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LPI Linux LPIC-1 101 and CompTIA Linux+

Search Text Files with Regular Expressions

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LPIC1 & CompTIA Linux+ Intro

Hardware Settings

Boot the System

Change Runlevels and Reboot or Shutdown the System

Design Hard Disk Layout

Install a Boot Manager

Manage Shared Libraries

Debian Package Management

RPM & YUM Package Management

Work on the Command Line

Process Text Streams Using Filters

Perform Basic File Management

Use Streams, Pipes, and Redirects

Create, Monitor, and Kill Processes

Process Priorities

Search Text Files with Regular Expressions

00:00:00 - And welcome to Nugget 103.7. Now this
00:00:05 - time we're going to learn a few things. We're actually going to use
00:00:07 - the tool that you've been using all along.
00:00:11 - We're going to learn to use grep. Now you've had on the job training for the
00:00:14 - past few nuggets all the way through this session, but we're
00:00:17 - going to learn about it officially. We're also going to learn
00:00:20 - about fgrep and egrep,
00:00:24 - which are obviously related to grep but separate tools in and
00:00:28 - of themselves. And then the majority of this section is going
00:00:33 - to be about the mighty regex.
00:00:38 - Now regular expressions or regex, as it's often called, are
00:00:42 - very, very useful ways to sort through text and pick and
00:00:48 - find and sort, analyze and -- regex is awesome stuff. It's
00:00:52 - also kind of confusing, so we're going to take time. At first we're just
00:00:56 - going to learn some simple regular expression tools.
00:01:00 - But I want you to really pay attention this nugget. It has a weight
00:01:03 - of two, but it will be invaluable to you as a Linux user,
00:01:08 - all right. So let's get down to business. Let's get grep,
00:01:10 - fgrep, and egrep -- we'll add one more to that, too,
00:01:15 - sed. We'll get those out of the way and then we'll spend the
00:01:17 - bulk of the time on regular expressions. So let's get started.
00:01:22 - Okay, first I want to start with the playing field. See, we're
00:01:25 - back in our handy dandy little terminal here, and let's
00:01:29 - see. I've made two files, so we'll look at file.txt and
00:01:34 - we'll look at file2.txt. And these are just some text
00:01:39 - files that I added a bunch of words to, see. So file two has these in it,
00:01:43 - file has these in it. All right, just so you know that that's what's
00:01:47 - in there and that's when I start working with grep and such.
00:01:50 - We're going to be manipulating those files --
00:01:53 - and apparently my speech. So let's clear the screen and we'll
00:01:57 - start with grep. Now there's two ways that you can invoke
00:02:00 - grep. What you can do is you can say grep and then what it
00:02:04 - is you're looking for. So we know, if you remember those, there
00:02:08 - was a couple words with two Os in it. So we'll grep oo from -- not
00:02:13 - fire -- from file.txt, all right. So basically we start with
00:02:17 - the grep, then what it is we're grepping for and where we're grepping
00:02:21 - from. And what we got out of there were these four words.
00:02:26 - It prints the whole line and it shows what was in, you know, what
00:02:29 - lines contained oo. Now you can do some things like
00:02:33 - grep -n to show line numbers.
00:02:37 - oo from file.txt, and there it tells you what line they're in.
00:02:41 - If you're working with code, sometimes that's really convenient.
00:02:44 - All right, but right there and you can also, let's see. Now grep
00:02:49 - is case sensitive. So if we were going to look for
00:02:55 - b-o-o from file.txt, it's not going to find the capital
00:03:00 - books. See up here that has a capital B. But grep has a flag for
00:03:04 - that, too -- it's I, which means case insensitive.
00:03:08 - Search for boo,
00:03:10 - file.txt, and see there it found both of these because it didn't
00:03:14 - care about what case it was, if it was upper case or lower case, all right. So that's
00:03:17 - how grep works. But if you remember, in the past few nuggets we've
00:03:21 - been using grep a different way. What we've been doing is piping
00:03:24 - the cont -- or the results of a command into grep. So
00:03:29 - let's say we were to type ls. You'd get these two files, right.
00:03:32 - If you type ls and then you use the pipe symbol
00:03:37 - into the command,
00:03:39 - grep for i-l-e all right. What we should get is both of those as
00:03:45 - a result. So instead of using a file
00:03:49 - for it to grep for boo or for whatever here, we've used the results
00:03:54 - of ls, which we know that those are the results of ls,
