By
Walter Alan
Zintz
.
[
Editor's Note:
We'll indicate a line-mode command in
the paragraph text by prefixing it with a colon
(
:
), the character used to execute
a single line-mode command from visual command mode.]
After the
:global
command, which
we discussed in the
previous
installment
of this tutorial series, the
:substitute
command is line mode's
most subtle and complex tool. When I've gone
over those
complexities we'll be ready to deal with techniques and tricks of
building powerful line-mode command strings.
Making Changes Within Lines
Most of you already know the
:substitute
command by its shortest abbreviation
:s
and use it in roughly this form:
s/previous/former/
%s/Smith/Lee and Smith/
to make some change within the line you are on, in the first case,
or change every instance in the file in the second. If you use
both forms you are already ahead of the game. Too many class
instructors and textbook writers try to tell you that the way to
change some phrase throughout the file is to type something like:
global/Smith/s//Lee and Smith/
This is wasteful nonsense. Both forms accomplish exactly the
same thing, but the second version involves extra typing for you
and an extra run through the file for your computer. I
t does not
matter that not every line in your file will contain a ``Smith''
to be changed -- the
:substitute
command will execute properly in either version, and quietly if
even one line in the file has a ``Smith'' it can change.
But neither form as it stands
is sure
to change every ``Smith'' in the file. The
:substitute
command is set up to
change only the first example of the string it finds on any
particular line, so a line in the text that originally read:
inure to Smith's benefit only if Smith shall
will be changed by either version of the command to read:
inure to Lee and Smith's benefit only if Smith shall
Line mode has a built-in solution for this problem: place a
lower-case letter ``g'' at the very end of the command, immediately
after the last ``/'' mark, in order to make the change on every such
string in each line. So typing this:
% substitute
/Smith/Lee and Smith/g
will make that text line come out as:
inure to Lee and Smith's benefit only if Lee and Smith shall
Finer tuning of the instances can be done by a little
trickery. Suppose you are working on tables, and want to change
only the very last ``k37'' on each line to ``q53''. This command
will do it:
% substitute /\(..*\)k37/\1q53
If this seems surprising, remember that in a search pattern
with a wild card, the editor always extends the match to the
greatest length it can. In this case that means the string
starting at the beginning of the line and ending with the last
``k37'' in the line.
Now you should be able to extend this example. What command
would change only the second-to-last ``k37'' on each line? This
requires a shrewd guess from you, so I've written a
solution
you can compare to
your own.
A Few More Metacharacters
You probably already know that you don't always have to type
the search pattern that indicates the text to be replaced by a
substitution command. If you want to reuse your very last search
pattern, whether that was in a substitution command or not, you
can use an empty search pattern string to stand for the last
search pattern, so the two commands below are actually
identical.
/Murphy/ substitute /Murphy/Thatcher/
/Murphy/ substitute //Thatcher/
Either command will go to the next line containing ``Murphy''
and there replace the first ``Murphy'' with ``Thatcher''.
Within a substitution command's search pattern to find the
text to be removed, all the normal search-pattern metacharacters
apply. So do two more that are reserved only for substitution
commands: the ``\('' and ``\)'' metacharacters.
These two metacharacters don't match anything themselves, so:
substitute /^The cat and small dog show/
substitute /^The \(cat\) and \(small dog
\) show/
are exactly the same command as far as they go. But the
substitution command remembers what it finds to match the text
between a pair of ``\('' and ``\)'' metacharacters, for use in
the replacement text. Whenever your replacement pattern contains
``\1'' the editor replaces that metacharacter with whatever
matched the characters that were between the first pair of ``\(''
and ``\)'' metacharacters. A ``\2'' in the replacement pattern
is removed and replaced by whatever was matched the characters
between the second pair. And so on -- you can have up to nine
pairs in one substitution command. These metacharacter pairs can
even be nested in the to-be-replaced text; the one that starts
first will be represented by ``\1'' and so on. So if you extend
that second substitution command above to read:
substitute /^The \(cat\) and \(small dog\) show/My \2-\1 fair
the substitution command will produce a line that begins:
My small dog-cat
fair
Or if you type:
substitute :up \(and \)\(over \)\(the sky\):\2\1\2\1\2\3
then your command will change the first line below to read
as the second line, just beneath it:
up and over the sky
over and over and over the sky
(I used the colon ``:'' character to separate the parts of
the command, instead of the slash ``/'' character, solely to
make it more readable for you. There is no danger of the
editor confusing ``/'' with ''\'' or ``l'' (el) with ``1''
(one) etcetera.)
