path: root/share/doc/usd/01.begin/u2

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.SH
II.  DAY-TO-DAY USE
.SH
Creating Files \(em The Editor
.PP
If you have to type a paper or a letter or a program,
how do you get the information stored in the machine?
Most of these tasks are done with
the
.UC UNIX
``text editor''
.UL ed .
Since
.UL ed
is thoroughly documented in 
.UL ed (1) 
and explained in
.ul
A Tutorial Introduction to the UNIX Text Editor,
we won't spend any time here describing how to use it.
All we want it for right now is to make some
.ul
files.
(A file is just a collection of information stored in the machine,
a simplistic but adequate definition.)
.PP
To create a file 
called
.UL junk
with some text in it, do the following:
.P1
.ta .65i
ed junk	\fR(invokes the text editor)\f3
a	\fR(command to ``ed'', to add text)\f3
.ft I
now type in
whatever text you want ...
.ft 3
\&.	\fR(signals the end of adding text)\f3
.P2
The ``\f3.\fR'' that signals the end of adding text must be
at the beginning of a line by itself.
Don't forget it,
for until it is typed,
no other
.UL ed
commands will be recognized \(em
everything you type will be treated as text to be added.
.PP
At this point you can do various editing operations
on the text you typed in, such as correcting spelling mistakes,
rearranging paragraphs and the like.
Finally, you must write the information you have typed
into a file with the editor command
.UL w :
.P1
w
.P2
.UL ed
will respond with the number of characters it wrote
into the file 
.UL junk .
.PP
Until the
.UL w
command,
nothing is stored permanently,
so if you hang up and go home
the information is lost.\(dg
.FS
\(dg This is not strictly true \(em
if you hang up while editing, the data you were
working on is saved in a file called
.UL ed.hup ,
which you can continue with at your next session.
.FE
But after
.UL w
the information is there permanently;
you can re-access it any time by typing
.P1
ed junk
.P2
Type a
.UL q
command
to quit the editor.
(If you try to quit without writing,
.UL ed
will print a
.UL ?
to remind you.
A second
.UL q
gets you out regardless.)
.PP
Now create a second file called 
.UL temp
in the same manner.
You should now have two files,
.UL junk
and
.UL temp .
.SH
What files are out there?
.PP
The
.UL ls
(for ``list'') command lists the names
(not contents)
of any of the files that
.UC UNIX
knows about.
If you type
.P1
ls
.P2
the response will be
.P1
junk
temp
.P2
which are indeed the two files just created.
The names are sorted into alphabetical order automatically,
but other variations are possible.
For example,
the command
.P1
ls -t
.P2
causes the files to be listed in the order in which they were last changed,
most recent first.
The
.UL \-l
option gives a ``long'' listing:
.P1
ls -l
.P2
will produce something like
.P1
-rw-rw-rw-  1 bwk  users 41 Jul 22 2:56 junk
-rw-rw-rw-  1 bwk  users 78 Jul 22 2:57 temp
.P2
The date and time are of the last change to the file.
The 41 and 78 are the number of characters
(which should agree with the numbers you got from
.UL ed ).
.UL bwk
is the owner of the file, that is, the person
who created it.
.UL users
is the name of the file's group.
The
.UL \-rw\-rw\-rw\- 
tells who has permission to read and write the file,
in this case everyone.
.PP
Options can be combined:
.UL ls\ \-lt
gives the same thing as
.UL ls\ \-l ,
but sorted into time order.
You can also name the files you're interested in,
and 
.UL ls
will list the information about them only.
More details can be found in 
.UL ls (1).
.PP
The use of optional arguments that begin with a minus sign,
like
.UL \-t
and
.UL \-lt ,
is a common convention for
.UC UNIX
programs.
In general, if a program accepts such optional arguments,
they precede any filename arguments.
It is also vital that you separate the various arguments with spaces:
.UL ls\-l
is not the same as
.UL ls\ \ \-l .
.SH
Printing Files
.PP
Now that you've got a file of text,
how do you print it so people can look at it?
There are a host of programs that do that,
probably more than are needed.