00:03:58 - right, file2 and file, and then we've used pipe to make that the input
00:04:03 - for grep. We learned about this in a previous nugget, but this is
00:04:06 - a way that you often use grep, all right. And then we grepped for i-l-e
00:04:11 - out of the results and we got both of them because both file names
00:04:14 - include i-l-e, all right. So you'll see me use these interchangeably. You know,
00:04:19 - we could also do the same thing that we did up here, if we were
00:04:22 - to say, cat file.txt. See, if you do that, you know that's
00:04:26 - what it contains. So if we did cat file.txt, piped it into grep oo,
00:04:32 - we would get the same results, see. Just as if we invoked
00:04:36 - it saying, use this file; it was the result of using cat on that
00:04:40 - file and then it grepped the results from there. That's a really
00:04:43 - commonly way that you use grep and I just wanted to show you that before
00:04:47 - we get too deep, so if I start doing things like using pipe it's not
00:04:50 - confusing. All right, so grep is one that we're familiar with and I want
00:04:54 - to show you some of the regular expressions that you can use
00:04:57 - with grep.
00:04:59 - There are some special characters that you can use inside
00:05:03 - in grep and they're called regular expression characters.
00:05:07 - Just, they're special characters. For example, let's say that we
00:05:11 - wanted to find all of the
00:05:16 - words that -- oh, let's see. In this, here's a good example. This
00:05:21 - is our file.txt, see these words. Now let's search
00:05:26 - for all the words that have p-l-e in them, all right. So we're going to grep
00:05:33 - p-l-e from file.txt. Now what do you think we'll
00:05:36 - find? We should have two results, right. Purple, p-l-e there; and
00:05:41 - plenty, p-l-e there. But what if we just wanted to search for
00:05:45 - words that started with p-l-e? Well that's possible, that's
00:05:49 - where regular expressions come into play. You would type something like grep
00:05:53 - and then the carrot symbol, shift six on the regular U.S. Keyboard.
00:05:58 - That signifies the beginning of a line. And then
00:06:03 - p-l-e file.txt.
00:06:06 - And it only showed plenty, not purple, because this is a regular
00:06:11 - expression that says, only at the beginning of a line. So it has to
00:06:14 - be the beginning of a line and then p-l-e, and it only found that one time,
00:06:18 - in the word plenty, all right. Now there is the same thing for end
00:06:22 - of the, end of the line. So our end of the line, grep,
00:06:28 - let's say
00:06:30 - s and end of the line, the dollar sign, okay.
00:06:37 - File.txt. Now what are we going to find? Well it found three of the words
00:06:42 - ended in s, see. So it found blades, books, and Books. And if we
00:06:47 - look, let's test it. Let's see how well it did.
00:06:52 - The whole file, there should only three that end in s. And sure
00:06:54 - enough, just these three -- blades, books, and Books all end in s.
00:06:59 - So s and then the end of the line character, all right. So that's what that dollar
00:07:04 - sign means. It's a regular expression showing end.
00:07:08 - All right, does that make sense?
00:07:11 - Now the period means any character. It can be any character.
00:07:15 - It has to be a character, but it can be any character. So let's say we want
00:07:19 - to look for any character and o from file.txt.
00:07:26 - Okay, it found all these because all these had something and then a O.
00:07:30 - Let's see if we can figure out something.
00:07:35 - How about
00:07:37 - grep any character p-l-e from file.txt. What
00:07:43 - do you think is going to happen?
00:07:46 - It found purple, but it didn't find plenty. And why didn't it find plenty?
00:07:50 - Well because if you look, there's no character before the p-l-e
00:07:53 - in plenty, right. This means any character, but it has to be
00:07:57 - a character.
00:07:59 - Now to get much more complicated with grepping, you're going
00:08:02 - to have to use a different tool, because grep doesn't understand
00:08:06 - all of the regular expressions that you can create. For that
00:08:09 - you need egrep, which is extended grep. Now you could also use
00:08:14 - grep -E, but since you use extended grep
00:08:18 - quite a bit, it's easier just to use egrep as a command all on its own.
00:08:23 - And the only difference, now it'll do all the things that grep will
00:08:25 - do. See, if we do grep.ple file.txt, it's going
00:08:30 - to show us the same result -- purple. What egrep will allow you to
00:08:34 - do, though, is string different regular expressions together
00:08:37 - and use some things like or and and and things like that. So let's do
