While I was going through a tutorial, I saw
$() being used in a Linux Shell command and wasn’t sure what it did.
$() is a command substitution
It turns out,
$() is called a command substitution. The command in between $() or backticks (“) is run and the output replaces
$(). It can also be described as executing a command inside of another command.
I find it helpful to see this in action, so here is an example:
Example of command substitution using $() in Linux:
As you can see below, if you run the following command,
hostname -f, on any AWS EC2 Linux instance, you’ll be given the hostname of your current EC2 instance.
ubuntu@172-33-44-55:~$ hostname -f ip-172-33-44-55.us-west-2.compute.internal
So, if you add
hostname -f inside of $(), the output of the hostname -f command will replace $(hostname -f).
echo "Hello world from AWS Hostname $(hostname -f)"
This will result in the following:
Hello world from AWS Hostname ip-172-33-44-55.us-west-2.compute.internal
Again, $() is a command substitution which means that it “reassigns the output of a command or even multiple commands; it literally plugs the command output into another context” (Source).