As the preceding examples show, the ``\('' and ``\)'' are not
too useful with plain-text search patterns; about their only real
value there is when you are searching for something that's
difficult to spell correctly, and don't want to type it into the
replacement pattern with possible spelling errors. (Spelling
errors aren't so dangerous in the to-be-replaced text, because
they only cause the pattern match to fail.)
These metacharacters save the day, t
hough, when you are
dealing with other search metacharacters in searching for text that
you will want to put back in. (Often the only way to specify the
exact spot you want the replacement done is to include some
neighboring text in the search pattern, and tell the editor that
after the neighboring text has been taken out it is to be put back
in right where it was.) Here are three examples of this kind of
substitution:
% substitute :\([Ss]ection\) \([0-9][0-9]*\):\1 No. \2:g
/\([Ss]ection\) \([0-9][0-9]*\)/ substitute ::\1 No. \2
% substitute ,[Aa]nswer: \([TtFf] \),ANSWER: \1,g
The first of these simply inserts ``No.'' in the middle of
phrases that are section numbers, throughout the document. But
the ``\('' and ``\)'' notation is essential to preserve the
section number in each case, and also to leave unchanged the
capitalization or noncapitalization of the first letter of
``section''.
The second command does the same thing, but only on the very
next line t
hat has a section number to change. The surprise here
is that I put the ``\('' and ``\)'' in the address pattern to
find the correct line. A line address doesn't use these
metacharacters, of course, but it does not object to them,
either. It just ignores them in its own line search, but does
pass them along when a following substitution command reuses the
last search pattern, as happens in this example.
The third example is useful in editing a list of answers to
exercises. It stops at each answer to a true-or-false question
and capitalizes the entire word ``answer''. The innovative aspect
of this command is that it finds the letter ``T'' or ``t'' or
``F'' or ``f'' following the word ``answer'', so it will not
change the capitalization where an answer is numerical rather
than true or false. And yet, the letter indicating whether
``true'' or ``false'' is the correct answer is not discarded as a
side effect. This is primarily an example of a change that can
be done more simply by using other metach
aracters in the
replacement pattern. Those other metacharacters are described
below.
Replacement-Pattern Metacharacters
The string of characters you want to put in via a substitution
command can use its own list of metacharacters. They're entirely
different from the metacharacters used in searching for a pattern
you want to take out of a line.
- &
In a replacement pattern, the ``&'' stands for the entire
text that was to be replaced. Use this when you want to add
rather than replace text. For example, to change ``kit'' to
``kit and kaboodle'' regardless of whether ``kit'' is
capitalized, use:
% substitute /[Kk]it/& and kaboodle/g
If you have
magic
turned off, you must backslash
the ``&'' to give it metavalue. With
magic
on,
backslash an ``&'' to insert it as a regular character.
- ~
The ``~'' character represents the replacement pattern you
used in your last substitution command. One good use for this
is in correcting various misspellings of a single word:
% substitute /[Ff]anstock clip/Fahnestock clip/g
% substitute /[Ff]ahnstock clip/~/g
% substitute /[Ff]ahnstocke clip/~/g
% substitute /[Ff]annstock clip/~/g
% substitute /[Ff]anestock clip/~/g
% substitute /[Ff]aenstock clip/~/g
If you have
magic
turned off, you must backslash
the ``~'' to give it metavalue. With
magic
on,
backslash a ``~'' to insert it as a regular character.
- \U
A ``\U'' metacharacter makes all letters following it
into capitals; to the end of the replacement pattern or until
another metacharacter turns it off. Here's how you'd use it
to change a section of your document to all capitals:
1 , substitute /.*/\U&
- \L
A ``\L'' gives the opposite effect of a ``\U'';
all following letters become lower case. You might use this
to decapitalize acronyms:
% substitute /FORTRAN and COBOL/\L&/g
- \E
Use ``\E'' to end the effect of a ``\U'' or ``\L''
metacharacter. Everything following the ``\E'' has the
same mix of capitalization as originally. For example, to
enclose a line of text in curly braces, and also change just
the first word to all capitals:
substitute :\([^ ]*\)\(.*\):{\U\1\E\2}
No ``\E'' is needed when you switch from ``\U'' to ``\L''
in the middle of a replacement pattern, or vice versa. When
either of those metacharacters appears, it automatically ends
the effect of the other. So if you have a list of book titles,
one title per line,
with only the first letters of the words capitalized, and you
want to change those titles to all capitals before the colon
in the middle of each title, and all lower cas
e after it,
just type:
% substitute ,\(.*\):\(.*\),\U\1:\L\2
- \u
- This metacharacter capitalizes just the single letter that
immediately follows it. If the character that immediately
follows it is not an alphabet letter, ``\u'' does nothing.
