What is the name for `<(...)` 'operator' in bash?

Solution 1:

This is called process substitution:

Process substitution is a form of redirection where the input or output of a process (some sequence of commands) appear as a temporary file.

Also from Bash Reference Manual → 3.5.6 Process Substitution:

Process substitution allows a process’s input or output to be referred to using a filename. It takes the form of

<(list)

or

>(list)

The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection. Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

Solution 2:

Also known as 'anonymous FIFO'.