bash: <<< Here string notation

Bash has a <<< operator to redirect a single string to stdin of a command:

tr -d o <<< "Hello world"
# Hell wrld

That is equivalent to echoing that string and piping it to the command:

echo "Hello world" | tr -d o
# Hell wrld

The supplied text undergoes expansion so can include command output if you like:

tr -d o <<< "$(date) :: Hello world"
# Wed 17 May 2023 14:51:15 BST :: Hell wrld