More About Files

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. 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 specifying 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 as follows. Press [Return] after each one. Terminate with the end-of-file marker control-d (^D).

pear 
banana 
apple 
^D

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 3A

Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. 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 
strawberry
^D

To read the contents of the file, type

% cat list1

You should now have two files. One contains six fruit names, the other contains four fruits. We will now use the cat command to join (concatenate) list1 and list2 into a new file called biglist. Type

% cat list1 list2 > biglist

This reads the contents of list1 and list2 in turn, then outputs 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

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.txt 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 | which, on a US keyboard, is above Enter on the right, with the backslash. 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, use wc (word count) with the option -l (ell) for number of lines only:

% who | wc -l

Exercise 3B

Using pipes, print all lines of list1 and list2 containing the letter ‘p’, sort the result, and print to a file sorted_plist.txt. Hint: from grep --help find an option to print only the line, omitting the file name.

grep -h p list? | sort > sorted_plist.txt

Summary