- \l
- The same as ``\u'', except that ``\l'' makes the
immediately following letter come out as lower case.
One more thing that's important to know about reusing patterns in
substitution commands. When all or part of a text-to-be-replaced
pattern is going to be used as a replacement pattern, or vice
versa, the command reuses the result of the original pattern,
after all the metacharacters have been evaluated in the original
situation. Since the metacharacters in either of those two types
of patterns have no meaning in the other type, it could hardly be
otherwise.
But when the reuse involves a text-to-be-replaced pattern
being used a second time as a text-to-be-replaced
pattern, or a
replacement pattern being reused as a replacement pattern, the
command brings along all the original metacharacters and
evaluates them afresh in the new situation. Thus, in either of
the cases in this paragraph, the second use is unlikely to
produce exactly the same results as the first use did.
Now another exercise for you. Suppose that lines 237 through
289 of a file have unknown capitalization--any line could be all
caps, all lower case, or some mixture. These lines are to be
changed so that the first letter of every word is a capital and
all other letters are lower case. To simplify this, words are
separated by space characters. What is the easy way to do this
with one line-mode substitution command? This exercise depends
on something I did not state directly, so don't feel bad if my
solution
is a little simpler
than yours.
Other Uses for Substitution
Despite the name, the
:substitute
command doesn't always take something out of the line and put
something else in its place. Here's an example that adds text at
the start of certain lines without removing anything:
537 , 542 substitute /^/WARNING: /
so that text which originally looked like this:
The primary output line carries very high voltage,
which does not immediately dissipate when power to
the system is turned off. Therefore, after turning
off the system and disconnecting the power cord,
discharge the primary output line to ground before
servicing the output section.
now looks like this:
WARNING: The primary output line carries very high voltage,
WARNING: which does not immediately dissipate when power to
WARNING: the system is turned off. Therefore, after turning
WARNING: off the system and disconnecting the power cord,
WARNING: discharge the primary output line to ground before
WARNING: servicing the output section.
It's just
as practical to pull some text out of lines without
putting anything back in its place. Here are two command lines
that do just that:
% substitute / uh,//g
. , $ substitute / *$
The latter command removes superfluous spaces at the ends of
lines. It doesn't need the final two slashes because there is no
suffix to be distinguished from a replacement pattern.
At times you might use both the previous principles, to create
:substitute
commands that
neither subtract nor add any text. Sound pointless?
Here's one such that I sometimes use
when I'm well along in writing one of these tutorials:
% substitute /^$
Now here's a different kind of exercise for you. I've already
given you the command, above. It obviously makes no change
whatsoever in the file. So why do I run this command? You need
a goodly dose of inspiration to answer this, so don't be
embarrassed if you have to look at my
answer
to this one.
A Start on Script Writing
Already you know enough about the editor to be able to
plan some fairly complex edits. Here's a short introduction
to the art of writing editing scripts for this editor.
BOTTOM-UP PROGRAMMING
. That's usually the
best way to build a complex editor command or command script.
That's a programmer's term that means putting all the little
details in separately and then pulling them all together into a
unified whole, rather than starting with a grand overall plan and
forcing the details to fit.
For example, reader R.T. of San Francisco, California asks how
to use the editor to automatically add HTML paragraph tags to
each paragraph of manuscripts. This requires inserting the
string ``<P>'' at the start of the first line of each
paragraph, and the string ``</P>'' at the end of the last
line. In these manuscripts, a completely empty line (not even a
non-print
ing character on it) separates one paragraph from
another.
