LINUX Tutorial Three

3.1 Redirection  

Most processes initiated by LINUX 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 [Enter] 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 'return' key or the 'end of file' (^D), copies it to the standard output (the screen).

In LINUX, 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. For example, move to your linuxstuff directory. Then, to create a file called list1 containing a list of fruit, type  

# cat > list1

Then type in the names of some fruit. Press [Enter] 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 3a

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

The symbol >> 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

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 vegetables. Press [Enter] 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.

3.4 Pipes

We have already seen the ls command for getting a file listing.

# ls

By default, this returns the files in sorted order, however the sorting is case-sensitive.

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

# ls > files.txt
# sort -f < files.txt

The -f flag in the sort -f command specifies that we will ignore case. However, this redirection approach is a bit slow and you have to remember to remove the temporary file called files.txt when you have finished. What you really want to do is connect the output of the ls 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

# ls | sort -f

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

To find out how many (non-hidden) files are in the current directory, type

# ls | wc -l

Note that a single command can apply multiple pipes. For example,

# cat list1 list2 | grep "p" | wc -l

will display the number of lines containing the letter "p" in files list1 and list2.

Exercise 3b

Using pipes, print all lines of /var/log/yum.log containing the word "fire", and sort the result.

Summary

Last Modified: January 2019