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unix v7是最后一个广泛发布的研究型UNIX版本

.NH
SPECIAL CHARACTERS
.PP
The editor
.UL ed
is the primary interface to the system
for many people, so
it is worthwhile to know
how to get the most out of
.UL ed
for the least effort.
.PP
The next few sections will discuss
shortcuts
and labor-saving devices.
Not all of these will be instantly useful
to any one person, of course,
but a few will be,
and the others should give you ideas to store
away for future use.
And as always,
until you try these things,
they will remain theoretical knowledge,
not something you have confidence in.
.SH
The List command `l'
.PP
.UL ed
provides two commands for printing the contents of the lines
you're editing.
Most people are familiar with
.UL p ,
in combinations like
.P1
1,$p
.P2
to print all the lines you're editing,
or
.P1
s/abc/def/p
.P2
to change 
`abc'
to
`def'
on the current line.
Less familiar is the
.ul
list
command
.UL l
(the letter `\fIl\|\fR'),
which gives slightly more information than
.UL p .
In particular,
.UL l
makes visible characters that are normally invisible,
such as tabs and backspaces.
If you list a line that contains some of these,
.UL l
will print each tab as
.UL \z\(mi>
and each backspace as
.UL \z\(mi< .
This makes it much easier to correct the sort of typing mistake
that inserts extra spaces adjacent to tabs,
or inserts a backspace followed by a space.
.PP
The
.UL l
command
also `folds' long lines for printing _
any line that exceeds 72 characters is printed on multiple lines;
each printed line except the last is terminated by a backslash 
.UL \*e ,
so you can tell it was folded.
This is useful for printing long lines on short terminals.
.PP
Occasionally the
.UL l
command will print in a line a string of numbers preceded by a backslash,
such as \*e07 or \*e16.
These combinations are used to make visible characters that normally don't print,
like form feed or vertical tab or bell.
Each such combination is a single character.
When you see such characters, be wary _
they may have surprising meanings when printed on some terminals.
Often their presence means that your finger slipped while you were typing;
you almost never want them.
.SH
The Substitute Command `s'
.PP
Most of the next few sections will be taken up with a discussion
of the
substitute
command
.UL s .
Since this is the command for changing the contents of individual
lines,
it probably has the most complexity of any 
.UL ed 
command,
and the most potential for effective use.
.PP
As the simplest place to begin,
recall the meaning of a trailing
.UL g
after a substitute command.
With
.P1
s/this/that/
.P2
and
.P1
s/this/that/g
.P2
the
first
one replaces the
.ul
first
`this' on the line
with `that'.
If there is more than one `this' on the line,
the second form
with the trailing
.UL g
changes
.ul
all
of them.
.PP
Either form of the
.UL s
command can be followed by
.UL p
or
.UL l
to `print' or `list' (as described in the previous section)
the contents of the line:
.P1
s/this/that/p
s/this/that/l
s/this/that/gp
s/this/that/gl
.P2
are all legal, and mean slightly different things.
Make sure you know what the differences are.
.PP
Of course, any
.UL s
command can be preceded by one or two `line numbers'
to specify that the substitution is to take place
on a group of lines. 
Thus
.P1
1,$s/mispell/misspell/
.P2
changes the 
.ul
first
occurrence of
`mispell' to `misspell' on every line of the file.
But
.P1
1,$s/mispell/misspell/g
.P2
changes 
.ul
every
occurrence in every line
(and this is more likely to be what you wanted in this
particular case).
.PP
You should also notice that if you add a
.UL p
or
.UL l
to the end of any of these substitute commands,
only the last line that got changed will be printed,
not all the lines.
We will talk later about how to print all the lines
that were modified.
.SH
The Undo Command `u'
.PP
Occasionally you will make a substitution in a line,
only to realize too late that it was a ghastly mistake.
The `undo' command
.UL u
lets you `undo' the last substitution:
the last line that was substituted can be restored to
its previous state by typing the command
.P1
u
.P2
.SH
The Metacharacter `\*.'
.PP
As you have undoubtedly noticed
when you use
.UL ed ,
certain characters have unexpected meanings
when they occur in the left side of a substitute command,
or in a search for a particular line.
In the next several sections, we will talk about
these special characters,
which are often called `metacharacters'.
.PP
The first one is the period `\*.'.
On the left side of a substitute command,
or in a search with `/.../',
`\*.' stands for
.ul
any
single character.
Thus the search
.P1
/x\*.y/
.P2
finds any line where `x' and `y' occur separated by
a single character, as in
.P1
x+y
x\-y
x\*By
x\*.y
.P2
and so on.
(We will use \*B to stand for a space whenever we need to
make it visible.)
.PP
Since `\*.' matches a single character,
that gives you a way to deal with funny characters
printed by
.UL l .
Suppose you have a line that, when printed with the
.UL l
command, appears as
.P1
 ....   th\*e07is   ....
.P2
and you want to get rid of the 
\*e07
(which represents the bell character, by the way).
.PP
The most obvious solution is to try
.P1
s/\*e07//
.P2
but this will fail. (Try it.)
The brute force solution, which most people would now take,
is to re-type the entire line.
This is guaranteed, and is actually quite a reasonable tactic
if the line in question isn't too big,
but for a very long line,
re-typing is a bore.
