# I/O Redirection in Linux

Hey there👋🏻! This article is all about Input Output Redirection in Linux. These are very useful when you want to save any output in a file or use a file as input to a program. We will also learn a bit about File Descriptor. 

### File Descriptor
>  A file descriptor is an integer number that uniquely represents open files in a system. The file descriptor for stdin, stdout and stderr is assigned as 0, 1 and 2 respectively.

When any command runs with a success then its file descriptor would be `0`. You can use `echo $?` to get the file descriptor of the previous command.
```
$ cat input
Rohini Chandra
$ echo $?
0
```

### Types of IO Redirections
There are 3 types of IO Redirects, mainly;
- Standard Input (stdin) -> Typing commands in a terminal.
- Standard Output (stdout) -> Output displayed in a terminal after executing a command.
- Standard Error (stderr) -> When a command is executed with an error. 

#### ✨Redirect Standard Output
To redirect command output to a file we use `>` and `>>` (greater than) symbols. `>` saves the output in a file. You can create the file while routing the output or use an existing one.

![o_redirect.PNG](https://cdn.hashnode.com/res/hashnode/image/upload/v1656765096604/4C-7E_Xzq.PNG align="center")

`echo` command simply displays the output in a terminal. If we want to route it to a file, we can do that using `>`. But this will replace the text if the same file is used again.  

To **append** output in an existing file, we use `>>`. Without overriding the data in an existing file, it will append the output in the same file.

![o_redirect2.PNG](https://cdn.hashnode.com/res/hashnode/image/upload/v1656765539498/NarCHWMCV.PNG align="center")

#### ✨Redirect Standard Input

To use a file as an input to a command we make use of `<` and `<<` (lesser than) symbols.
One use case is when we want to use a SQL file to populate a database.
```
mysql -u username -p database_name < file.sql
```

#### ✨Redirect Standard Error

To redirect error output to a file we use `2>` where `2` represents stderr. This means that only errors will be routed to the file.
`ls` command lists all the files and directories of a current working directory. We will make use of this command to get a better understanding.

![e_redirect.PNG](https://cdn.hashnode.com/res/hashnode/image/upload/v1656771138279/57VBq8SdS.PNG align="center")

Here in the above example, we try to list **input_directory** and **output_directory** where we don't have the output_directory directory. So, the error will be sent to the `error` file and `ls` will list the output for input_directory.

`2>&1` means error and success output will be redirected to the same file.

```
$ ls input_directory output_directory > error 2>&1
$ cat error
ls: cannot access 'output_directory': No such file or directory
input_directory:
input

```
If we want error and output in two different files,
```
ls input_directory output_directory > success 2> error
```

Thank you for reading this blog 😇!