Skip to main content

Redirection in Linux and Unix


 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 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 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.

 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 {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

For Practice

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

Appending to a file

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

 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
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 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

For Practice

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

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
who
list users currently logged in



Comments

Popular posts from this blog

CentOS / Redhat : Configure CentOS as a Software Router with two interfaces

Linux can be easily configured to share an internet connection using iptables. All you need to have is, two network interface cards as follows: a) Your internal (LAN) network connected via eth0 with static ip address 192.168.0.1 b) Your external WAN) network is connected via eth1 with static ip address 10.10.10.1  ( public IP provided by ISP ) Please note that interface eth1 may have public IP address or IP assigned by ISP. eth1 may be connected to a dedicated DSL / ADSL / WAN / Cable router: Step # 1: Enable Packet Forwarding Login as the root user. Open /etc/sysctl.conf file # vi /etc/sysctl.conf Add the following line to enable packet forwarding for IPv4: net.ipv4.conf.default.forwarding=1 Save and close the file. Restart networking: # service network restart Step # 2: Enable IP masquerading In Linux networking, Network Address Translation (NAT) or Network Masquerading (IP Masquerading) is a technique of transceivin

Virtual Box and Alt/Tab Keys

I use virtual box for all my testing activities. It comes too often that I have a virtual box VM window open & I want to switch to my host machine to see some stuff like tutorials etc.. If you press the alt+tab combination it just works inside the VM & doesn't switches to host machine. In these scenarios you can press the host key once ( not hold it ) & then whatever you press goes to host machine. So in general where host key is the default Right Ctrl, just press Right Ctrl once & now press the alt+tab & it will switch you out to host machine. This is really helpful when you have the VM windows open or you're working on seamless mode. Hope it help others too.

Set date and time in Linux

There are few ways to set the date and time on Linux command line. In order to do this, you must login as root and execute the following methods as follow: For you to remember the syntax, issue the command “date” first [root@linuxtechtips ~]# date Mon Aug 20 18:30:29 SGT 2012 Let say you want to change it to Sept 6, 2012, 3pm, just follow the pattern above [root@linuxtechtips ~]# date 090615002012 Thu Sep  6 15:00:00 SGT 2012 where as: 09 = month (September) 06 = day 15 = hour 00 = min 2012 = year Now it’s set, as simple as that: [root@linuxtechtips ~]# date Thu Sep  6 15:00:01 SGT 2012 Another example, you want it to change to 20th of December, 2012, 10:45pm [root@linuxtechtips ~]# date 122022452012 Thu Dec 20 22:45:00 SGT 2012 Viola!!! [root@linuxtechtips ~]# date Thu Dec 20 22:45:03 SGT 2012 Now if you want to challenge yourself, then you can use this as well: Using our example date above, use the date comman