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.
$varExpands to the value bound to var. This is the main way to use variables in command invocations.
$"var"$'var'Expands to the value bound to var. This is useful to disambiguate the variable name when concatenating it with another value, such as ‘$"var"-suffix’.
$#varExpands to the length of the value bound to var. Raises an error if the value is not a sequence (see Sequences in The Emacs Lisp Reference Manual).
$(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.
$<command>As with ‘${command}’, evaluates the Eshell command invocation
command, but writes the output to a temporary file and
returns the file name.
$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 in The Emacs Lisp Reference Manual).
$#var[hello]Returns the length of the cdr of the element of var whose
car is equal to "hello".