Redirection
Most processes initiated by UNIX
commands write to the standard output (that is, they write to the terminal
screen), and many take their input from the standard input (that is, they read
it from the keyboard). There is also the standard error, where processes write
their error messages, by default, to the terminal screen.
We have already seen one use
of the cat command
to write the contents of a file to the screen.
Now type cat without specifying a file to read
% cat
Then type a few words on the
keyboard and press the [Return] key.
Finally hold the [Ctrl]
key down and press [d] (written as ^D for short) to end the input.
What has happened?
If you run the cat command without specifing a file to read,
it reads the standard input (the keyboard), and on receiving the 'end of file'
(^D), copies it to the standard output (the screen).
In UNIX, we can redirect both
the input and the output of commands.
Redirecting the Output
We use the > symbol to
redirect the output of a command. For example, to create a file called list1 containing a list of fruit, type
% cat >
list1
Then type in the names of
some fruit. Press [Return] after each one.
pear
banana
apple
^D {this means press [Ctrl] and [d] to stop}
What happens is the cat
command reads the standard input (the keyboard) and the > redirects the
output, which normally goes to the screen, into a file called list1
To read the contents of the
file, type
% cat list1
For Practice
Using the above method,
create another file called list2 containing the following fruit:
orange, plum, mango, grapefruit. Read the contents of list2
Appending to a
file
The form >> appends
standard output to a file. So to add more items to the file list1,
type
% cat >>
list1
Then type in the names of
more fruit
peach
grape
orange
^D (Control D to stop)
To read the contents of the
file, type
% cat list1
You should now have two
files. One contains six fruit, the other contains four fruit.
We will now use the cat
command to join (concatenate) list1 and list2 into a new file called biglist.
Type
% cat list1 list2
> biglist
What this is doing is reading
the contents of list1 and list2 in turn, then outputing the text to
the file biglist
To read the contents of the
new file, type
% cat biglist
Redirecting the Input
We use the < symbol to
redirect the input of a command.
The command sort
alphabetically or numerically sorts a list. Type
% sort
Then type in the names of
some animals. Press [Return] after each one.
dog
cat
bird
ape
^D (control d to stop)
The output will be
ape
bird
cat
dog
Using < you can redirect
the input to come from a file rather than the keyboard. For example, to sort
the list of fruit, type
% sort <
biglist
and the sorted list will be
output to the screen.
To output the sorted list to
a file, type,
% sort <
biglist > slist
Use cat to read the contents
of the file slist
Pipes
To see who is on the system
with you, type
% who
One method to get a sorted
list of names is to type,
% who >
names.txt
% sort < names.txt
This is a bit slow and you
have to remember to remove the temporary file called names when you have
finished. What you really want to do is connect the output of the who command
directly to the input of the sort command. This is exactly what pipes do. The
symbol for a pipe is the vertical bar |
For example, typing
% who | sort
will give the same result as
above, but quicker and cleaner.
To find out how many users
are logged on, type
% who | wc -l
For Practice
Using pipes, display all
lines of list1 and list2 containing the letter 'p', and sort
the result.
Summary
|
Command
|
Meaning
|
|
command > file
|
redirect standard output to a file
|
|
command >> file
|
append standard output to a file
|
|
command < file
|
redirect standard input from a file
|
|
command1 | command2
|
pipe the output of command1 to the input of command2
|
|
cat file1 file2 > file0
|
concatenate file1 and file2 to file0
|
|
sort
|
sort data
|
|
who
|
list users currently logged in
|
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