00:08:42 - something kind of complicated, all right. Let's type egrep, and now
00:08:48 - this is going to be a regular expression with some spaces in between
00:08:52 - or you know some complicated things, so we're going to put it in
00:08:54 - quotes. Just it's good to put it in quotes or single quotes
00:08:58 - if you're afraid that it's going to be the name of a variable.
00:09:01 - In fact, let's do single quotes. That way, after we
00:09:04 - use the name of a variable, it won't be substituted. It'll just
00:09:07 - use literally what we put in there. So we're going to search for
00:09:11 - something that begins with either b
00:09:18 - or d. Now I'll go through this in a second. B or d, close the single
00:09:24 - quotes, in file.txt. So what we have here, we put
00:09:29 - it in single quotes just because it was an expression, it's just
00:09:33 - what we're searching for.
00:09:36 - The carrot symbol means the beginning of a line
00:09:39 - and then inside these parentheses is where we're giving a choice.
00:09:43 - So it's going to be b, and then the pipe symbol inside a
00:09:47 - regular expression means or, d; close parentheses. So it's going
00:09:52 - to evaluate that. So it's going to have to begin the line with
00:09:55 - either b or d. File.txt is what we're searching in. So we hit
00:10:00 - Enter, and it's going to show us all of the words that started
00:10:04 - with either b or d. See, so that's kind of, kind of nice. But what
00:10:10 - if we didn't want blades? So then we can string things together.
00:10:15 - Egrep, type the same sort of thing because we want it all the
00:10:19 - words that begin with
00:10:21 - b or d,
00:10:25 - and then we're going to say, and then after that have
00:10:34 - o-o. Single quotes,
00:10:38 - file.txt. And so now it found all of the letters that
00:10:44 - start -- or all of the words in -- or all the lines that start with b or
00:10:48 - d, and then immediately have o-o after them. See how we've
00:10:53 - strung those regular expressions together. Now it gets kind of complicated.
00:10:57 - You could also do a range. So instead of b or d, let's say
00:11:03 - that we want --
00:11:07 - you can actually use grep for a range. I believe that it's safe to
00:11:11 - use egrep.
00:11:13 - And we're going to search for all the words that start -- now
00:11:18 - to do a range, you're going to open a bracket here, okay, the
00:11:21 - square brackets. So all of them that start with anywhere from
00:11:26 - a to k.
00:11:30 - Oh, but what did I forget. I want it to start, the line to start
00:11:34 - with that. Any letters a through k
00:11:38 - in file.txt. So what do we have here. Again, single quotes,
00:11:42 - we just put it inside single quotes, and we're going to search for
00:11:46 - any word, the line that begins with any range between a and k.
00:11:51 - So a, b, c, d, e, f, g, h, i, j, k in file.txt. And what do
00:11:56 - we get. We got blade, book, books, kitten, doogy, and fast with the
00:12:02 - dollar sign in there, okay. So all of those, are all those in between a and
00:12:07 - k? Yes. Now what is missing?
00:12:10 - Well if you, let's
00:12:12 - look. Cat file.txt.
00:12:14 - Now it didn't find this Books because it's a capital. We
00:12:20 - just did the lower case range, all right.
00:12:24 - Let's get super crazy. Let's start nesting. This will be the last
00:12:28 - one we do on egrep here because we're going to get crazy.
00:12:31 - Now there are other, other regular expressions but as long as
00:12:34 - you can string together some craziness here, you should be,
00:12:36 - you should be good. Let's do egrep,
00:12:40 - open with the single parentheses here, something that begins with
00:12:45 - either
00:12:49 - a through k or
00:12:54 - A through K.
00:12:58 - See what I did? See the difference there?
00:13:01 - Single quote. File.txt.
00:13:04 - So now what did it do? It did the same thing it did last time,
00:13:10 - except it also found this because we said -- again, now this is really
00:13:15 - complicated, but we said what begins with and it would probably --
00:13:22 - we could also --
00:13:25 - if you like this better, the way it, if it makes it more clear.
00:13:31 - You could do it that way and it does the same thing. So again, for
00:13:35 - everybody trying to keep track here, it's saying the begins
00:13:39 - with, so the line begins with, and then match up these parentheses,
00:13:43 - everything inside here that is either a through k lower case.
00:13:48 - The pipe symbol means or, A through K upper case. And then we
00:13:54 - told it file.txt. So see how we keep stringing these things
00:13:57 - together with egrep. We have a lot more, extended grep
