We use the > symbol to redirect the output of a command. For ex####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 catwithout 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. ample, 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
As exercise, using the above method, try to 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
Notice that > writes over the content of the file, while >> writes at the end of the content of the file.