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Redirection in Linux and Unix


 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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