include problem

[unix-history] / .ref-BSD-3 / usr / doc / adv.ed / ae5

.NH
CUT AND PASTE WITH UNIX COMMANDS
.PP
One editing area in which non-programmers
seem not very confident
is in what might be called
`cut and paste' operations _
changing the name of a file,
making a copy of a file somewhere else,
moving a few lines from one place to another in a file,
inserting one file in the middle of another,
splitting a file into pieces,
and
splicing two or more files together.
.PP
Yet most of these operations are actually quite easy,
if you keep your wits about you
and go cautiously.
The next several sections talk about cut and paste.
We will begin with the
.UX
commands
for moving entire files around,
then discuss
.UL ed
commands
for operating on pieces of files.
.SH
Changing the Name of a File
.PP
You have a file named 
`memo'
and you want it to be called
`paper'
instead.
How is it done?
.PP
The
.UX
program that renames files
is called
.UL mv
(for `move');
it `moves' the file from one name to another, like this:
.P1
mv  memo  paper
.P2
That's all there is to it:
.UL mv
from the old name to the new name.
.P1
mv  oldname  newname
.P2
Warning: if there is already a file around with the new name,
its present contents will be
silently
clobbered
by the information from the other file.
The one exception is that you can't move a file
to itself _
.P1
mv  x  x
.P2
is illegal.
.SH
Making a Copy of a File
.PP
Sometimes what you want is a copy of a file _
an entirely fresh version.
This might be because you want to work on a file, and
yet save a copy in case something gets fouled up,
or just because you're paranoid.
.PP
In any case, the way to do it is with the
.UL cp
command.
.UL cp \& (
stands for `copy';
the
.UC UNIX
system
is big on short command names,
which are appreciated by heavy users,
but sometimes a strain for novices.)
Suppose you have a file called
`good'
and
you want to save a copy before you make some
dramatic editing changes.
Choose a name _
`savegood'
might be acceptable _ then type
.P1
cp  good  savegood
.P2
This copies
`good'
onto
`savegood',
and you now have two identical copies of the file
`good'.
(If
`savegood'
previously contained something,
it gets overwritten.)
.PP
Now if you decide at some time that you want to get
back to the original state of
`good',
you can say
.P1
mv  savegood  good
.P2
(if you're not interested in
`savegood'
any more), or
.P1
cp  savegood  good
.P2
if you still want to retain a safe copy.
.PP
In summary, 
.UL mv
just renames a file;
.UL cp
makes a duplicate copy.
Both of them clobber the `target' file
if it already exists, so you had better
be sure that's what you want to do
.ul
before
you do it.
.SH
Removing a File
.PP
If you decide you are really done with a file
forever, you can remove it
with the
.UL rm
command:
.P1
rm  savegood
.P2
throws away (irrevocably) the file called
`savegood'.
.SH
Putting Two or More Files Together
.PP
The next step is the familiar one of collecting two or more
files into one big one.
This will be needed, for example,
when the author of a paper
decides that several sections need to be combined
into one.
There are several ways to do it,
of which the cleanest, once you get used to it,
is a program called
.UL cat .
(Not 
.ul
all
.UC UNIX 
programs have two-letter names.)
.UL cat
is short for
`concatenate', which is exactly
what we want to do.
.PP
Suppose the job is to combine the files
`file1'
and
`file2'
into a single file called
`bigfile'.
If you say
.P1
cat  file
.P2
the contents of
`file'
will get printed on your terminal.
If you say
.P1
cat  file1  file2
.P2
the contents of
`file1'
and then the contents of
`file2'
will
.ul
both
be printed on your terminal,
in that order.
So
.UL cat
combines the files, all right,
but it's not much help to print them on the terminal _
we want them in 
`bigfile'.
.PP
Fortunately, there is a way.
You can tell
the system
that instead of printing on your terminal,
you want the same information put in a file. 
The way to do it is to add to the command line
the character
.UL >
and the name of the file
where you want the output to go.
Then you can say
.P1
cat  file1  file2  >bigfile
.P2
and the job is done.
(As with
.UL cp
and
.UL mv ,
you're putting something into
`bigfile',
and anything that was already there is destroyed.)
.PP
This ability to
`capture' the output of a program
is one of the most useful aspects of
the 
.UC UNIX
system.
Fortunately it's not limited to the
.UL cat 
program _
you can use it with 
.ul
any
program that prints on your terminal.
We'll see some more uses for it in a moment.
.PP
Naturally, you can combine several files,
not just two:
.P1
cat  file1  file2  file3  ...  >bigfile
.P2
collects a whole bunch.
.PP
Question:
is there any difference between
.P1
cp  good  savegood
.P2
and
.P1
cat  good  >savegood
.P2
Answer: for most purposes, no.
You might reasonably ask why there are two programs
in that case,
since
.UL cat
is obviously all you need.
The answer is that 
.UL cp
will do some other things as well,
which you can investigate for yourself
by reading the manual.
For now we'll stick to simple usages.
.SH
Adding Something to the End of a File
.PP
Sometimes you want to add one file to the end of another.
We have enough building blocks now that you can do it;
in fact before reading further it would be valuable
if you figured out how.
To be specific,
how would you use
.UL cp ,
.UL mv
and/or
.UL cat
to add the file
`good1'
to the end of the file
`good'?
.PP
You could try
.P1
cat  good  good1  >temp
mv  temp  good
.P2
which is probably most direct.
You should also understand why
.P1
cat  good  good1  >good
.P2
doesn't work.
(Don't practice with a good `good'!)
.PP
The easy way is to use a variant of
.UL > ,
called
.UL >> .
In fact,
.UL >> 
is identical to
.UL >
except that instead of clobbering the old file,
it simply tacks stuff on at the end.
Thus you could say
.P1
cat  good1  >>good
.P2
and
`good1'
is added to the end of
`good'.
(And if
`good'
didn't exist,
this makes a copy of
`good1'
called
`good'.)