This looks pretty easy. All that seems to be needed is to go
to each empty line, then move up to the preceding line to insert
the end-of-paragraph string and down to the following line to put
in the start-of-paragraph string. But there are flaws in the
obvious command to do this:
global /^$/ - substitute :$:</P>: | ++ substitute /^/<P>/
The first problem is that when the editor goes to the empty
first line that commonly begins a file, it will be unable to move
up a line to do the first substitution. No substitution is
needed there, of course, but since the editor doesn't leave that
empty first line, moving down two lines will put it on the second
line of the following paragraph -- definitely the wrong place for
a start-of-paragraph tag. There are several ways to fix this
problem:
- Have the editor
:mark
the empty
line before leaving it to execu
te (or attempt to execute) the
first substitution. Then it can go to the marked line (which
works even if the editor never left it) and drop down one line to
perform the second substitution.
- Change the address of that second substitution command from
``++'' to ``/./'' in order to move forward to the next nonempty
line, which will be the first line of the following paragraph
whether the search starts from the empty line or the line above
it.
- Run two separate
:global
searches, each of which executes one of the two substitution
commands.
Problem number two is that there may be several empty lines
between two paragraphs, since HTML interpretation is not affected
by them. If the editor is on the first of two or more
consecutive empty lines, the command I first proposed above will
perform its second substitution on the second empty line just
below it. When it moves to the second previously-empty line, it
will run the first substitution co
mmand on the empty line it just
left. (Yes, the second line is no longer empty, but it has
already been marked by the
:global
command before any substitutions are done.) That is, a stretch
of text that initially looked like this:
at this meeting, so be sure to be there!
At next month's meeting we'll hear from the new
and should have been edited to look like this:
at this meeting, so be sure to be there!</P>
<P>At next month's meeting we'll hear from the new
actually turns out like this:
at this meeting, so be sure to be there!</P>
</P>
<P>
<P>At next month's meeting we'll hear from the new
It may look as though this hazard can be defeated by
modifying the number two solution to the first problem above.
That is, the address for both substitutions will be a search
pattern that looks for a line that already has some text
on it. This works properly when the editor is on the fi
rst
of two consecutive empty lines. From the second line,
though, it runs its substitution commands on lines that have
already been given their tags, so the sample text now looks
like this:
at this meeting, so be sure to be there!</P></P>
<P><P>At next month's meeting we'll hear from the new
COMPLEX CONDITIONALS
. What's really needed
here is double-conditional execution. That is, substitution
commands must run on a given line only if both of these
conditions are true:
- The line to be substituted is adjacent to the empty line.
- The line to be substituted is not itself empty.
In this case, the editor can handle it. The
:global
portion of the command line
takes care of the first condition if the substitution commands'
addresses move exactly one line in each direction from the empty
line. (Of the three proposed solutions to the first difficulty
encountered, numbers one and three both do th
is much.) To
satisfy the second condition, make the substitution commands
remove one character from the existing line -- and then replace
it, of course. This ensures that if there is no character to
remove because the line is empty, the substitution command will
fail on that line and do nothing.
Either the first or third solution can be adapted to satisfy
that second condition. I've used the third solution in the
example commands below, because the technique is easier to follow
than it would be with the first solution:
global /^$/ + substitute /^./<P>&/
global /^$/ - substitute :.$:&</P>:
Bottom-up techniques can be continued if there are yet other
special needs to be accommodated. Reader R.T. may have headlines
and subheads mixed in with the paragraphs, and may already have
appropriate HTML tags at the beginnings and ends of those heads
and subheads. As an exercise, how would you adapt the commands
just above so they would not add a para
graph tag where any text
already begins or ends with an HTML tag? Hint -- an HTML tag
always begins with a ``<'' and ends with a ``>'' character.
This is a very minor change, so you probably will not need to
look at my
solution
except
to confirm your own answer.
A LITTLE TRICKERY
. At times a command needs
to be supercharged by way of a far out use of
substitution--something perfectly legitimate, but never intended
by the people who wrote this editor. Here are a few that you may
find useful.
You can't make a substitution that extends over more than a
single line--not directly, that is. Any attempt to put a
``newline'' character in either the to-be-replaced pattern or the
replacement pattern of a substitution command will fail. But by
combining the
global
and
substitute
commands, you can often
get the effect of a substitution that spills over a line
ending.
