Docs for "$(cmd)" and $(cmd)

CHANGELOG.rst

@@ -9,6 +9,7 @@ Deprecations and removed features
 
 Scripting improvements
 ----------------------
+- fish's command substitution syntax has been extended: ``$(cmd)`` now has the same meaning as ``(cmd)`` but it can be used inside double quotes, to prevent line splitting of the results. (:issue:`159`).
 - ``string collect`` supports a new ``--allow-empty`` option, which will output one empty argument in a command substitution that has no output (:issue:`8054`). This allows commands like ``test -n (echo -n | string collect --allow-empty)`` to work more reliably.
 
 Interactive improvements

doc_src/cmds/string-collect.rst

@@ -19,9 +19,11 @@ Description
 
 ``string collect`` collects its input into a single output argument, without splitting the output when used in a command substitution. This is useful when trying to collect multiline output from another command into a variable. Exit status: 0 if any output argument is non-empty, or 1 otherwise.
 
+A command like ``echo (cmd | string collect)`` is mostly equivalent to a quoted command substitution (``echo "$(cmd)"``). The main difference is that the former evaluates to zero or one elements whereas the quoted command substitution always evaluates to one element due to string interpolation.
+
 If invoked with multiple arguments instead of input, ``string collect`` preserves each argument separately, where the number of output arguments is equal to the number of arguments given to ``string collect``.
 
-Any trailing newlines on the input are trimmed, just as with ``"$(cmd)"`` substitution in sh. ``--no-trim-newlines`` can be used to disable this behavior, which may be useful when running a command such as ``set contents (cat filename | string collect -N)``.
+Any trailing newlines on the input are trimmed, just as with ``"$(cmd)"`` substitution. Use ``--no-trim-newlines`` to disable this behavior, which may be useful when running a command such as ``set contents (cat filename | string collect -N)``.
 
 With ``--allow-empty``, ``string collect`` always prints one (empty) argument. This can be used to prevent an argument from disappearing.
 
@@ -34,6 +36,11 @@ Examples
 
 ::
 
+    >_ echo "zero $(echo one\ntwo\nthree) four"
+    zero one
+    two
+    three four
+
     >_ echo \"(echo one\ntwo\nthree | string collect)\"
     "one
     two

doc_src/language.rst

@@ -569,6 +569,8 @@ The output of a command (or an entire :ref:`pipeline <pipes>`) can be used as th
 
 When you write a command in parenthesis like ``outercommand (innercommand)``, the ``innercommand`` will be executed first. Its output will be taken and each line given as a separate argument to ``outercommand``, which will then be executed. [#]_
 
+A command substitution can have a dollar sign before the opening parenthesis like ``outercommand $(innercommand)``. This variant is also allowed inside double quotes. When using double quotes, the command output is not split up by lines.
+
 If the output is piped to :ref:`string split or string split0 <cmd-string-split>` as the last step, those splits are used as they appear instead of splitting lines.
 
 The exit status of the last run command substitution is available in the :ref:`status <variables-status>` variable if the substitution happens in the context of a :ref:`set <cmd-set>` command (so ``if set -l (something)`` checks if ``something`` returned true).
@@ -588,7 +590,7 @@ Examples::
 
     # Set the ``data`` variable to the contents of 'data.txt'
     # without splitting it into a list.
-    begin; set -l IFS; set data (cat data.txt); end
+    set data "$(cat data.txt)"
 
     # Set ``$data`` to the contents of data, splitting on NUL-bytes.
     set data (cat data | string split0)

doc_src/tutorial.rst

@@ -400,11 +400,11 @@ Unlike other shells, fish does not split command substitutions on any whitespace
     -lgobject-2.0
     -lglib-2.0
 
-If you need a command substitutions output as one argument, without any splits, use ``string collect``::
+If you need a command substitutions output as one argument, without any splits, use quoted command substitution::
 
     > echo "first line
     second line" > myfile
-    > set myfile (cat myfile | string collect)
+    > set myfile "$(cat myfile)"
     > printf '|%s|' $myfile
     |first line
     second line|