I saw the syntax in this question: Birth is empty on ext4
sudo debugfs -R "stat <$(stat -c %i /home/user/path/to/file)>" /dev/sda5
I know command substitution $() and ``.
I kind of vaguely know the <() and >() syntax.
But I've never seen <$()>.
So what does that do?
<$()> would be valid bash syntax for commands that read from a file
(< part) where file names are created using command substitution
($() part) that redirect output to some other file (>
part). Example:
$ echo text > FILE
$ wc < "$(echo FILE)" > WC_OUT
$ cat WC_OUT
1 1 5
However, in the answer you linked to <$()> is used in the parameters
to debugfs. In man debugfs it says:
stat filespec
Display the contents of the inode structure of the inode filespec.
and:
The filespec argument may be specified in two forms. The first form is an inode number surrounded by angle brackets, e.g., <2>
In that case:
sudo debugfs -R "stat <$(stat -c %i FILE)>" /dev/sda5
would be expanded by the shell to the equivalent of this:
sudo debugfs -R "stat <4476834>" /dev/sda5
which is a valid debugfs command. Check it with echo:
$ echo sudo debugfs -R "stat <$(stat -c %i FILE)>" /dev/sda5
sudo debugfs -R stat <4476834> /dev/sda5
debugfs since I assumed that no bash command would parse an argument as <123456789>.
Commented
Feb 22, 2020 at 18:37
debugfs is not Bash commad, that is it's not Bash built-in command. It's an external program that could run in all shells, even in these that do not implement < and > and on multiple operating systems.
Commented
Feb 22, 2020 at 18:49
< >is being passed todebugfs.<$(...)>would be shell syntax (not meaningful or valid, but shell syntax nonetheless) -- but inside double quotes,<and>are literal characters; the shell leaves them alone.