File redirection in bash

Question

What's the difference between using > and >>?

How can I use both < and > (redirection operators) in a single command?

Answer 1

> redirects data from stdout (read standard out) to a file.

ls -a > my-files.txt

will take the output of ls -a and put it into a file named my-files.txt deleting/overwriting the file if it exists. Using >> instead of > in the example above will not overwrite the file if it exists but add the output of ls -a to the end of my-files.txt

< on the other hand sends data to a programs stdin (read standard in). If I have a file called my-files.txtand I want to know how many words it contains I can send it to a program called wc. This program accepts data on it's stdin so to the data to it I do

wc -w < my-files.txt

And lastly If I want to save the output of that command to a new file I can use both like so

wc -w < my-files.txt > wordcount.txt

Answer 2

Try,
Advanced Bash-Scripting Guide
An in-depth exploration of the art of shell scripting

1. Special Characters, Quoting
2. IO Redirection
3. Introduction to Variables and Parameters