Introduction
Bash brace expansion is an incredibly valuable feature once you commit it to memory and begin using it on a regular basis. It will save you immense amounts of typing. This also works in Zsh.
How it works
When you provides a set of braces {} with comma-separated values inside,
Bash will expand each one into a separate argument.
The simple example to demonstrate this is using echo like this:
echo Hello {Jack,Jill,Bill}
# Outputs: Hello Jack Jill Bill
echo Hello Ja{ke,net,cob,red}
# Outputs: Hello Jake Janet Jacob Jared
echo Hello S{u,uzann,uzi}e
# Outputs: Hello Sue Suzanne Suzie
echo Hello {J,K,T}immy
# Outputs: Hello Jimmy Kimmy Timmy
The brace expansion will take the current word that the braces are in, and expand it to create a unique word for each value inside the braces.
You can also have it auto increment numbers and characters. For example
echo {1..10}
# Outputs: 1 2 3 4 5 6 7 8 9 10
echo {1..10..2} # Increment by 2 each time
# Outputs: 1 3 5 7 9
echo {a..e}
# Outputs: a b c d e
echo {a..z..2} # a through z incrementing by 2 each time
# Outputs: a c e g i k m o q s u w y
echo {A..z..2} # Capital letters come before lowercase in ASCII
# Outputs: A C E G I K M O Q S U W Y [ ] _ a c e g i k m o q s u w y
Practical examples
Here are a few practical examples of how I use brace expansion regularly.
Changing a file extension
# Change a .txt to a .md
mv myfile.{txt,md}
Rename directory for backup
# Add .backup to a directory name
mv /my/directory{,.backup}
Installing similarly named packages
# Installs Java JRE + JDK + JavaFX on Fedora
sudo yum install java-1.8.0-openjdk{,-devel,-openjfx}
# Install multiple PHP packages on Ubuntu
sudo apt install php-{common,mbstring,mysql}
Make several directories at once
mkdir -p $HOME/my_new_project/{src,docs,build}
Conclusion
You should have a basic understanding of how Bash brace expansion works and how you can apply it to some common tasks. What other kind of uses can you think of?