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4.1 Dollars Expansion

Eshell has different $ expansion syntax from other shells. There are some similarities, but don't let these lull you into a false sense of familiarity.

$var
Expands to the value bound to var. This is the main way to use variables in command invocations.
$#var
Expands to the length of the value bound to var. Raises an error if the value is not a sequence (see Sequences).
$(lisp)
Expands to the result of evaluating the S-expression (lisp). On its own, this is identical to just (lisp), but with the $, it can be used in a string, such as ‘/some/path/$(lisp).txt’.
${command}
Returns the output of command, which can be any valid Eshell command invocation, and may even contain expansions.
$var[i]
Expands to the ith element of the value bound to var. If the value is a string, it will be split at whitespace to make it a list. Again, raises an error if the value is not a sequence.
$var[: i]
As above, but now splitting occurs at the colon character.
$var[: i j]
As above, but instead of returning just a string, it now returns a list of two strings. If the result is being interpolated into a larger string, this list will be flattened into one big string, with each element separated by a space.
$var["\\\\" i]
Separate on backslash characters. Actually, the first argument – if it doesn't have the form of a number, or a plain variable name – can be any regular expression. So to split on numbers, use ‘$var["[0-9]+" 10 20]’.
$var[hello]
Calls assoc on var with "hello", expecting it to be an alist (see Association Lists).
$#var[hello]
Returns the length of the cdr of the element of var who car is equal to "hello".