Understanding the opening, writing, and closing of a file

Let's understand the opening, closing, and writing of a file.

Write a Shell script file_01.sh, shown as follows:

# We will open file for writing purpose 
# We are assigning descriptor number 3 for file sample_out.txt 
exec 3> sample_out.txt 
# We are sending output of command "echo" to sample_out.txt file 
echo "This is a test message for sample_out.txt file" >&3 
# Run command date & store output in file sample_out.txt 
date >&3 
# Closing file with file descriptor 3 
exec 3<&- 

Save the file, give the permission to execute, and run the script as follows:

    $ chmod u+x file_01.sh
    $ ./file_01.sh
    $ cat sample_out.txt

This should produce the following output:

    This is a test message ...

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