Let's
suppose that you have to alter a long document so that
all references to ``Acme Distributors'' are changed to ``Barrett
and Sons''. A simple substitution command will make most of these
changes, but it will miss those instances where ``Acme'' appears
at the end of one line and the next line starts with
``Distributors''. A followup pair of substitutions, to replace
``Acme'' wherever it appears at the end of a line and to replace
``Distributors'' when it starts a line, would wreak havoc--this
document also refers to ``Acme Supply Co.'' and to three other
companies whose names end with ``Distributors''.
But we can handle this problem nicely with the following two
command strings:
global /Acme$/ + substitute /^Distributors/and Sons
global /^and Sons/ - substitute /Acme$/Barrett
The first command goes to every line that ends with ``Acme''
and then moves forward one line--if and only if that next line
begins with ``Distributors'', it is changed to begin with ``and
So
ns''. The next command reverses the process to change ``Acme''
to ``Barrett'', but only in the right instances. (Note well that
the second command searches for ``and Sons'', not
``Distributors'', because the first command has changed those
line-split ``Acme Distributors'' to ``Acme and Sons''.)
Often it is a good strategy to start with a change you
definitely don't want in order to wind up with what you do want.
Suppose you are a technical writer who has just finished writing
a number of lengthy photo captions full of phrases like ``the
light spot in the upper righthand corner'' and ``dark areas near
the lower lefthand edge''. Along comes the news that the Art
Director has decided to flop all the photos: print them in
mirror-image form. Suddenly, everything that was on the right is
now on the left, and vice versa.
Your captions will be accurate again if you change every
``lefthand'' to read ``righthand'' and vice versa. But how to do
that without wading through the whole text and making eac
h change
individually? The obvious pair of substitutions will not
work:
% substitute /lefthand/righthand/g
% substitute /righthand/lefthand/g
The second command doesn't just change the original instances
of ``righthand'' to ``lefthand''; it also reverses every change
your first command made--now everything is described as being on
the lefthand side. But the following three substitution commands
will do the job nicely.
% substitute /lefthand/QQQQ/g
% substitute /righthand/lefthand/g
% substitute /QQQQ/righthand/g
By making the first command change ``lefthand'' temporarily to
``QQQQ'' (or any other string you know will not be found in your
document), you keep those changes safe from the effect of your
second command. Then, after that second command has finished,
the third command changes those Q strings to what you had wanted
in the first place.
It can even make sense to type in things incorrectly, then
change them to what
you want via substitution. When I'm writing
documents in plain ASCII, to be printed without any formatting, I
often use a line across the page to separate major sections of
the document. But where others are satisfied with just a string
of hyphens, or another single character, I pretty things up with
multicharacter dividers like:
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
-+--+--+--+--+--+--+--+--+--+-
*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~
[][][][][][][][][][][][][][][]
Not that I have the patience and concentration to type in
page-wide lines of alternating characters, especially when I would
have to constantly get on and off the shift key, too. No, I just
use my repeat key to fill the line with whatever character will
begin my eventual multicharacter pattern. For those four
patterns above, I would have repeat-keyed in these four lines,
respectively:
------------------------------
------------------------------
******************************
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[
T
hen I only have to run a simple repeated substitution to get
the line I actually want. Here are the commands I would run on
the four lines above, respectively:
substitute /--/-=/g
substitute /---/-+-/g
substitute /\*\*/*\~/g
substitute /\[\[/[]/g
SEMI-AUTOMATIC SUBSTITUTIONS
. At times
you'll have to make changes that are so dependent on human
judgment that no substitution tricks could possibly do exactly
what's wanted. In those cases there are two ways to have the
editor partially automate those changes.
The first is to use a variant form
of the
substitute
command that presents
each potential substitution on your screen, and asks you to
accept or reject it. All you need to do is put a lower-case
``c'' at the end of your substitution command, in the same place
you would put a ``g'' to change every instance on each line, like
either of these examples:
% substitute /^someth
ing/something else/c
% substitute /something/something else/gc
The editor will then display the lines where substitutions are
to be made on your screen, one at a time. Each line will have
``^'' marks below the text to be removed, like this:
something in the air. The consensus is that
^^^^^^^^^
and if there are two or more places on the line where the
substitution could be performed, the line will be displayed on
your screen two or more times, with a different potential
substitution marked each time.. After displaying a line on your
screen, the editor will wait for you to type something ending
with a carriage return. If whatever you type begins with a
lower-case ``y'', the change will be made. If it begins with
anything else, the text will be left as it is.
