The vi/ex Editor, Part 3: The global Command
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:
The 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.)
The command list cannot include an undo or another global
command.
If you include a command that escapes to the shell, it must
be the last command on the line. (Putting two or more
shell-escape commands in one command list will not work.)
This makes it possible to use pipes (symbolized by the |
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.
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:
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
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.
.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.
(DIR) Solutions
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.
(DIR) Part 4: The Substitute Command
(DIR) Back to the index