.PP
One simple thing is to use the editor,
since printing is often done just before making changes anyway.
You can say
.P1
ed junk
1,$p
.P2
.UL ed
will reply with the count of the characters in 
.UL junk
and then print all the lines in the file.
After you learn how to use the editor,
you can be selective about the parts you print.
.PP
There are times when it's not feasible to use the editor for printing.
For example, there is a limit on how big a file
.UL ed
can handle
(several thousand lines).
Secondly, 
it
will only print one file at a time,
and sometimes you want to print several, one after another.
So here are a couple of alternatives.
.PP
First is
.UL cat ,
the simplest of all the printing programs.
.UL cat
simply prints on the terminal the contents of all the files
named in a list.
Thus
.P1
cat junk
.P2
prints one file, and
.P1
cat junk temp
.P2
prints two.
The files are simply concatenated (hence the name
.UL cat '') ``
onto the terminal.
.PP
.UL pr
produces formatted printouts of files.
As with 
.UL cat ,
.UL pr
prints all the files named in a list.
The difference is that it produces 
headings with date, time, page number and file name
at the top of each page,
and
extra lines to skip over the fold in the paper.
Thus,
.P1
pr junk temp
.P2
will print
.UL junk
neatly,
then skip to the top of a new page and print
.UL temp
neatly.
.PP
.UL pr
can also produce multi-column output:
.P1
pr -3 junk 
.P2
prints
.UL junk
in 3-column format.
You can use any reasonable number in place of ``3''
and 
.UL pr
will do its best.
.UL pr
has other capabilities as well;
see
.UL pr (1).
.PP
It should be noted that
.UL pr
is
.ul
not
a formatting program in the sense of shuffling lines around
and justifying margins.
The true formatters are
.UL nroff
and
.UL troff ,
which we will get to in the section on document preparation.
.PP
There are also programs that print files
on a high-speed printer.
Look in your manual under
.UL lpr .
.SH
Shuffling Files About
.PP
Now that you have some files in the file system
and some experience in printing them,
you can try bigger things.
For example,
you can move a file from one place to another
(which amounts to giving it a new name),
like this:
.P1
mv junk precious
.P2
This means that what used to be ``junk'' is now ``precious''.
If you do an
.UL ls
command now,
you will get
.P1
precious
temp
.P2
Beware that if you move a file to another one
that already exists,
the already existing contents are lost forever.
.PP
If you want
to make a
.ul
copy
of a file (that is, to have two versions of something),
you can use the 
.UL cp
command:
.P1
cp precious temp1
.P2
makes a duplicate copy of 
.UL precious
in
.UL temp1 .
.PP
Finally, when you get tired of creating and moving
files,
there is a command to remove files from the file system,
called
.UL rm .
.P1
rm temp temp1
.P2
will remove both of the files named.
.PP
You will get a warning message if one of the named files wasn't there,
but otherwise
.UL rm ,
like most
.UC UNIX
commands,
does its work silently.
There is no prompting or chatter,
and error messages are occasionally curt.
This terseness is sometimes disconcerting
to new\%comers,
but experienced users find it desirable.
.SH
What's in a Filename
.PP
So far we have used filenames without ever saying what's
a legal name,
so it's time for a couple of rules.
First, filenames are limited to 14 characters,
which is enough to be descriptive.\(dg
.FS
\(dg In  4.2 BSD the limit was extended to 255 characters.
.FE
Second, although you can use almost any character
in a filename,
common sense says you should stick to ones that are visible,
and that you should probably avoid characters that might be used
with other meanings.
We have already seen, for example,
that in the
.UL ls
command,
.UL ls\ \-t
means to list in time order.
So if you had a file whose name
was
.UL \-t ,
you would have a tough time listing it by name.
Besides the minus sign, there are other characters which
have special meaning.
To avoid pitfalls,
you would do well to 
use only letters, numbers and the period
until you're familiar with the situation.
.PP
On to some more positive suggestions.
Suppose you're typing a large document
like a book.
Logically this divides into many small pieces,
like chapters and perhaps sections.