Even this substitution variant may not give you enough
control. You may need to see more than one line to verify your
judgment, or the text to be put in may vary from one place to
another. In those c
ases, you can use one of the qualities of the
:global
command. This is a
simplified version of the technique our programmer friend Hal (in
the first installment of this tutorial) used to work over the
problem points that Lint found in the code he was updating.
If you are editing in screen mode, as usual, you must start by
typing a capital ``Q'' to go into line mode. From line mode's
colon prompt, give a command like the following (if you want to
make the same substitution as in our previous examples):
global /something/ visual
This command
will bring you in turn to each line
in the
file that contains the string ``something'' and put you in screen
-editing mode there. After you've looked around, and made a
substitution if you think it justified, typing a capital ``Q''
takes you out of screen-editing mode and returns you to the
global
command, which
takes you the
next marked line and resumes screen editing.
There is an indirect
hazard
in leaving screen
editing mode, though. And that brings us to the whole dismal
subject of preventing loss of your changes, or of your entire
file, while you are in the editor.
Don't Lose Your Files
The vi/ex editor is not strong on protecting you from the
consequences of your own mistakes. In part that's just the
natural result of giving you a lot of editing power. But
when it comes to losing all the changes you've made in a file
during a session, or even losing the original file you started
with, the editor could be a lot more responsible without
hamstringing your subtle edits. Still, there are ways you can
comfortably protect yourself from those hazards, and many of
those ways I explain below.
IN EMERGENCIES
. Consider one of the editor's
safety features that can accidentally but quite easily turn into
a disaster. You may alr
eady know that when you edit with this
editor, you are working on a copy of the file, not the original.
Your changes do not affect the original unless you use the
:write
command, for which
:w
is the shortest abbreviation, or
leave the editor in a normal way. That's a good precaution
against making a mistake with this powerful editor that mangles
your file, and then discovering you have no backup copy.
That copy you are working on lives in a volatile place,
though, where it can easily be erased when the system crashes or
your link into the system goes down. That could cost you all the
additions and changes you'd made in that session with the editor.
Your first line of defense against this is to run the
:write
command often--every time
you run it, your current edited version replaces the previous
version in the stable permanent-storage area on disk.
And if you don't intend to change the original? Your ed
ited
version is to be a new file, with the original left untouched?
Well, you can use a modified form of writing the file, by typing
:write nufile
where ``nufile'' is
whatever you want the name of the edited file to be. (That can
be a path name, in case you don't want to put the new file in
your current directory.) This will write the revised version
only to the new file (creating the file if necessary), and leave
the original file untouched.
That method of preserving the original file is dangerous,
though. If you forget even once to add the filename to the
:write
command, your original file
is wiped off the disk. That's why this technique is best
reserved for occasions where you do want to change the original
file, but also want a copy of some partially edited stage of the
file saved elsewhere. When you get the file to the state you
want to save as a copy, run both of these commands:
write nufile
write
and then go back to editing and writing to the file as usual.
The sane way to protect your original file from any changes is
to start your editing with a
:file
nufile
command, for which
:f
nufile
is the shortest abbreviation. From that
point on, the editor considers ``nufile'' to be the name of the
file you are working on, and writes go automatically to that
filename. (Incidentally, if you ever forget whether you have
changed the file name, or what you've changed it to, a
:file
command by itself, without a
file name, will tell you what the editor thinks the file's
current name is, among other interesting facts.)
CRASHES WILL HAPPEN
. Still, a crash may catch
you by surprise, with a lot of additions and changes that you
have not written to any file.. To protect against this, the
editor always attempts to save your current working copy of the
file when a crash is imminent. You can
even launch an emergency
save yourself when you face a sticky situation, such as being
unable to do a normal write because it would exceed your
filespace quota. Just type a
:preserve
command (or its
:pre
abbreviation) and the working
copy is normally preserved. There are a few gotchas to watch
for, though.
The preservation function puts the saved copy in a specific
directory, and it will fail if that directory does not exist or
is not writable. (The path name of that directory varies
between versions of the editor, although
/var/preserve
seems to be a common choice on modern
Unix systems.) To check writability, run a
:preserve
command from a short
file, as a test. If the result is a message something like
this:
Can't open /var/preserve
Preserve failed!
there is a problem you will have to take up with your system
administrator. (To speed up that discussion, bring along the
path name of the directory that couldn't be opened.) If the
message reads like this:
File preserved.
so far, so good. The next question is whether the editor has
preserved an accurate copy or a pile of garbage--some editor
implementations are broken in this area. To check this, recover
the file you've just preserved.
