Could someone explain me this syntax for mapfile input redirection?
mapfile -t array < <(./inner.sh)
(from this StackOverflow answer by gniourf_gniourf)
As I understand, the first "<" is to take input from the output of whatever is on the right side of it. But what is the <( ... ) syntax? Why is the second "<" required?
The reason the linked example looks like this:
wc <(cat /usr/share/dict/linux.words)
And the mapfile answer looks like this:
mapfile -t array < <(./inner.sh)
Is because of the difference in the wc and mapfile commands and how the substituted process needs to be given to command.
As anishsane says the expansion of <(command) is a filename and can be used anywhere a filename can be used.
wc takes filenames as arguments so it can be used directly as an argument.
mapfile reads from standard input so to get it to read from a specific file you use you redirect standard input from the file </path/to/somefile but as I just said the expansion of <(command) is a filename so you can use that in the input redirection.
However, you cannot just concat the two bits directly (the way you can with a file name/path) because << is also a valid shell construct (a here document) and that would be ambiguous to the shell. So to avoid that you need to space out the two < characters and end up with < <(command) (which is analogous to < file which is perfectly legal).
wc filenamesyntax.<(...)is process substitution syntax. From logical perspective,<(...)gets expanded to a filename, & anything read from that file is the output of command between the parenthesis. You can check other examples on that page or try your own examples.