UnixWorld Online: Tutorial: Article No. 009: Part 3

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The vi/ex Editor, Part 3: The global Command

By Walter Alan Zintz.

Questions regarding this article should be directed to the author at walter@ccnet.com.

The Wondrous global Command

If you're surprised that I made no mention of global in the preceding installment of this tutorial--well, global is not an address. It's actually a line-mode command, and it's much more powerful than most users suspect.

Even experienced users of line mode usually think of global along these lines: ``If you type global and then a search pattern and then a line-mode command, all on one line, then the editor finds every line in the file that contains that pattern and runs the command on every one of those lines''. That is, typing:

global /^Chapter [1-9]/ delete

is expected to find and delete every line in the file that starts with a chapter heading. This example will do just that, and so will many other such uses of the command. But spectacular failures will happen from time to time--typing:

global /^Chapter [1-9]/ write >> t.of.contents

definitely will not append each of the marked lines to a file named ``t.of.contents'', as moderately-experienced users might expect. (It's likely to overflow your file system quota instead.)

The Details of global Operations

More important, misunderstanding the global command keeps users from exploiting more than a small fraction of that command's power. But you don't have to live with the limitations of ignorance on this--here's the full story in plain terms:

Searching where you tell it to look:

As a line-mode command, global can be preceded by an address or pair of addresses. Its default is to search the entire file, but if you start your command as 257 , 382 global then it will only search through lines 257 through 382 inclusive. Any line-mode addresses can begin a global command, so starting with ?^Exercises? +++ , $ global will restrict the pattern search and line marking to a stretch beginning three lines past the last previous line that starts with the string ``Exercises'', and ending at the end of the file.

Marking either hits or misses:

Typing the command name as global or g will definitely cause it to mark every line in the search area that contains the pattern. But typing it as global! or g! or v reverses the procedure--now it will only mark lines that do not contain the search string. So if you are editing a copy of a log file of error messages, and only the lines that begin with ``Error 3b:'' are of interest, you can eliminate all the unwanted lines by typing:

   global! /^Error 3b:/ delete
Choose your own search pattern delimiter:

Since this command always searches the file (or the section of it that you select) from top to bottom, you can use almost any punctuation character to mark the start and end of your search pattern. There's no need to use ? or / characters to indicate a direction for the search. If you want to eliminate lines that contain three consecutive slash marks, any of:

   global +///+ delete
   global ;///; delete
   global ]///] delete

will be a simpler choice than using slashes as delimiters and backslashing all three of the slashes you are searching for. (However, using ! as you delimiter is dangerous, because global is likely to mistake your delimiter for the switch that tells it to find only lines that do not contain the search pattern.)

Of course this applies only to the search pattern that goes right after the global command name, the one that says which lines to mark. If you use any search patterns before the command name, to say which area of the file is to be searched, then use ? and / delimiters as usual.

Global searches that seem senseless can be very useful:

At times it's wise to have global or global! run a search over just one line in a file. This is the basis for conditional execution of line-mode commands. As a simple example, you may find yourself editing files from outside your organization that are sometimes (but not always) sent to you with an extra, empty last line, as a spacer. You need to remove that last line, if and only if it is empty. You could go the end of each file and look, but it's easier to have the editor do the checking and (where necessary) the deletion, so you type:

   $ global /^$/ delete

It can also be useful to have global mark every line in the area of the file you tell it to search! Our put-upon programmer, Hal (in the first installment of this tutorial) used this when he had to reverse the order of the lines in one file. His command line, which would look like this if typed out in unabbreviated form:

   global /^/ move 0

begins by marking each line that has a start-of-line point, which makes every line qualify. Next it goes to the first line and moves it up right after the fictitious line zero--a no-op, of course. But then it moves the second line to the same place, pushing the former first line down one position in the file. As it does the same with the third line, the fourth line, etcetera, it's changing the order of the lines to the exact opposite of the order they were in at the start.

One global can run many commands:

You can put several commands on the line after a global command and its search pattern. After marking the appropriate lines, global will then go to each marked line and run all of the commands you've given it, in the order you gave them. Just separate these commands with a vertical bar (``|'') character. If you type:

   global /^CHAPTER/ substitute /APTER/apter/ | copy $

the editor will go to each line that starts with a chapter heading, change ``CHAPTER'' to ``Chapter'', and then copy the line (now beginning ``Chapter'' instead of ``CHAPTER'') to the end of the file. The order in which you put those two commands is important -- the substitute command must come first so the subsequent copy command will copy the decapitalized version of the line, not the original all-caps version.

