LINUX Tutorial Four

4.1 Redirection  

The standard for most processes initiated by LINUX commands is write to the standard output (that is, they write to the terminal screen), and take their input from the standard input (that is, they read it from the keyboard).
There is also a third output channel, called 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 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.

windows In Windows the 'End of File' is [Ctrl+Z] .
EOF End of file is Often Written as EOF
As everything in LINUX is a file we can redirect to a socket(internet), printer, alternative screen, etc!

4.2 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 (Control 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

Exercise 4a

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

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

4.3 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 vegetables. Press [Return] after each one.

carrot
beetroot
artichoke
^D (control d to stop)

The output will be

artichoke
beetroot
carrot

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

4.4 Redirecting the Errors  

We use the 2> symbols to redirect the errors of a command. We can also use the symbols 2>> to redirect the output by appending the output to th eend of a file. For example, the ls command will report an error if the file to be listed is not found or for instance if the command does not have the permission to list the file.
Type  

% ls xxx > lsErrors

% ls /root >> lsErrors

What happens is the ls command can't find the file xxx and reports and error which is redirected to the file lsErrors. The /root directory is usually a protected directory which a regular user does not have permission to list, hence this command will also report an error which is redirected to the lsErrors file

To read the contents of the file, type

% cat lsErrors

The output will be

ls: cannot access xxx: No such file or directory
ls: cannot open directory /root: Permission denied

4.5 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

Efficiency In the who followed by sort example the commands are executed sequentially. A pipe however executes the commands concurrently. This can improve efficiency.

To download a news feed as a single file, type

% curl http://feeds.bbci.co.uk/news/world/rss.xml

To discover if Portugal is mentioned in the news we can use the grep command

% curl http://feeds.bbci.co.uk/news/world/rss.xml > tmp
% grep Portugal < tmp ; rm tmp

This can be very slow as we have to wait for the entre file to download before searching for Portugal. What you really want to do is connect the output of the curl command directly to the input of the grep command and have both commands operating concurrently so that as soon as reference to Portugal has been downloaded it wll reported by grep on the screen.

Pipes do this so we can simply do

% curl http://feeds.bbci.co.uk/news/world/rss.xml | grep Portugal

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

Exercise 4b

a2ps -Phockney textfile is the command to print a postscript file to the printer hockney.

Using pipes, print all lines of list1 and list2 containing the letter 'p', sort the result, and print to the printer hockney.

Answer available here

Summary