This is where the metacharacter `\*.' comes in handy.
Since `\*e07' really represents a single character,
if we say
.P1
s/th\*.is/this/
.P2
the job is done.
The `\*.' matches the mysterious character between the `h' and the `i',
.ul
whatever it is.
.PP
Bear in mind that since `\*.' matches any single character,
the command
.P1
s/\*./,/
.P2
converts the first character on a line into a `,',
which very often is not what you intended.
.PP
As is true of many characters in
.UL ed ,
the `\*.' has several meanings, depending
on its context.
This line shows all three:
.P1
\&\*.s/\*./\*./
.P2
The first `\*.' is a line number,
the number of
the line we are editing,
which is called `line dot'.
(We will discuss line dot more in Section 3.)
The second `\*.' is a metacharacter
that matches any single character on that line.
The third `\*.' is the only one that really is
an honest literal period.
On the
.ul
right
side of a substitution, `\*.'
is not special.
If you apply this command to the line
.P1
Now is the time\*.
.P2
the result will
be
.P1
\&\*.ow is the time\*.
.P2
which is probably not what you intended.
.SH
The Backslash `\*e'
.PP
Since a period means `any character',
the question naturally arises of what to do
when you really want a period.
For example, how do you convert the line
.P1
Now is the time\*.
.P2
into
.P1
Now is the time?
.P2
The backslash `\*e' does the job.
A backslash turns off any special meaning that the next character
might have; in particular,
`\*e\*.' converts the `\*.' from a `match anything'
into a period, so
you can use it to replace
the period in
.P1
Now is the time\*.
.P2
like this:
.P1
s/\*e\*./?/
.P2
The pair of characters `\*e\*.' is considered by
.UL ed
to be a single real period.
.PP
The backslash can also be used when searching for lines
that contain a special character.
Suppose you are looking for a line that contains
.P1
\&\*.PP
.P2
The search
.P1
/\*.PP/
.P2
isn't adequate, for it will find
a line like
.P1
THE APPLICATION OF ...
.P2
because the `\*.' matches the letter `A'.
But if you say
.P1
/\*e\*.PP/
.P2
you will find only lines that contain `\*.PP'.
.PP
The backslash can also be used to turn off special meanings for
characters other than `\*.'.
For example, consider finding a line that contains a backslash.
The search
.P1
/\*e/
.P2
won't work,
because the `\*e' isn't a literal `\*e', but instead means that the second `/'
no longer \%delimits the search.
But by preceding a backslash with another one,
you can search for a literal backslash.
Thus
.P1
/\*e\*e/
.P2
does work.
Similarly, you can search for a forward slash `/' with
.P1
/\*e//
.P2
The backslash turns off the meaning of the immediately following `/' so that
it doesn't terminate the /.../ construction prematurely.
.PP
As an exercise, before reading further, find two substitute commands each of which will
convert the line
.P1
\*ex\*e\*.\*ey
.P2
into the line
.P1
\*ex\*ey
.P2
.PP
Here are several solutions;
verify that each works as advertised.
.P1
s/\*e\*e\*e\*.//
s/x\*.\*./x/
s/\*.\*.y/y/
.P2
.PP
A couple of miscellaneous notes about
backslashes and special characters.
First, you can use any character to delimit the pieces
of an
.UL s
command: there is nothing sacred about slashes.
(But you must use slashes for context searching.)
For instance, in a line that contains a lot of slashes already, like
.P1
//exec //sys.fort.go // etc...
.P2
you could use a colon as the delimiter _
to delete all the slashes, type
.P1
s:/::g
.P2
.PP
Second, if # and @ are your character erase and line kill characters,
you have to type \*e# and \*e@;
this is true whether you're talking to
.UL ed
or any other program.
.PP
When you are adding text with
.UL a
or
.UL i
or
.UL c ,
backslash is not special, and you should only put in
one backslash for each one you really want.
.SH
The Dollar Sign `$'
.PP
The next metacharacter, the `$', stands for `the end of the line'.
As its most obvious use, suppose you have the line
.P1
Now is the
.P2
and you wish to add the word `time' to the end.
Use the $ like this:
.P1
s/$/\*Btime/
.P2
to get
.P1
Now is the time
.P2
Notice that a space is needed before `time' in
the substitute command,
or you will get
.P1
Now is thetime
.P2
.PP
As another example, replace the second comma in
the following line with a period without altering the first:
.P1
Now is the time, for all good men,
.P2
The command needed is
.P1
s/,$/\*./
.P2
The $ sign here provides context to make specific which comma we mean.
Without it, of course, the
.UL s
command would operate on the first comma to produce
.P1
Now is the time\*. for all good men,
.P2
.PP
As another example, to convert
.P1
Now is the time\*.
.P2
into
.P1
Now is the time?
.P2
as we did earlier, we can use
.P1
s/\*.$/?/
.P2
.PP
Like `\*.', the `$'
has multiple meanings depending on context.
In the line
.P1
$s/$/$/
.P2
the first `$' refers to the
last line of the file,
the second refers to the end of that line,
and the third is a literal dollar sign,
to be added to that line.
.SH
The Circumflex `^'
.PP
The circumflex (or hat or caret)
`^' stands for the beginning of the line.
For example, suppose you are looking for a line that begins