You're not limited to just two commands in a global command line; there is no maximum on the number of commands there. The maximum string length for the command list varies with the editor version you're using, but I've never encountered a limit of less then 256 characters. There are a few restrictions on what the command list can contain, though:

Commands don't have to run on the lines global marks.

Using global is essentially the same as moving to each marked line manually, then typing in the command string while you are there. Just as you no longer expect every command you type in to operate on the line you are on when you type it, you don't have to have the commands in a global string operate entirely on the marked lines. Here are three points to note regarding this:

  1. Any command in a global command line can take its own address or addresses, just as it could if it were typed in as a separate command. So this command string:

          global /^XX/ - copy $ | /ZZ$/ , +5 delete

    is entirely legitimate. It goes to each line that begins with two capital X's, then copies the line just before that one to the end of the file, and finally goes forward to the next line that ends with two capital Z's and deletes that line and the five lines that follow it.

  2. Even if you give no addresses for the commands in a global string, default addresses for those commands may make them operate on other than the marked line. That's the fault in that global command string in the introduction to this installment of my tutorial that tries to write individual lines to another file. Because the default address for the write command is the entire file, this command will write the entire file the user is editing to the end of the other file, once for every line that global has marked. The correct way to write individual lines is to type:

          global /^Chapter [1-9]/ . write >> t.of.contents

    where the dot address in front of the write command tells it to write only the line it is on.

  3. But even if you take a command that has the current line as its default address, and put it in the string following global without giving it an address of its own, it can still operate on different lines from the ones globalhas marked if it is not the first command in the string. The reason: each subsequent command in a global takes as the current line whatever line the command before it left as the current line.

    In my earlier example about wanting to both change the capitalization of lines beginning with ``CHAPTER'' and copy those lines to the end of the file, the task was easy because the lines were to be copied in their changed state. But what if the user wanted only the lines in the midst of the file decapitalized, while the ones copied to the end of the file were to remain all-caps? It might seem obvious to simply reverse the order of the two commands, so the copy command was executed first, before the substitute command was called to change the capitalization, like this:

          global /^CHAPTER/ copy $ | substitute /APTER/apter/

    Surprisingly, that would produce the opposite of the effect that was intended. That is, it would decapitalize the copied lines at the end of the file, but leave the marked lines in the midst of the file all-caps. The reason? The copy command leaves the last line of the copy text block, not the original text block, as the current line. So after the copy command has run, the substitute command, using the command's default address (the current line) because it has not been given an explicit address, would operate on the copy line rather than the original.

    But there is one thing that no amount of current-line shifting can change. Wherever in the file the command string may leave the current line, when the commands have finished running, global will go to the next marked line without fail. The only way any of the commands in the string can prevent this is by deleting the next marked line -- in that case, global will merely go on to the next marked line that has not been deleted. And even this fact has uses that might not be obvious.

    Let's say you want to thin out the lines in a file, by deleting every second line. You can do it by typing:

          global /^/ + delete

    This global starts off by marking every line. When it goes to line 1, the command it executes will delete line 2. The next undeleted marked line is line 3, where its command deletes line 4, and so on. Or if you want to delete two-thirds of the lines in your file, type:

          global /^/ + , ++ delete

A Few More Uses for global Commands

The examples above are designed to show not only the working principles of the global command, but also some of the less-obvious tricks it can do. But I couldn't fit every important trick in above. Here are some more that are well worth knowing.

Keeping Count. At times it's a good idea to follow global with a string of commands that have absolutely nothing to do with the lines that global has marked. The most common occasion for this comes when you need to repeat a line-mode command a certain number of times.

At tradeshows I'm often invited to test a system right there on the show floor. I can't carry a 10,000-line test file along with me in every media and format any system might require, so I type in a ten-line file on the spot and expand it by telling the editor ten times to make a copy of the entire file and put that copy at the end of the present file. (Each such copy doubles the file's size, so I wind up with 10,240 lines.)

But that requires accurate counting. If I'm off by even one on the number of times I type that command in, I get a half-size or double-size file that ruins my test results. Instead of trying to count without an error, I let the editor do the counting for me. After I've typed in the initial ten lines, I give one global command:

global /^/ % copy $

This tells the editor to search the ten lines of the file, mark every line that has a start-of-line (which means every line, of course), and then go to each of those ten lines and run the subsequent command to make a whole-file copy. This guarantees that the command will run exactly ten times.

