It is called process substitution and specific to Bash (not POSIX shell compliant). The output of the commands in parenthesis will become runtime (file) inputs

It is called process substitution and specific to Bash (not POSIX shell compliant). The output of the commands in parenthesis will become runtime (file) inputs for the diff command. More: https://www.gnu.org/software/bash/manual/html_node/Process-Substitution.html https://www.gnu.org/software/bash/manual/html_node/Process-S...