs. So what xargs is going to do is just echo each one of those back in a line. But here is what I want to show you. We can actually instead of just saying we want to echo this, we can actually put a placeholder. We'll specify it with the capital I option, then we can use the two curly braces and then we can actually place this somewhere in position. So for example, I can have buy more: and then it will drop in the value where I've put this placeholder. See what it does? So now we have the ability to actually not just take that argument, but to take that argument and put it inside something, to put in a specific place rather than just forcing it to just use the argument by itself.

You don't have to use these curly braces. That's the most common one, but you can actually specify absolutely anything you want here. So I could have fruit. You just want to make it something that's unique. :FRUIT. Right? And it will drop it in there. So all you're doing is saying all right, with the -I option here is what I am going to declare as my placeholder and wherever you see that, that's where the argument ought to go. There is one last catch to using xargs. And that's that xargs, when it tries to figure out where the arguments are, it will split things up based on either spaces or control characters.

This can cause some problems. So for example, let's say I do ls of my Library directory. So inside Library, let's take a look at what's in there. Let's just grab the first few of those by piping this through grep. So in grep I'll look for A, followed by anything that comes after it, and so it will just show me those first four. Now, let's take that list and let's pipe that into xargs and let's just have it echo it back to us. That's all we're going to do. We'll use the n1 option, so it will break it. Each item that it finds, each argument, will get sent through to echo one time.

See what happened? Application Support got broken in half. Autosave Information got broken in half. It thought they were each different arguments, because it split it up on the space. So you see why this causes the problem. In order to get around this what we need do is the -0 option and that will tell it to split on null characters as separators instead of on spaces and new lines. So now when I hit Return, now it correctly does break them up. The main place that you are going to use that -0 option is with file names.

If you're working with file names, you probably want to use that -0 option to be safe. So it's a bit of a quirk, but it is something that you have to keep in mind as you're passing things to it is, is there a chance that there will be a space in the data that I'm passing in that would screw things up? That gives you a good explanation of what xargs does. Next, let's try some examples of how we can put it into action and make it work for us.

Show transcript

This video is part of

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Unix for Mac OS X Users

82 video lessons · 25424 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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