Writing Comments in Bash Scripts
In Bash everything after the hash mark (#) and until the end of the line is considered to be a comment. Adding comments to your Bash scripts will save you a lot of time and efforts when you look at your code in the future.
Pushd and Popd Commands in Linux
pushd and popd are commands that allow you to work with directory stack and change the current working directory in Linux and other Unix-like operating systems.
How to Increment and Decrement Variable in Bash (Counter)
One of the most common arithmetic operations when writing Bash scripts is incrementing and decrementing variables. This is most often used in loops as a counter, but it can occur elsewhere in the script as well.
The sequence of characters (#!) is called shebang and is used to tell the operating system which interpreter to use to parse the rest of the file.