u2

unix v7是最后一个广泛发布的研究型UNIX版本

if you give the
command
.UL pwd ,
it will print something like
.P1
/usr/your\(hyname
.P2
This says that you are currently in the directory
.UL your-name ,
which is in turn in the directory
.UL /usr ,
which is in turn in the root directory
called by convention just
.UL / .
(Even if it's not called
.UL /usr
on your system,
you will get something analogous.
Make the corresponding changes and read on.)
.PP
If you now type
.P1
ls /usr/your\(hyname
.P2
you should get exactly the same list of file names
as you get from a plain
.UL ls  :
with no arguments,
.UL ls
lists the contents of the current directory;
given the name of a directory,
it lists the contents of that directory.
.PP
Next, try
.P1
ls /usr
.P2
This should print a long series of names,
among which is your own login name
.UL your-name .
On many systems, 
.UL usr
is a directory that contains the directories
of all the normal users of the system,
like you.
.PP
The next step is to try
.P1
ls /
.P2
You should get a response something like this
(although again the details may be different):
.P1
bin
dev
etc
lib
tmp
usr
.P2
This is a collection of the basic directories of files
that
the system
knows about;
we are at the root of the tree.
.PP
Now try
.P1
cat /usr/your\(hyname/junk
.P2
(if
.UL junk
is still around in your directory).
The name
.P1
/usr/your\(hyname/junk
.P2
is called the
.UL pathname
of the file that
you normally think of as ``junk''.
``Pathname'' has an obvious meaning:
it represents the full name of the path you have to follow from the root
through the tree of directories to get to a particular file.
It is a universal rule in
the
.UC UNIX
system
that anywhere you can use an ordinary filename,
you can use a pathname.
.PP
Here is a picture which may make this clearer:
.P1 1
.ft R
.if t .vs 9p
.if t .tr /\(sl
.if t .tr ||
.ce 100
(root)
/ | \e
/  |  \e
/   |   \e
  bin    etc    usr    dev   tmp 
/ | \e   / | \e   / | \e   / | \e   / | \e
/  |  \e
/   |   \e
adam  eve   mary
/        /   \e        \e
             /     \e       junk
junk temp
.ce 0
.br
.tr //
.P2
.LP
Notice that Mary's
.UL junk
is unrelated to Eve's.
.PP
This isn't too exciting if all the files of interest are in your own
directory, but if you work with someone else
or on several projects concurrently,
it becomes handy indeed.
For example, your friends can print your book by saying
.P1
pr /usr/your\(hyname/chap*
.P2
Similarly, you can find out what files your neighbor has
by saying
.P1
ls /usr/neighbor\(hyname
.P2
or make your own copy of one of his files by
.P1
cp /usr/your\(hyneighbor/his\(hyfile yourfile
.P2
.PP
If your neighbor doesn't want you poking around in his files,
or vice versa,
privacy can be arranged.
Each file and directory has read-write-execute permissions for the owner,
a group, and everyone else,
which can be set
to control access.
See
.UL ls (1)
and
.UL chmod (1)
for details.
As a matter of observed fact,
most users most of the time find openness of more
benefit than privacy.
.PP
As a final experiment with pathnames, try
.P1
ls /bin /usr/bin
.P2
Do some of the names look familiar?
When you run a program, by typing its name after the prompt character,
the system simply looks for a file of that name.
It normally looks first in your directory
(where it typically doesn't find it),
then in
.UL /bin
and finally in
.UL /usr/bin .
There is nothing magic about commands like
.UL cat
or
.UL ls ,
except that they have been collected into a couple of places to be easy to find and administer.
.PP
What if you work regularly with someone else on common information
in his directory?
You could just log in as your friend each time you want to,
but you can also say
``I want to work on his files instead of my own''.
This is done by changing the directory that you are
currently in:
.P1
cd /usr/your\(hyfriend
.P2
(On some systems,
.UL cd
is spelled
.UL chdir .)
