<(command) is being replaced by a path to a file which contains the output of the command: $ ls <(echo A) /dev/fd/63 $ cat <(echo A) A So diff <(c

<(command) is being replaced by a path to a file which contains the output of the command: $ ls <(echo A) /dev/fd/63 $ cat <(echo A) A So diff <(command1) <(command2) becomes diff /dev/fd/62 /dev/fd/63