RESCUING SAVED FILES
. There are two ways to
recover a rescued working copy, whether it was saved in a crash
or because you used the
:preserve
command. Usually you recover it from your shell command line, by
entering the editor with a ``-r'' flag:
vi -r novel.chap3
vi -r
The first of these commands puts you in the editor, with the
latest rescued copy of your file ``novel.chap3'' before you. The
latter command doesn't put you in the editor at all; it displays
a list of all files that have been emergency-saved and then
returns you to the shell command line. This list is useful wh
en,
say, the system crashed while you were editing a file that you
hadn't given a name. (Yes, you can enter the editor without
giving a filename to edit; the editor will simply bring up an
empty buffer and assume you will give it a name later.) In this
case the preservation process will give the file a name, and you
must know this name to recover it.
I said that the first command would bring up the
latest
rescued copy of the file you named. If the
system has been staggering for a while, there may be more than
one occasion when either you or the system caused the editor to
preserve the working copy of that file. If the latest version is
not the best copy, you can discard it and pull up the next most
recent version, without leaving the editor. Just give a
:recover
command (or its
:rec
abbreviation) to have the
current rescued version discarded and replaced by the next-latest
of the saved copies. (When you're already in the editor
, there's
no need to give the name of the file to recover if it is the same
as that of the file you're editing at the time. The editor
assumes the current filename unless you give another name as an
argument following the
:recover
command.) If this isn't the best copy either, you can continue
this process.
When you've recovered a file either way, look it over. If the
editor version you're using has a broken preservation function,
you'll only find garbage characters or a display like this:
LOST
LOST
LOST
LOST
LOST
If that be the case, the file you preserved is hopelessly lost
and you'd better have a talk with your system administrator about
getting a better version of the editor. But if what you see
looks like what you had, then all you have to do is write the
copy you've recovered to a file somewhere--the preserved copy was
erased when you used one of the recovery commands, so it can't be
recovered that way again.
And that brings up the l
ast gotcha. You may believe that any
of the three commands
ZZ
or
:x
or
:wq
will check whether the working
copy needs to be written to the file, write it only if necessary,
and then quit the editor. Actually, the last of the three, the
:wq
command, always writes to the
file regardless, and is the only one you should use.
The first two attempt some checking, but their checks are not
very complete. In particular, they and the
:quit
command often check for
nothing more than the flag that is set when an editing change is
made to the current working copy and unset when the working copy
is written to the file. You are courting disaster if you ever
use the
ZZ
or
:x
commands, or if you use
:quit
carelessly.
The gotcha in the case of a recovered file is that pulli
ng a
new file into the buffer, whether normally or by recovering an
emergency-saved copy, is not an editing change. If your version
of the editor has a weak version of
ZZ
or
:x
then its casual check will
show no reason to write the file, and all your carefully
recovered work will be lost for good when the command closes the
editor without writing the file. So always use
:wq
or separate
:write
and
:quit
commands to end an editing
session.
A FEW MORE HAZARDS AND SOLUTIONS
.
Worse yet
can befall you. You may accidentally lose
both your own editing changes and the original file you were
working from.
Suppose one of your global editing commands went astray and
trashed your working copy of the file, but didn't happen to
affect the part that is on your screen. If you then wrote the
working copy to the file, the garb
age replaced your original file
contents. Oh, misery! And with any but the smallest file, it's
not practical to look over the working copy carefully before each
:write
command.
Or perhaps you did discover the disaster before you wrote the
working copy to the file. Seeing that undoing the errors was not
feasible, you decided either to run an
:edit!
command to bring up a fresh
copy of the original file, or a
:quit!
to leave the editor. In
either case, the ``!'' at the end of the command tells the editor
to ignore the garbage changes that have not been written to the
file.
But since you were not creating an editor script here, you
probably typed the short form of your command, either
:e!
or
:q!
. At least you tried to.
Perilously placed between the ``e'' and ``q'' keys on a standard
keyboard is the ``w'' key. If you accidentally typed
:w!
instead of what you intended,
you told the editor to overwrite the original file with that
trashed version, and ignore any anti-write protections you may
have set. Oh, misery cubed!
