What is $()
In a shell script or terminal command, the
$() syntax is known as “command substitution.” It allows you to execute a command and use its output as part of another command or assigned to a variable. In the context of
$(docker ps -q), it executes the
docker ps -q command and captures its output (the container IDs of running containers in this case). Therefore we can combine shell commands.
For example, if
docker ps -q returns two container IDs:
docker stop $(docker ps -q) is equivalent to running
docker stop 1234abcd 5678efgh. The command substitution allows you to dynamically pass the output of one command as an argument to another command.