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The PATH variable

From: Unix for Mac OS X Users

Video: The PATH variable

In the last movie, we saw how commands and programs are really just files that are being executed, and we saw that we type echo 'Hello world' that what it's actually doing is executing a file that is located in bin/echo. I write the argument. Those are doing exact same thing. This is just a shortcut for finding this file and executing it. So how does Unix do that? How does Unix know to find echo in this location or to put it another way, if I create another file named echo, how does Unix know not to use that one instead, or what if I wanted it to use that one instead? How would I go about doing that? Well, the way that Unix manages all of this is with a variable called PATH.

The PATH variable

In the last movie, we saw how commands and programs are really just files that are being executed, and we saw that we type echo 'Hello world' that what it's actually doing is executing a file that is located in bin/echo. I write the argument. Those are doing exact same thing. This is just a shortcut for finding this file and executing it. So how does Unix do that? How does Unix know to find echo in this location or to put it another way, if I create another file named echo, how does Unix know not to use that one instead, or what if I wanted it to use that one instead? How would I go about doing that? Well, the way that Unix manages all of this is with a variable called PATH.

$PATH will show you the current value of your path and the path is always a colon-separated list of file paths and this is the list that Unix will use when trying to locate commands to execute. The early entries take precedence over the later ones. So what it does is it starts at the left. When we type echo, it says, "Oh! I'll look in user bin. Do I find echo there? No. All right, I'll look in bin. Do I find echo there?" Well, in this case, it does and so it runs it. If it didn't though, then it would look in usr/sbin, sbin, usr/local/bin, and finally user/X11/bin.

If it didn't find it in any of those places, then it would say "Oh, the command is not found." You can see that behavior. If you just type some junk on the screen and hit Return, it comes up and says command not found. What it actually did was checked all of those directories in order and then said, "Nope, it wasn't there. It must not be a command that's available to me." Now, you can change this path in bash. We do that with just typing PATH, no dollar sign in front of it, equals, and then the path that we want it to be, Different shells choose a slightly different technique for setting the path. I am just going to show you the bash way.

Let's just grab this string here and I'll use Command+C and then Command+V to paste it and that will give me everything except that very last path. So I am just leaving off that last path and the colon that's before it. Now, if we go up and do echo $PATH, you can see my path has changed and I've omitted that last directory. What that means is that any commands or programs that were in that last directory, Unix will no longer look there. It won't be able to find them. I have essentially taken them out or uninstalled them. We can do the same thing by adding our own directories. If we have other places we want it to check, we can add that in there.

Frequently, when we do web development, we want to add a MySQL path so it'll look inside our MySQL folder to find all those MySQL commands. That's a very common use of this. Let me show you an extreme example. If we set our path equal to just simply usr/ local/bin and let's echo our path, that's it! It's just going to check this one place. We could use echo, we could try that as an example, but echo is such a fundamental and that it's actually really built into the system. The system will find it anyway. As a different example, let me use ls.

That's another pretty fundamental one but if we try ls now, it says oops. Command not found. It didn't find the ls command anymore. Now, you could change your path back and just paste that back in, or I want to show you that if you close the window and you reopen it now, now if we say ls, we get our directory listing. That's because setting the path in the way that we did only lasts for the current session. It doesn't stick around. In Chapter 7, we are going to talk about configuring your environment and you'll be able to set a path there that will stick. So if you want to know how to do that, Chapter 7 will cover how to make a path last.

But I want to make sure that I show you here is the concept of the path so that you will understand how it's translating these commands into executing the files that may be located in different locations. There is a last thing. Let me also just mention that the command that we learned in the last movie, which is the which command, so if we say which ls, it tells us which version of ls we are using. What that means is there may be several versions here, but I'm using your PATH variable and I am telling you which one I'm going to give you. So that's important. which is slightly different than whereis because which uses our path to say this is the one that's active.

So it's a really good command to know. So for example, maybe we have several different versions of Ruby installed. Well then we could say which ruby, and it will come back and tell us the version of Ruby that we will get if we were to type a command that started with Ruby. Okay, now that we have those fundamental concepts of how commands and programs work, we are ready to actually start diving in and learning some useful commands.

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This video is part of

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Unix for Mac OS X Users

82 video lessons · 25437 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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