Now when you use a filename in something like
.UL cat
or
.UL pr ,
it refers to the file in your friend's directory.
Changing directories doesn't affect any permissions associated
with a file \(em
if you couldn't access a file from your own directory,
changing to another directory won't alter that fact.
Of course,
if you forget what directory you're in, type
.P1
pwd
.P2
to find out.
.PP
It is usually convenient to arrange your own files
so that all the files related to one thing are in a directory separate
from other projects.
For example, when you write your book, you might want to keep all the text
in a directory called
.UL book .
So make one with
.P1
mkdir book
.P2
then go to it with
.P1
cd book
.P2
then start typing chapters.
The book is now found in (presumably)
.P1
/usr/your\(hyname/book
.P2
To remove the directory
.UL book ,
type
.P1
rm book/*
rmdir book
.P2
The first command removes all files from the directory;
the second
removes the empty directory.
.PP
You can go up one level in the tree of files 
by saying
.P1
cd ..
.P2
.UL .. '' ``
is the name of the parent of whatever directory you are currently in.
For completeness,
.UL . '' ``
is an alternate name
for the directory you are in.
.SH
Using Files instead of the Terminal
.PP
Most of the commands we have seen so far produce output
on the terminal;
some, like the editor, also take their input from the terminal.
It is universal in
.UC UNIX
systems
that the terminal can be replaced by a file
for either or both of input and output.
As one example,
.P1
ls
.P2
makes a list of files on your terminal.
But if you say
.P1
ls >filelist
.P2
a list of your files will be placed in the file
.UL filelist
(which
will be created if it doesn't already exist,
or overwritten if it does).
The symbol
.UL >
means ``put the output on the following file,
rather than on the terminal.''
Nothing is produced on the terminal.
As another example, you could combine
several files into one by capturing the output of
.UL cat
in a file:
.P1
cat f1 f2 f3 >temp
.P2
.PP
The symbol
.UL >>
operates very much like
.UL >
does,
except that it means
``add to the end of.''
That is,
.P1
cat f1 f2 f3 >>temp
.P2
means to concatenate
.UL f1 ,
.UL f2 
and
.UL f3
to the end of whatever is already in
.UL temp ,
instead of overwriting the existing contents.
As with
.UL > ,
if 
.UL temp
doesn't exist, it will be created for you.
.PP
In a similar way, the symbol
.UL <
means to take the input
for a program from the following file,
instead of from the terminal.
Thus, you could make up a script of commonly used editing commands
and put them into a file called
.UL script .
Then you can run the script on a file by saying
.P1
ed file <script
.P2
As another example, you can use
.UL ed
to prepare a letter in file
.UL let ,
then send it to several people with
.P1
mail adam eve mary joe <let
.P2
.SH
Pipes
.PP
One of the novel contributions of
the
.UC UNIX
system
is the idea of a
.ul
pipe.
A pipe is simply a way to connect the output of one program
to the input of another program,
so the two run as a sequence of processes \(em
a pipeline.
.PP
For example,
.P1
pr f g h
.P2
will print the files
.UL f ,
.UL g ,
and
.UL h ,
beginning each on a new page.
Suppose you want
them run together instead.
You could say
.P1
cat f g h >temp
pr <temp
rm temp
.P2
but this is more work than necessary.
Clearly what we want is to take the output of
.UL cat
and
connect it to the input of
.UL pr .
So let us use a pipe:
.P1
cat f g h | pr
.P2
The vertical bar 
.UL |
means to
take the output from
.UL cat ,
which would normally have gone to the terminal,
and put it into
.UL pr
to be neatly formatted.
.PP
There are many other examples of pipes.
For example,
.P1
ls | pr -3
.P2
prints a list of your files in three columns.
The program
.UL wc
counts the number of lines, words and characters in
its input, and as we saw earlier,
.UL who
prints a list of currently-logged on people,
one per line.