You are not lost yet, though, if you have been editing along
in screen mode all the while. At any time you can type a short
sequence to put the working copy back the way it was when you
started this editing session. Then you only need to write the
working copy to the file to expunge the trash there.
Start by typing
Q
to leave
screen mode and drop back to line mode. Now, line mode has an
undo
command that works the way the
screen mode
u
command does, but on
line mode commands. That is, it reverses the effect of the
latest line mode command that changed (or may have changed) the
working copy. One line mode command that may well change the
working copy is
visual
, of cour
se.
And -- surprise -- when you typed
vi
novel.chap3
from your shell prompt to enter the
editor, your shell actually launched
ex
(for which
vi
is just an alias) and the editor
gave itself an initial
visual
command to boost you into screen mode.
So all the time you've been editing, the editor has been
holding a complete copy of the original file, in case you go back
to line mode and want to reverse the effect of that initial
visual
command. (That's one reason
the editor's buffer takes up so much more room than you'd expect
in swap space.) If you want to see the complete command sequence
to restore the original working copy and return to visual mode,
using shortest abbreviations and showing carriage returns as new
lines, here it is:
Qu
w
vi
One last hazard, which may seem childish to experienced Unix
users but trips
up many a refugee from single user systems.
Unless you're on one of those rare Unix implementations that
offers file locking, there is little to prevent another user on
the system from editing the same file at the same time as you
do.
You will each be editing on a separate working copy, so there
will be nothing to tell you that someone else is also editing the
same file. But each time you write your changed version to the
file, you will wipe out whatever changes the other user has
already written to file, and vice versa. The ultimate victor in
this unknowing war will be the user who finishes editing last.
The other user can come back an hour later and find no indication
that he/she ever touched the file.
There's no real technical solution to this danger. You'll
just have to coordinate carefully with other users on files
that more than one of you may have occasion to edit.
Reader Feedback
One of our readers
raised a
significa
nt point about
this
technique
; important enough to deserve a reply published in
this article.
Dear Walter...
In your tutorial you write that you can use the command
global/XXX/visual
to search for the pattern "XXX" and edit/move around (remember,
Hal needed this command to edit the linted spaghetti-code...)
But there's one problem: suppose I found, after the 10th XXX of
100, that I do not want to view the remaining 90 occurences. It
works as long as I don't type 'Q'. But now I want to view/edit
the code where my lint report is something like "illegal", I have
to type Q and then global/illegal/visual.
And now there's the problem: typing Q doesn't prompt for a new
input, it moves to the 11th occurence of "XXX".
Do you know my problem? Is there a way to stop vi moving on with
the global command after typing Q?
Thanks a lot in advance!
Chris...
As Chris clearly realizes, ordinarily there is no problem with
omitting the remaining 90 stops. Each time th
is command puts you
into visual mode somewhere in the file, you are not restricted to
fixing one limited problem. You may move anywhere in the file,
edit whatever you like, and keep doing this as long as you
please. When you finally finish all the edits you've decided to
do, you can write the file and quit the editor in your usual
way--the suspended
global
command
will silently disappear at this point.
But going into a second string of
visual
mode stops from a new
global
command, as Chris wants to
do, requires finesse. Obviously, it's not possible to use the
Q
command to return to the
line-mode command prompt until every one of those 100 lines has
been visited and the first
global
has terminated.
The best way out of this predicament starts with writing your
changes to the file. Then, instead of typing
Q
type an
:edit
command. This brings up a
fresh copy of the file you are editing, but since you've just
done a write, the fresh copy is identical to the one you've been
working on. Because you haven't left the editor, most of the
state is saved--contents of the named buffers, any maps and/or
abbreviations, values of the
set
command options. You do wipe out a few small items like the
contents of the unnamed buffer--and, of course, that suspended
global
command.
Now you can use the
Q
command to
go into line mode, then run that second
global
command.
In The Next Installment
In this tutorial to date, you've undoubtedly seen some aspects
of the editor that you wish had been designed differently. The good
news is that many of these features are yours to change at
will--without hacking up the source code and recompiling.
In
Part 5 of this tutorial
, I'll elucidate
the editor's built-in facilities for setting up your own editing
environment, and the many factors you can modify this way.
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