global CommandQuestions regarding this article should be directed to the author at walter@ccnet.com.
global CommandIf 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.)
global OperationsMore 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:
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.
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
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.
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.
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:
global keyword and the following list of
commands all must be on one line. (That is, on one physical
line, with no carriage returns in it. If that one line is too
long for your terminal's screen width, the terminal may wrap it
around to occupy two or more lines on your screen, but this
will not cause a problem.)undo
or another global command.| character) in your shell-escape command string,
without having the editor mistake the pipe symbol for the
separator between two editor commands in your
global command line.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:
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.
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.
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 global has 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
global CommandsThe 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.
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. .pp 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.
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.
Last Modified: Thursday, 16-May-96 07:12:11 PDT