Physically it must be divided too,
for 
.UL ed
will not handle really big files.
Thus you should type the document as a number of files.
You might have a separate file for each chapter,
called
.P1
chap1
chap2
.ft R
etc...
.P2
Or, if each chapter were broken into several files, you might have
.P1
chap1.1
chap1.2
chap1.3
\&...
chap2.1
chap2.2
\&...
.P2
You can now tell at a glance where a particular file fits into the whole.
.PP
There are advantages to a systematic naming convention which are not obvious
to the novice
.UC UNIX 
user.
What if you wanted to print the whole book?
You could say
.P1
pr chap1.1 chap1.2 chap1.3 ......
.P2
but you would get tired pretty fast, and would probably even make mistakes.
Fortunately, there is a shortcut.
You can say
.P1
pr chap*
.P2
The
.UL *
means ``anything at all,''
so this translates into ``print all files
whose names begin with 
.UL chap '',
listed in alphabetical order.
.PP
This shorthand notation
is not a property of the
.UL pr
command, by the way.
It is system-wide, a service of the program
that interprets commands
(the ``shell,''
.UL sh (1)).
Using that fact, you can see how to list the names of the files in the book:
.P1
ls chap*
.P2
produces
.P1
chap1.1
chap1.2
chap1.3
\&...
.P2
The
.UL *
is not limited to the last position in a filename \(em
it can be anywhere
and can occur several times.
Thus
.P1
rm *junk* *temp*
.P2
removes all files that contain
.UL junk
or
.UL temp
as any part of their name.
As a special case,
.UL *
by itself matches every filename,
so
.P1
pr *
.P2
prints all your files
(alphabetical order),
and
.P1
rm *
.P2
removes
.ul
all files.
(You had better be
.IT  very 
sure that's what you wanted to say!)
.PP
The
.UL *
is not 
the only pattern-matching feature available.
Suppose you want to print only chapters 1 through 4 and 9.
Then you can say
.P1
pr chap[12349]*
.P2
The
.UL [...]
means to match any of the characters inside the brackets.
A range of consecutive letters or digits can be abbreviated,
so you can also do this 
with
.P1
pr chap[1-49]*
.P2
Letters can also be used within brackets:
.UL [a\-z]
matches any character in the range
.UL a
through
.UL z .
.PP
The
.UL ?
pattern matches any single character,
so
.P1
ls ?
.P2
lists all files which have single-character names,
and
.P1
ls -l chap?.1
.P2
lists information about the first file of each chapter
.UL chap1.1 \&, (
.UL chap2.1 ,
etc.).
.PP
Of these niceties,
.UL *
is certainly the most useful,
and you should get used to it.
The others are frills, but worth knowing.
.PP
If you should ever have to turn off the special meaning
of
.UL * ,
.UL ? ,
etc.,
enclose the entire argument in single quotes,
as in
.P1
ls \(fm?\(fm
.P2
We'll see some more examples of this shortly.
.SH
What's in a Filename, Continued
.PP
When you first made that file called
.UL junk ,
how did 
the system
know that there wasn't another
.UL junk
somewhere else,
especially since the person in the next office is also
reading this tutorial?
The answer is that generally each user 
has a private
.IT directory ,
which contains only the files that belong to him.
When you log in, you are ``in'' your directory.
Unless you take special action,
when you create a new file,
it is made in the directory that you are currently in;
this is most often your own directory,
and thus the file is unrelated to any other file of the same name
that might exist in someone else's directory.
.PP
The set of all files
is organized into a (usually big) tree,
with your files located several branches into the tree.
It is possible for you to ``walk'' around this tree,
and to find any file in the system, by starting at the root
of the tree and walking along the proper set of branches.
Conversely, you can start where you are and walk toward the root.
.PP
Let's try the latter first.
The basic tools is the command
.UL pwd
(``print working directory''),
which prints the name of the directory you are currently in.
.PP
Although the details will vary according to the system you are on,
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 mental adjustment 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.\(dg
.FS
\(dg The c shell instead reads a file called 
.UL .login
.
.FE
We'll get back to the shell in the section
on programming.