Redirections and file descriptors in bash

Following are some redirection options in bash that are not widely used but are very useful

- To redirect both stdout and stdin to a file in bash
     echo tan &> filename 
 OR
     echo tan > filename 2>&1

- To redirect stdout of a command to stderr
       echo tan 2>&1

- Following command will open a file for reading and writing and assign the interger file description N to it. If the file does not exist then it will be created
    9<>filename
Following is an example of how you can use this
     exec 9<>temp     # open file temp and assign file descriptor 9 to it
     echo aman >&9  # Write aman to the file
     exec 9>&-         # Close the file descriptor Now read the file and it will contain string aman
     cat temp

Before closing the file descriptor, you can see it opened

    cd /proc/$$/fd 



Closing File Descriptors

n<&-

Close input file descriptor n.

0<&-, <&-

Close stdin.

n>&-

Close output file descriptor n.

1>&-, >&-

Close stdout