What Does <() Do in Bash

What does (command args) mean in the shell?

<() is called process substitution in the manual, and is similar to a pipe but passes an argument of the form /dev/fd/63 instead of using stdin.

< reads the input from a file named on command line.

Together, these two operators function exactly like a pipe, so it could be rewritten as

find /bar -name *foo* -print0 | while read line; do
  ...
done

Difference between ${} and $() in Bash

The syntax is token-level, so the meaning of the dollar sign depends on the token it's in. The expression $(command) is a modern synonym for `command` which stands for command substitution; it means run command and put its output here. So

echo "Today is $(date). A fine day."

will run the date command and include its output in the argument to echo. The parentheses are unrelated to the syntax for running a command in a subshell, although they have something in common (the command substitution also runs in a separate subshell).

By contrast, ${variable} is just a disambiguation mechanism, so you can say ${var}text when you mean the contents of the variable var, followed by text (as opposed to $vartext which means the contents of the variable vartext).

The while loop expects a single argument which should evaluate to true or false (or actually multiple, where the last one's truth value is examined -- thanks Jonathan Leffler for pointing this out); when it's false, the loop is no longer executed. The for loop iterates over a list of items and binds each to a loop variable in turn; the syntax you refer to is one (rather generalized) way to express a loop over a range of arithmetic values.

A for loop like that can be rephrased as a while loop. The expression

for ((init; check; step)); do
    body
done

is equivalent to

init
while check; do
    body
    step
done

It makes sense to keep all the loop control in one place for legibility; but as you can see when it's expressed like this, the for loop does quite a bit more than the while loop.

Of course, this syntax is Bash-specific; classic Bourne shell only has

for variable in token1 token2 ...; do

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(Somewhat more elegantly, you could avoid the echo in the first example as long as you are sure that your argument string doesn't contain any % format codes:

date +'Today is %c. A fine day.'

Avoiding a process where you can is an important consideration, even though it doesn't make a lot of difference in this isolated example.)

How does cat EOF work in bash?

This is called heredoc format to provide a string into stdin. See https://en.wikipedia.org/wiki/Here_document#Unix_shells for more details.

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From man bash:

Here Documents

This type of redirection instructs the shell to read input from
the current source until a line
containing only word (with no trailing
blanks) is seen.

All of the lines read up to that point are then used as the
standard input for a command.

The format of here-documents is:

          <<[-]word
                  here-document
          delimiter

No parameter expansion, command substitution, arithmetic expansion, or
pathname expansion is performed on
word. If any characters in word are
quoted, the
delimiter is the result of quote removal on word, and the lines
in the here-document are not expanded.
If word is unquoted, all lines of the
here-document are subjected to parameter expansion, command
substitution, and arithmetic
expansion. In the latter case, the
character sequence \<newline> is
ignored, and \ must be used to quote the characters \, $, and `.

If the redirection operator is <<-, then all leading tab characters
are stripped from input lines and the
line containing delimiter. This
allows here-documents within shell scripts to be indented in a natural fashion.

What is the done function/meaning in the bash while loop?

Intuitively, I thought the done is some kind of boundary maker that told you the while field is over.

Correct; done ends a while-loop (or for-loop or similar).

Here it show that done can take the data from a variable.

No; the < $input (meaning < name.csv) is a redirection being applied to the whole while-loop. So it's the while-loop as a whole, not the done specifically, that takes input from name.csv.

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

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.

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