Not that this trick is limited to files that have exactly as many lines as the number of times I want to command to be repeated. If I had typed in a twenty-line file, I could copy it ten times by giving my global as:

1 , 10 global /^/ % copy $

Moving Around Automatically. At times you may need to handle a series of editing problems in a file, where the edits must be dealt with one by one, not with a global editing script. But moving to each spot where work needs to be done can be a very tedious business. If there is a text pattern that identifies each place that needs editing, or if you can write a script to insert such a pattern, as Hal did at the start of this tutorial's first installment, then global can move you around automatically.

You may recall that Hal used a script to mark up the legacy source code, putting each lint warning at the end of the source line to which it referred, preceded by "XXX" to make the affected lines identifiable. Suppose that the nefarious Vice President for Information Systems comes back to Hal to demand that each warning be investigated, to see whether the code can be rewritten to eliminate the warning.

Should Hal just leaf through the code, searching for XXX patterns to guide him to the trouble spots? Hal knows that with the spaghetti code he's facing, the actual problem may be a long way from the line lint has designated. In traveling to the actual trouble spot he may have passed several XXX patterns along the way, so searching for the next XXX in the file may bring him to a site he's already dealt with, or may miss a number of XXX sites that he passed when he moved forward to get to the actual problem spot on the previous fix. Besides, because he frequently does pattern searching while fixing a problem, he can't depend on a visual-mode n command to use the XXX pattern he needs to find; he must type the pattern in afresh each time.

But Hal knows a way around all this--dropping back to line mode (by typing a capital letter Q from visual mode) and giving a simple global command:

global /XXX/ visual | write

This command brings Hal to the first "XXX" line, where it puts him into visual mode to do his editing. When the edit is finished, Hal simply types a capital letter Q and the editor takes him to the second "XXX" line and puts him into visual mode there, no matter how much moving around Hal did during the first edit, and so on through the list of "XXX" lines. As frosting on the cake, the write command automatically writes the changed file to disk after each individual edit.

Now You Give it a Try

Here are a few exercises you can try to solve, before you start using advanced global tactics in your own editing. To keep things rolling I've provided at least one solution to each exercise, and also a hint on the last (and toughest) problem.

Copy and Decapitalize. Let's think back to the user who wanted to find each line in the file that begins with "CHAPTER", then copy each such line to the end of the file just as it is, and finally change the start of each original line (in mid-file) from "CHAPTER" to "Chapter" while leaving the copied lines (at the end of the file) beginning "CHAPTER".

We've already learned that this cannot be done with either of:

global /^CHAPTER/ substitute /APTER/apter/ | copy $
global /^CHAPTER/ copy $ | substitute /APTER/apter/

What global command (or commands) would it take to do what's desired here? Finding a solution to this is not difficult when there are so many workable ones.

A Precise String Length. An old friend who does some pretty tricky work with troff often needs to insert a string of backslashes in a line--up to 64 of them in a row. The count of backslashes must be exactly right or troff will choke. How can he get these strings exactly right without tedious counting and checking?

Let's say he needs to put 16 backslashes in line 217, right before the string "n(PDu". What command(s) should he use to get them in there without hand counting. My solution is pretty plain once you know which commands to use.

Numbering Paragraphs. A documentation writer has divided each section of a document into paragraphs, and as a troff user, marks the start of each paragraph by a line that contains the macro ".pp" only. That is, a break between paragraphs looks like this:

which is the only way that argon gas can be dissolved
in this liquid.

The problem of energizing the argon to fluorescence while
it is dissolved was first approached by applying a strong

How can this tech writer use the vi editor to number the paragraphs in each section? (If this seems far-fetched to you, consider that I once got a phone call from a Unix guru asking how to do just this.) To keep the problem simple, let's say that there are never more than 35 paragraphs in a section, and that they should be numbered with Roman numerals.

This problem is still difficult enough that I'm offering you two hints. The first is that global is essential here. Look at the second hint only if you're about to give up and check my solution.

Coming Up Next

In the next part of this tutorial, I'll cover the less-known aspects of the other line-mode commands for dealing with text and files. If you're a little overwhelmed with all that I've said about global, you'll be pleased to know that substitute is notably simpler, and all the remaining commands are very much simpler, than global.

After that, future parts of this tutorial will deal with visual mode; easier and more fun than line mode any day.

Copyright © 1995, 1996 Walter Alan Zintz. All Rights Reserved.
Edited by Becca Thomas / Online Editor / UnixWorld Online / editor@unixworld.com

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Last Modified: Thursday, 16-May-96 07:12:11 PDT