00:14:00 - gives us a lot more things like ranges and the or symbol and that
00:14:04 - sort of thing. So that's how you string regular expressions
00:14:07 - together. And then lastly,
00:14:10 - we have fgrep. Now fgrep is much different.
00:14:15 - Fgrep is fast grep, it's often called fast grep, and the
00:14:19 - reason is because let's say we wanted to search for --
00:14:23 - do you remember way back in the beginning we searched for the words
00:14:26 - that end
00:14:29 - in -- where's my dollar sign --
00:14:32 - end in s file.txt. Actually here, let me, just to refresh your
00:14:37 - memory here. So if we do this, it's going to show us all the
00:14:40 - words that end in s. Because remember the dollar sign is
00:14:43 - the regular expression symbol for the end of the line, right.
00:14:47 - But if we did that same thing with fgrep,
00:14:52 - s and dollar sign file.txt, it's not going to find anything.
00:14:58 - Because fast grep doesn't recognize regular expressions
00:15:02 - at all. Where fgrep is really useful is where you don't want regular
00:15:06 - expressions to be evaluated. For example, let's say we're trying
00:15:10 - to search for f-a-s
00:15:14 - in file.txt. Again, we're trying to find this
00:15:18 - line, right, that's what we're trying to do. Well if we use regular grep, it's
00:15:22 - not going to show anything. Because remember, it's evaluating
00:15:24 - this as, find me any line that has f-a as the last line or
00:15:30 - as the last character in the line, and that's not what we're
00:15:33 - looking for. If you use fgrep,
00:15:36 - f-a
00:15:39 - dollar sign file.txt, it's going to find it because
00:15:43 - it ignores all that stuff. So fgrep is useful if you're doing a situation
00:15:47 - that it might be interpreted as regular expressions and
00:15:50 - you don't want it to. Alright, so that rounds up the
00:15:54 - grep, egrep, and fgrep. So grep is the regular tool that you use. Egrep
00:15:59 - is extended grep, which provides a lot more functionality like the or
00:16:02 - symbol and all these regular expressions that aren't supported
00:16:05 - regularly. And then fgrep is kind of the opposite. It's fast grep.
00:16:09 - It just searches for this literal string that you type in on
00:16:12 - the line right there. And those are the three greps. So really
00:16:16 - quickly here, remember grep is the one that uses some
00:16:22 - regular expressions. Fgrep
00:16:24 - only interprets literal strings that you supply to it.
00:16:32 - And egrep is extended
00:16:36 - regex, or regular expressions. So egrep is the one that you
00:16:40 - can do all that fancy stuff in. And again, egrep does the same thing
00:16:43 - that grep does. So if you aren't sure if grep supports the
00:16:47 - regular expressions that you're trying to use, you can use
00:16:49 - egrep. That's not going to harm you if you're using regular expressions.
00:16:52 - This just supports more than grep does out of the box. All right, so those
00:16:56 - are the three greps. Just try to remember what the difference is
00:17:00 - between them. It's easy for me to remember F as in fast
00:17:03 - and E as in extended, and then grep is just regular grep.
00:17:08 - All right. So while grep is normally used or is always used to search
00:17:13 - through files for specific things using regular expressions
00:17:16 - or not, sed or stream editor is the tool that you use for editing
00:17:21 - things on the command line. Now this seemingly simple tool can
00:17:24 - be very, very powerful. We're just going to introduce it here so you can do
00:17:28 - some simple manipulation with it. And basically you type
00:17:31 - sed, and then -e is to tell it you're going to edit
00:17:35 - something. And then in single quotes, you give it the command
00:17:39 - that you're going to do or your action. It starts with the action.
00:17:42 - We're going to substitute, so s means substitute, then
00:17:46 - a forward slash, and then you pick what you're going to substitute.
00:17:50 - So let's say we want to search for all occurrences of o-o. Now
00:17:54 - this can be regular expressions, too. So we'll start
00:17:58 - with simple text. So we're going to search for o-o, and another forward slash,
00:18:02 - and we're going to replace it with zero zero, okay. And then
00:18:08 - we close that
00:18:10 - and the single quotation to close it. And then what file are we going to
00:18:16 - work on. Well, we're going to work on our trusty file.txt,
00:18:20 - all right. So what this is going to do, if we hit Enter it actually
00:18:23 - puts it onto the screen here. It doesn't actually change the file.
00:18:26 - If we wanted to create a file, remember redirection. We would
00:18:29 - have added