Thus
.P1
who | wc
.P2
tells how many people are logged on.
And of course
.P1
ls | wc
.P2
counts your files.
.PP
Any program
that reads from the terminal
can read from a pipe instead;
any program that writes on the terminal can drive
a pipe.
You can have as many elements in a pipeline as you wish.
.PP
Many
.UC UNIX
programs are written so that they will take their input from one or more files
if file arguments are given;
if no arguments are given they will read from the terminal,
and thus can be used in pipelines.
.UL pr
is one example:
.P1
pr -3 a b c
.P2
prints files
.UL a ,
.UL b
and
.UL c
in order in three columns.
But in
.P1
cat a b c | pr -3
.P2
.UL pr
prints the information coming down the pipeline,
still in
three columns.
.SH
The Shell
.PP
We have already mentioned once or twice the mysterious
``shell,''
which is in fact
.UL sh (1).
The shell is the program that interprets what you type as
commands and arguments.
It also looks after translating
.UL * ,
etc.,
into lists of filenames,
and
.UL < ,
.UL > ,
and
.UL |
into changes of input and output streams.
.PP
The shell has other capabilities too.
For example, you can run two programs with one command line
by separating the commands with a semicolon;
the shell recognizes the semicolon and
breaks the line into two commands.
Thus
.P1
date; who
.P2
does both commands before returning with a prompt character.
.PP
You can also have more than one program running
.ul
simultaneously
if you wish.
For example, if you are doing something time-consuming,
like the editor script
of an earlier section,
and you don't want to wait around for the results before starting something else,
you can say
.P1
ed file <script &
.P2
The ampersand at the end of a command line
says ``start this command running,
then take further commands from the terminal immediately,''
that is,
don't wait for it to complete.
Thus the script will begin,
but you can do something else at the same time.
Of course, to keep the output from interfering
with what you're doing on the terminal,
it would be better to say
.P1
ed file <script >script.out &
.P2
which saves the output lines in a file
called
.UL script.out .
.PP
When you initiate a command with
.UL & ,
the system
replies with a number
called the process number,
which identifies the command in case you later want
to stop it.
If you do, you can say
.P1
kill process\(hynumber
.P2
If you forget the process number,
the command
.UL ps
will tell you about everything you have running.
(If you are desperate,
.UL kill\ 0
will kill all your processes.)
And if you're curious about other people,
.UL ps\ a
will tell you about
.ul
all
programs that are currently running.
.PP
You can say
.P1 1
(command\(hy1; command\(hy2; command\(hy3) &
.P2
to start three commands in the background,
or you can start a background pipeline with
.P1
command\(hy1 | command\(hy2 &
.P2
.PP
Just as you can tell the editor
or some similar program to take its input
from a file instead of from the terminal,
you can tell the shell to read a file
to get commands.
(Why not? The shell, after all, is just a program,
albeit a clever one.)
For instance, suppose you want to set tabs on
your terminal, and find out the date
and who's on the system every time you log in.
Then you can put the three necessary commands
.UL tabs , (
.UL date ,
.UL who )
into a file, let's call it
.UL startup ,
and then run it with
.P1
sh startup
.P2
This says to run the shell with the file
.UL startup
as input.
The effect is as if you had typed 
the contents of
.UL startup
on the terminal.
.PP
If this is to be a regular thing,
you can eliminate the 
need to type
.UL sh :
simply type, once only, the command
.P1
chmod +x startup
.P2
and thereafter you need only say
.P1
startup
.P2
to run the sequence of commands.
The
.UL chmod (1)
command marks the file executable;
the shell recognizes this and runs it as a sequence of commands.
.PP
If you want 
.UL startup
to run automatically every time you log in,
create a file in your login directory called
.UL .profile ,
and place in it the line
.UL startup .
When the shell first gains control when you log in,
it looks for the 
.UL .profile
file and does whatever commands it finds in it.
We'll get back to the shell in the section
on programming.