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grep: Searching for matching expressions

From: Unix for Mac OS X Users

Video: grep: Searching for matching expressions

In this chapter we're going to take a look at some of the most useful and powerful Unix commands. Up until now we've really been covering the fundamentals and getting familiar with Unix, but now we are at the point where we can start doing some serious work, and the most powerful tool in the Unix toolbox by far is grep. Grep is a powerful way for us to search for text which matches patterns that we specify, and we aren't just talking about searching for simple text strings. We can define complex matching patterns by using regular expressions, and regular expression are actually part of how grep gets its name.

grep: Searching for matching expressions

In this chapter we're going to take a look at some of the most useful and powerful Unix commands. Up until now we've really been covering the fundamentals and getting familiar with Unix, but now we are at the point where we can start doing some serious work, and the most powerful tool in the Unix toolbox by far is grep. Grep is a powerful way for us to search for text which matches patterns that we specify, and we aren't just talking about searching for simple text strings. We can define complex matching patterns by using regular expressions, and regular expression are actually part of how grep gets its name.

Grep stands for Global Regular Expression Print. If you think back to the movie where we talked about Unix text editors, I told you that the earliest Unix text editor was called ed or Ed, and if you were working inside the ed text editor and you wanted to search for something, the way you did it was by typing a g, then a forward slash and then a regular expression that described what you were going to search for, another forward slash and then p, for print. And G stood for global, meaning globally search for it, and p for print meant to output the results, and the shorthand that described that pattern, global regular expression print, become shortened to just be rep and it is these regular expressions often just called regex or regexp for short that give grep its power.

Grep and regular expressions have been around since the earliest days of Unix. You might have guessed that since it was available in this early text editor, ed, but they were actually created for Unix. By now though, grep and regular expression was spread to most programming languages, into many text editors as well, so they're useful in a lot of context. It's really worth trying to learn how they work. We'll focus on regular expressions in the next movie. First, let's get familiar with grep by using simple text strings. So to begin with, notice that I'm in my user directory and I'll just change into my Unix files directory and let's take a look at what's in there.

File that we're going to start out working with is fruits.txt. That's just a simple list of fruit that we created earlier and what we're going to do be doing is search inside this file for an expression. We're going to start with just searching by a simple text string to begin with. And what you also might say is that you're going to grep that file. It is perfectly acceptable to use grep as a verb. So we're going to grep this file for text string. The way that we will do that is with grep, space, then the expression that we want to search for, we're going to search for Apple.

Now this is still an expression, even though it's the simplest expression possible. It's the literal characters, a-p-p-l-e, but it is still a regular expression that it's passing in. So grep for Apple inside the file fruit.txt. That's all there it is to it. We hit Return and it comes back and what it returns to us are the results. The results that we're seeing are the three lines that contain a match. Not just the three words, the three lines. What I mean by that is in our case each of our lines only had one word on it, but if that first line had been Tom likes apples and the second line was Mary likes pineapples, then it would return the entire line, not just the single word apple.

It would have returned the line that had a match, so don't be confused by that. We're actually seeing the entire line that gets returned. It's also a case-sensitive match. So for example, if we search for apple with a capital A, well, then it comes back and it says no matches, because it checked for case-sensitive search and looked for apple exactly as we specified it and it wasn't there. If we wanted to be insensitive, we can pass an option for that, I. That's a case insensitive search and now it doesn't care. It doesn't care if I have upper or lower case a in my expression and it doesn't care whether it's upper or lowercase a in the results.

It will still match. I is probably the single most common option that you would use with grep, but there are a number of others. You can use the man pages to look through all of them. Let me show you a few of the more useful ones. Notice for example that it matched both Apple and pineapple. If we were to use the -w option, that will match only on whole words. I'm going to take away the capital A to make sure that that still matches. Now notice it didn't find the pineapple. It only found the two whole word matches apple and apple and it's smart about how it finds words. It uses spaces and punctuation and line returns to know where the word breaks are.

Now we also could write a more complex regular expression that would do the same thing and we'll that little bit later, but this is sort of the quick and easy way. Another nice one is the V option. So with the V, we'll get our lines that don't match. I is the reverse, the inversion of our regular search. So notice now apple and pineapple are missing from the list. We've essentially filtered them out. So all lines that do not match is what we're looking for. We could do the in option. In option is nice because it gives us line numbers. So it numbers the line, so it says, all right, I found a match in line 5 and here's the line.

I found the match in line 6 and here's the line. A match in line 13 and here's the one. Now you may not actually be interested in seeing those results. Maybe what you're interested in is actually just counting them and you can do that with the C option. grep c, it comes back and says ah, I found it three times. An example of a place that where that might be more useful, if we do grep inside lorem_ipsum, let's search for lore. So it's our long fake Latin text. It comes back and it says, ah, the word lorem occurs in there 44 times.

Now notice also how fast that was. It's really, really fast about being able to search in there looking for this text. That's part of the power of Unix, is being able to do these things and do them in a really, really fast way.

Show transcript

This video is part of

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

82 video lessons · 26917 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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