00:18:31 - the greater than newfile.txt and it would put the results in
00:18:35 - a file. But I actually wanted to show you what it does here.
00:18:38 - So it's taken all of our two Os and changed it to two zeros,
00:18:43 - see, just like we told it to do in the file, which is really,
00:18:46 - really cool. That's exactly what it did. So it took -- we
00:18:50 - substituted o-o for zero zero. If you want to do anything a little
00:18:55 - more creative with sed that you want to use extended regular
00:19:00 - expressions like we did with egrep, well there's
00:19:03 - a command for that, too. Basically, you need to type sed minus
00:19:07 - r for regular expressions and edit, and then you can start
00:19:11 - doing all kinds of crazy things. Like we want to substitute
00:19:16 - anything that
00:19:20 - begins with either B
00:19:24 - or b.
00:19:27 - So it begins with either B or b.
00:19:31 - We want to substitute that with
00:19:35 - capital C, in file.txt. So now what is this going to do? Well what
00:19:41 - it did, it took all of our words that started with either
00:19:45 - B or b, again upper case or lower case B, and replaced it with
00:19:49 - an upper case C. And sure enough, we have clades, Cooks, cooks and cook.
00:19:54 - So you can use regular expressions, even complicated ones,
00:19:57 - right inside sed, the stream editor. But you have to remember
00:20:01 - to use that R flag if you're going to use any complicated
00:20:05 - regular expressions. A few simple ones, just like with grep,
00:20:08 - a few simple ones you can use without that flag. But regular
00:20:11 - expressions, extended regular expressions, you have to use that
00:20:14 - R flag along with the E flag, alright. So that's how you can actually
00:20:17 - change the file. Rather than just finding it, you can make changes
00:20:21 - by using this substitute command in using sed as your stream
00:20:26 - editor. All right, so we've gone over grep; fgrep; egrep; sed, the
00:20:31 - stream editor; and now you're ready to go over the regular expressions,
00:20:35 - the big scary part that I told you about. Well it turns out, pretty much
00:20:39 - already did. I tricked you into it there while we were going over
00:20:42 - the other things. We covered what regular expressions are. I
00:20:45 - mean, this is quite a set of regular expressions right here.
00:20:49 - And you know what's going on here. We're substituting this
00:20:54 - regular expression for this regular expression, and this actually
00:20:58 - just a, you know, we're substituting it for that and close it. So you've done,
00:21:02 - you've done regular expressions throughout this entire nugget.
00:21:05 - So the only thing that I might say is man regex, just
00:21:10 - to give you some more examples of some things that you
00:21:15 - can do. And it gets a lot more complicated than you'll need
00:21:17 - for this nugget, but it's one of those things if you can learn
00:21:21 - regular expressions, you will just be a guru that everybody will
00:21:24 - ask you to write these really complicated forums -- or not forums, formulas --
00:21:29 - for them to do because not many people really understand
00:21:32 - regular expressions that well. But after this nugget, you're
00:21:36 - one of the few that at least understand it somewhat.
00:21:39 - So you've done it. You learned all about grep; the fast
00:21:43 - grep that just does literal searches; egrep for extended regular
00:21:48 - expressions searching; and sed, the stream editor, that
00:21:52 - is a lot more powerful than it seems by the little command line
00:21:56 - editing that we just did. But sed, the stream editor, is amazingly
00:22:00 - powerful when you're doing scripting. And throughout all of that,
00:22:03 - you learned the big and powerful regular expressions, at least quite
00:22:08 - an introduction to it. Again, I don't think anybody can ever
00:22:12 - know everything there is to know about regular expressions,
00:22:15 - but it's a great tool and you should learn as much as you can
00:22:18 - about it. I hope that this has been informative for you, and I'd
00:22:22 - like to thank you for viewing..

Perform Basic File Editing Operations Using vi

Create Partitions and Filesystems

Maintain the Integrity of Filesystems

Control Mounting and Unmounting of Filesystems

Manage Disk Quotas

Manage File Permissions and Ownership

Create and Change Hard and Symbolic Links

Find System Files and Place Files in the Correct Location

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Shawn Powers

Shawn Powers

CBT Nuggets Trainer

Certifications:
LPIC-1; CompTIA Linux+, A+; Cisco CCNA

Area Of Expertise:
Linux

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