UNIX Tutorial Three

3.1 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 specifing 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 copies it to the standard output (the screen) until it receives the 'end of file' (^D).

In UNIX, we can redirect both the input and the output of commands.

3.2 Redirecting the Output

We use the > symbol to redirect the output of a command to a file. For example, to create a file called list1.txt containing a list of fruit, type

$ cat > list1.txt

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.txt

To read the contents of the file, type

$ cat list1.txt

Exercise 3a

Using the above method, create another file called list2.txt containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2.txt

3.2.1 Appending to a file

The form >> appends standard output to a file. So to add more items to the file list1.txt, type

$ cat >> list1.txt

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.txt

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.txt and list2.txt into a new file called biglist.txt. Type

$ cat list1.txt list2.txt > biglist.txt

What this is doing is reading the contents of list1.txt and list2.txt in turn, then outputing the text to the file biglist.txt

To read the contents of the new file, type

$ cat biglist.txt

3.3 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.txt

and the sorted list will be output to the screen.

To output the sorted list to a file, type,

$ sort < biglist.txt > slist.txt

Use cat to read the contents of the file slist.txt

3.4 Pipes

To see which users have recently logged into this computer, type

$ last

One method to get a sorted list of names is to type,

$ last > users.txt
$ sort < users.txt

This is a bit slow and you have to remember to remove the temporary file called users.txt when you have finished. What you really want to do is connect the output of the last 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

$ last | sort

will give the same result as above, but quicker and cleaner.

To find out how many times users have logged on, type

$ last | wc -l

Exercise 3b

Using pipes, display all lines of list1.txt and list2.txt containing the letter 'p', and sort the result.

Answer available here

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
last	list recent users