e2

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.H1
Writing text out as a file \(mi the Write command ``w''
.H2
.PG
It's likely that we'll want to save our text for later use.
To write out the contents of the buffer onto a file,
we use the
.ul
write
command
.X1
w
.X2
followed by the filename we want to write on.
This will copy the buffer's contents
onto the specified file
(destroying any previous information on the file).
To save
the text on a file named ``junk'', for example, type
.X1
w junk
.X2
Leave a space between ``w'' and the file name.
.ul
Ed
will respond by printing
the number of characters it wrote out.
In our case,
.ul
ed
would respond with
.X1
68
.X2
(Remember that blanks and the newline character at the end of each
line are included in the character count.)
Writing a file just makes a copy of the text \(mi the
buffer's contents are not disturbed, so we can go on adding
lines to it.
This is an important point.
.ul
Ed
at all times works on a copy
of a file, not the file itself.
No change in the contents
of a file takes place until you give a ``w'' command.
(Writing out the text onto a file from time to time as it is being
created is a good idea, since if the system crashes or if you make some horrible mistake, you will lose
all the text in the buffer but any text that was written onto
a file is relatively safe.)
.H1
Leaving ed \(mi the Quit command ``q''
.H2
.PG
To terminate a session with
.ul
ed,
save the text you're working on
by writing it onto a file using the ``w'' command,
and then type the command
.X1
q
.X2
which
stands for
.ul
quit.
The system will respond with ``%''.
At
this point your buffer vanishes, with all its text,
which is why you want to write it out before quitting.
.H1
Exercise 1:
.H2
.PG
Enter
.ul
ed
and
create some text using
.X1
a
.li
. . . text . . .
.li
\fB.\fR
.X2
Write it out using ``w''.
Then leave
.ul
ed
with the ``q'' command, and print the file,
to see that everything worked.
(To print a file, say
.X1
pr filename
.X2
or
.X1
cat filename
.X2
in response to ``%''.
Try both.)
.H1
Reading text from a file \(mi the Edit command ``e''
.H2
.PG
A common way to get text into the buffer is to read it
from a file in the file system.
This is what you do to edit text
that you saved with the 
``w''
command in a previous session.
The
.ul
edit
command ``e''
fetches the entire contents of a file into the buffer.
So if we had saved the three lines
``Now is the time'', etc.,
with a ``w'' command in an earlier session,
the 
.ul
ed
command
.X1
e junk
.X2
would fetch the entire contents of the file ``junk''
into the buffer, and respond
.X1
68
.X2
which is the number of characters in ``junk''.
.ul
If anything was already in the buffer, it is deleted first.
.PG
If we use the ``e'' command to read a file into the buffer,
then we need not use a file name after a subsequent ``w'' command;
.ul
ed
remembers the last file name used in an ``e'' command,
and ``w'' will write on this file.
Thus a common way to operate is
.X1
ed
e file
[editing session]
w
q
.X2
.PG
You can find out at any time what file name
.ul
ed
is remembering by typing the 
.ul
file
command ``f''.
In our case,
if we typed
.X1
f
.X2
.ul
ed
would reply
.X1
junk
.X2
.H1
Reading text from a file \(mi the Read command ``r''
.H2
.PG
Sometimes we want to read a file into the buffer
without destroying anything that is already there.
This is done by the
.ul
read
command ``r''.
The command
.X1
r junk
.X2
will read the file ``junk'' into the buffer;
it adds it
to the end of whatever is already in the buffer.
So if we do a read after
an edit:
.X1
e junk
r junk
.X2
the buffer will contain
.ul
two
copies of the text (six lines).
.X1
Now is the time
for all good men
to come to the aid of their party.
Now is the time
for all good men
to come to the aid of their party.
.X2
Like the ``w'' and ``e'' commands, ``r'' prints the
number of characters read in, after the reading operation is complete.
.PG
Generally speaking, ``r'' is much less used than ``e''.
.H1
Exercise 2:
.H2
.PG
Experiment with the ``e'' command \(mi
try reading and printing various files.
You may get an error ``?'',
typically because you spelled the file name wrong.
Try alternately reading and appending to see that they work
similarly.
Verify that
.X1
ed filename
.X2
is exactly equivalent to
.X1
ed
e filename
.X2
What does
.X1
f filename
.X2
do?
.H1
Printing the contents of the buffer \(mi the Print command ``p''
.H2
.PG
To
.ul
print
or list the contents of the buffer (or parts
of it) on the terminal, we use the print command
.X1
p
.X2
The way this is done is as follows.
We specify the lines where
we want printing to begin and where we want it to end,
separated by a comma, and
followed by the letter ``p''.
Thus to print the first two lines of the buffer, for
example, (that is, lines 1 through 2) we say
.X1
1,2p	(starting line=1, ending line=2 p)
.X2
.ul
Ed
will respond with
.X1
Now is the time
for all good men
.X2
.PG
Suppose we want to print
.ul
all
the lines in the buffer.
We could use ``1,3p'' as above if we knew there were exactly
3 lines in the buffer.
But in general, we don't
know how many there are, so what do we use for the ending
line number?
.ul
Ed
provides a shorthand symbol for ``line number of
last line in buffer'' \(mi the dollar sign ``$''.
Use it this
way:
.X1
1,$p
.X2
This will print
.ul
all
the lines in the buffer (line 1 to last line.)
If you want to stop the printing before it is finished,
push the DEL or Delete key;
.ul
ed
will type
.X1
?
.X2
and wait for the next command.
.PG
To print the
.ul
last
line of the buffer, we could use
.X1
$,$p
.X2
but
.ul
ed
lets us abbreviate this to
.X1
$p
.X2
We can print any single line by typing the line
number followed by a ``p''.
Thus
.X1
1p
.X2
produces the response
.X1
Now is the time
.X2
which is the first line of the buffer.
.PG
In fact,
.ul
ed
lets us abbreviate even further:
we can print any single line by typing
.ul
just
the line number \(mi no need to type the letter ``p''.
So if we say
.X1
$
.X2
.ul
ed
will print the last line of the buffer for us.
.PG
We can also use ``$'' in combinations like
.X1
$\(mi1,$p
.X2
which prints the last two lines of the buffer.
This helps when we want to see how far we got in typing.
.H1
Exercise 3:
.H2
.PG
.H2
As before, create some text using the append command and
experiment with the ``p'' command.
You will find, for example,
that you can't print line 0 or a line beyond
the end of the buffer, and that attempts
to print a buffer in reverse order by saying
.X1
3,1p
.X2
don't work.