Saturday, 15 October 2016

Difference between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1


If you are a regular Linux user then you might already know of this or atleast seen these your shell scripts. For eg.
  • ls -a 2>/dev/null
These are called redirection. There are various other functions for redirections and in this post we are going to see and understand them.

File descriptors

In simple words, when you open a file, the operating system creates an entry to represent that file and store the information about that opened file. So if there are 100 files opened in your OS then there will be 100 entries in OS (somewhere in kernel). These entries are represented by integers like (...100, 101, 102....). This entry number is the file descriptor. So it is just an integer number that uniquely represents an opened file in operating system. If your process opens 10 files then your Process table will have 10 entries for file descriptors.

File descriptors  0,1 and 2 are generally given to std input,  std output and std error streams for a process. So you cannot generally use them for anything else. 
At the file descriptor level, 
  • stdin is defined to be file descriptor 0
  • stdout is defined to be file descriptor 1, and 
  • stderr is defined to be file descriptor 2.

 NOTE : The FD (File descriptor) table in per process table. So 2 different process can have same FD on calling open(). This is abstraction on user side. On kernel side of things each opened file has a unique FD and two open files cannot have same FD.  Please don't get confused.

Now that we know standard file descriptors created for each table lets see the redirection functions.

Redirection Basics

The > operator redirects the output usually to a file but it can be to a device. You can also use >> to append.

  •  > file redirects stdout to file
  • 1> file redirects stdout to file
  • 2> file redirects stderr to file
  • &> file redirects stdout and stderr to file

NOTE : If you don't specify a number then the standard output stream is assumed but you can also redirect errors

NOTE :  As mentioned before you can use >> instead of > to append . 


/dev/null is the null device it takes any input you want and throws it away. It can be used to suppress any output. 

Long story short - If you want any stream ignored redirect it to /dev/null

Now that we know the basics lets understand the functions
  • 2>/dev/null : This means redirect stderr of the process to /dev/null which essentially mean discard it.
  • 2>&- : This will essentially close the output of stderr for a process.
  • 2>&1: This combines stdout and stderr into single stream.
  • |& : This is just an abbreviation for 2>&1 | (Added in bash4)
  • &>/dev/null : This is an abbreviation for > /dev/null 2>&1 which essentially means combine stdout and stderr and redirect it to /dev/null (discard it)
  • >/dev/null : This is again an abbreviation for 1 > /dev/null which means discard standard output. Incase you missed my point sometime back if you do not give a FD standard output stream is assumed.

Have written a script that attempts to recover accidentally deleted files that makes use of redirections -

 Related Links

How to associate a static IP to your AWS EC2 instance using Elastic IP Addresses


When you start up an EC2 instance by default you get a public IP and a public DNS by which you can reach your server from internet. However when you restart your EC2 instance your IP and your public DNS (which is infact your hostname) gets changed and that may not always be way you desire. So in this post we will see how to associate a static IP to your AWS EC2 instance using Elastic IP Addresses.

Elastic IP Addresses

An Elastic IP address is a static IP address designed for dynamic cloud computing. An Elastic IP address is associated with your AWS account. With an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account. 

An Elastic IP address is a public IP address, which is reachable from the Internet. If your instance does not have a public IP address, you can associate an Elastic IP address with your instance to enable communication with the Internet; for example, to connect to your instance from your local computer.

How to associate a static IP to your AWS EC2 instance using Elastic IP Addresses

If you have an EC2 instance running you can see the current public IP address and your public DNS entry.

And you can reach your EC2 instance using (highlighted in image above)For eg I have deployed a webapp I can access it using URL - 
Please note these URLs are for demo purpose and may not be live when you are reading this. This is just to give you idea about what changes when you assign an elastic IP address.

As I mentioned earlier this will change when you reboot your EC2 instance. Lets see how we can assign an elastic IP.

Go to Elastic IP under Network & Security and select "Allocate New Address" and confirm.

You can see a new entry in the list with your newly assigned elastic IP. Select the entry , go to actions and select "Associate Address"

You can select either your instance your your network interface. I have selected my running EC2 instance. Then click associate.

NOTE : Careful! Note you will get a new public IP and public DNS (you will loose the previous non static one).

You will see the new IP, public DNS and Elastic Ip in EC2 dashboard.

That's it! You can now access your EC2 instance with this new IP and public DNS.  For eg -
Note how the IP has changed and what was before. And this will remain same even when you reboot!

PS : WebDynamo is a sameple webapp ( that I have deployed on tomcat on my EC2 instance for demo purpose. Please refer links in Related Links section below for more details.

Related Links

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