$() vs () in Bash

In Bash, there are two ways to execute commands within a command substitution context: $() and ().

$()

The $(command) syntax is known as command substitution. It allows the output of a command to be substituted as an argument to another command or assigned to a variable.

For example,

echo "The current working directory is $(pwd)."

This will output "The current working directory is /path/to/current/directory."

()

The command in $(command) can also be written inside a subshell using () instead of $().

output=$(echo "hello, world")
echo "$output"

This is equivalent to:

output=`echo "hello, world"`
echo "$output"

The ( and ) within the subshell run the commands inside the subshell in a separate environment from the outer shell.

When to use $() vs ()

Use $() when you want to capture the output of a command and use it in another command or assign it to a variable.

Use () when you want to group commands together and run them in a separate environment as a unit.

Conclusion

Understanding the differences between $() and () can help simplify writing bash scripts and executing commands in the terminal. It is a powerful feature of Bash and can be a time saver for developers.