Since 'eshell-named-command-hook' already makes execution of commands
extensible, "which" should be too.  This makes sure that "which" returns
the right result for quoted commands like "/:cat".

* lisp/eshell/em-alias.el (eshell-aliases-file): Allow it to be nil.
(eshell-read-aliases-list, eshell-write-aliases-list): Check if
'eshell-aliases-file' is nil.
(eshell-maybe-replace-by-alias--which): New function...
(eshell-maybe-replace-by-alias): ... use it.

* lisp/eshell/esh-cmd.el (eshell-named-command-hook): Update docstring.
(eshell/which): Make extensible.
(eshell--find-plain-lisp-command, eshell-plain-command--which): New
functions.
(eshell-plain-command): Use 'eshell--find-plain-lisp-command'.

* lisp/eshell/esh-ext.el (eshell-explicit-command--which): New
function...
(eshell-explicit-command): ... unise it.
(eshell-quoted-file-command--which): New function...
(eshell-quoted-file-command): ... use it.
(eshell-external-command--which): New function.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/which/plain/eshell-builtin)
(esh-cmd-test/which/plain/external-program)
(esh-cmd-test/which/plain/not-found, esh-cmd-test/which/alias)
(esh-cmd-test/which/explicit, esh-cmd-test/which/explicit/not-found)
(esh-cmd-test/which/quoted-file)
(esh-cmd-test/which/quoted-file/not-found): New tests.

* test/lisp/eshell/eshell-tests-helpers.el (with-temp-eshell-settings):
Don't load or save aliases.
(eshell-command-result--match,eshell-command-result--match-explainer)
(eshell-command-result-match): New functions.

* lisp/eshell/em-script.el (eshell-source-file): Make obsolete.
(eshell--source-file): Adapt from 'eshell-source-file'...
(eshell-script-initialize, eshell/source, eshell/.): ... use it.
(eshell-princ-target): New struct.
(eshell-output-object-to-target, eshell-target-line-oriented-p): New
implementations for 'eshell-princ-target'.
(eshell-execute-file, eshell-batch-file): New functions.

* lisp/eshell/esh-mode.el (eshell-mode): Just warn if we can't create
the Eshell directory.

* test/lisp/eshell/em-script-tests.el (em-script-test/execute-file):
(em-script-test/execute-file/args), em-script-test/batch-file): New
tests.

* test/lisp/eshell/eshell-tests-helpers.el (with-temp-eshell-settings):
New function...
(with-temp-eshell): ... use it.

* doc/misc/eshell.texi (Control Flow): Update documentation.

* etc/NEWS: Announce this change (bug#70847).

This was documented to work by calling the output function with 'nil',
but that was never actually implemented.  Instead, for compatibility,
we now support a new (optional) close function.

* lisp/eshell/esh-io.el (eshell-virtual-targets): Update docstring.
(eshell-generic-target): New struct...
(eshell-function-target): ... inherit from it, and rename from
'eshell-virtual-target'.
(eshell-get-target): Handle already-created 'eshell-generic-target'.
(eshell-close-target): Call the target's close function if present.

* test/lisp/eshell/esh-io-tests.el (esh-io-test/virtual/device-close):
New test.

* doc/misc/eshell.texi (Redirection): Document the new behavior.

This also adds the ability to suppress module loading/unloading
messages, which will be necessary to support running Eshell scripts as
batch scripts.

* lisp/eshell/esh-mode.el (eshell-mode): Move module
loading/initialization to...

* lisp/eshell/esh-module.el (eshell-load-modules)
(eshell-initialize-modules): ... here.
(eshell-module-loading-messages): New option.
(eshell-module--feature-name): Improve docstring.
(eshell-unload-modules): Display a real warning if unable to unload a
module.

* test/lisp/eshell/eshell-tests-helpers.el (with-temp-eshell)
(eshell-command-result-equal):
* test/lisp/eshell/eshell-tests-unload.el (load-eshell): Silence Eshell
loading messages.

* lisp/eshell/esh-cmd.el (eshell-do-eval): Don't defer when all the
processes are done.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/pipeline-wait/nested-pipes): New test.

Erase all existing duplicates instead of just the last duplicate entry
when 'eshell-hist-ignoredups' is set to 'erase'.  Multiple duplicates
can exist in case 'eshell-hist-ignoredups' was set to something else
than 'erase' in the past or if the history file contains duplicates
(bug#71107).

* lisp/eshell/em-hist.el (eshell-add-input-to-history): Remove all
duplicates from history ring.

* test/lisp/eshell/em-hist-tests.el
(em-hist-test/add-to-history/erase-existing-dups): New test.

* lisp/eshell/esh-cmd.el (eshell/funcall): New function...

* test/lisp/eshell/em-glob-tests.el (em-glob-test/expand/splice-results)
(em-glob-test/expand/no-splice-results)
(em-glob-test/expand/explicitly-splice-results)
(em-glob-test/expand/explicitly-listify-results):
* test/lisp/eshell/esh-var-tests.el
(esh-var-test/quoted-interp-lisp-indices)
(esh-var-test/quoted-interp-cmd-indices)
(esh-var-test/interp-convert-quoted-var-number)
(esh-var-test/quoted-interp-convert-var-number)
(esh-var-test/quoted-interp-convert-quoted-var-number)
(esh-var-test/last-status-var-lisp-command)
(esh-var-test/last-result-var-split-indices)
(esh-var-test/last-arg-var-split-indices): ... use it in tests.

* doc/misc/eshell.texi (List of Built-ins): Describe this command.

* lisp/eshell/esh-ext.el (eshell-explicit-remote-commands)
(eshell-explicit-command): Update docstrings.
(eshell--local-prefix): New constant.
(eshell-handle-remote-command): Remove.
(eshell-quoted-file-command): New function...
(eshell-ext-initialize): ... add it as a hook.
(eshell-remote-command): Support running commands on localhost.
(eshell-connection-local-command): Rename from
'eshell-external-command'.
(eshell-external-command): New implementation calling
'eshell-remote-command' or 'eshell-connection-local-command' as
appropriate.

* test/lisp/eshell/esh-ext-tests.el
(esh-ext-test/explicitly-local-command): Update test.

* doc/misc/eshell.texi (Remote Access): Update documentation.

* etc/NEWS: Update announcement.

* lisp/eshell/esh-util.el (eshell-split-filename): Escaping shouldn't
matter for splitting the name (no other shells handle it like this).

* test/lisp/eshell/em-glob-tests.el
(em-glob-test/convert/quoted-start-directory): New test.

* lisp/eshell/em-glob.el (eshell-glob-convert): Use 'concat' instead of
'file-name-concat' to avoid extraneous slashes.
(eshell-extended-glob): Bail out if we didn't find a glob after all.

* test/lisp/eshell/em-glob-tests.el (tramp): Require.
(em-glob-test/convert/remote-start-directory): Use the mock remote
connection.
(em-glob-test/remote-user-directory): New test.