* test/lisp/eshell/em-hist-tests.el (em-hist-test/check-history-file):
Fix typo.

* test/lisp/eshell/em-hist-tests.el (em-hist-test/check-history-file):
New function.  Use it throughout this file.
(em-hist-test/history-append): Rename to...
(em-hist-test/write-history/append): ... this.
(em-hist-test/history-read): Rename to...
(em-hist-test/write-history/overwrite): ... this.
(em-hist-test/write-history/append-multiple-eshells)
(em-hist-test/write-history/overwrite-multiple-shells): New tests.
(em-hist-test/write-history/read-only): Check the resulting history.

* lisp/eshell/em-hist.el (eshell-hist--new-items): New variable.
(eshell-hist-initialize): Initialize 'eshell-hist--new-items' to 0.
(eshell/history): Change the behavior of 'history -a' to "append new
history in current buffer to history file".  Clarify the help text of
'history -r'.
(eshell-add-input-to-history): Increase counter of new history items.
(eshell-read-history): Respect 'eshell-hist-ignoredups' option.
(eshell-write-history): If the optional argument APPEND is non-nil,
appending new history items rather than the whole history.

* test/lisp/eshell/em-hist-tests.el (em-hist-test/history-append)
(em-hist-test/history-read): New tests (bug#66768).

* lisp/eshell/esh-arg.el (eshell-get-marker, eshell-insert-marker)
(eshell-complete-marker-ref): New functions...
(eshell-special-ref-alist): ... Add them to the new "marker" entry.

* test/lisp/eshell/esh-arg-tests.el
(esh-arg-test/special-reference/marker)
(esh-arg-test/special-reference/nested)
(esh-arg-test/special-reference/lisp-form):
* test/lisp/eshell/em-cmpl-tests.el
(em-cmpl-test/special-ref-completion/type)
(em-cmpl-test/special-ref-completion/marker): New tests.

* doc/misc/eshell.texi (Arguments): Document the new special ref type.

* etc/NEWS: Announce this change (bug#66458).

* lisp/eshell/esh-arg.el (eshell-current-argument-plain): New variable.
(eshell-parse-special-reference): Use 'eshell-parse-arguments'.
(eshell-get-buffer): New function.
(eshell-insert-buffer-name): Properly quote the buffer name.

* lisp/eshell/esh-proc.el (eshell-read-process-name): Move to "Special
references" section.
(eshell-insert-process): Properly quote the process name.

* lisp/eshell/em-extpipe.el (eshell-parse-external-pipeline):
* lisp/eshell/esh-io.el (eshell-parse-redirection): Don't do anything
when 'eshell-argument-plain' is non-nil.

* test/lisp/eshell/esh-arg-tests.el
(esh-arg-test/special-reference/quoted)
(esh-arg-test/special-reference/var-expansion): New tests.
(esh-arg-test/special-reference/special): Rename to...
(esh-arg-test/special-reference/special-characters): ... this.

* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-tests--deftest):
Properly quote the buffer name.
(em-extpipe-test-4, em-extpipe-test-7): Use 'eshell-get-buffer'.

* lisp/eshell/esh-util.el (eshell--unmark-string-as-output): New
function...

* lisp/eshell/esh-mode.el (eshell-mode): ... use it.

* test/lisp/eshell/eshell-tests.el (eshell-test/yank-output): New test
(bug#66469).

This really just generalizes Eshell's previous support for iterative
evaluation of a single current command to a list of multiple commands,
of which at most one can be in the foreground (bug#66066).

* lisp/eshell/esh-cmd.el (eshell-last-async-procs)
(eshell-current-command): Make obsolete in favor of...
(eshell-foreground-command): ... this
(eshell-background-commands): New variable.
(eshell-interactive-process-p): Make obsolete.
(eshell-head-process, eshell-tail-process): Use
'eshell-foreground-command'.
(eshell-cmd-initialize): Initialize new variables.
(eshell-add-command, eshell-remove-command)
(eshell-commands-for-process): New functions.
(eshell-parse-command): Make 'eshell-do-subjob' the outermost call.
(eshell-do-subjob): Call 'eshell-resume-eval' to split this command
off from its parent forms.
(eshell-eval-command): Use 'eshell-add-command'.
(eshell-resume-command): Use 'eshell-commands-for-process'.
(eshell-resume-eval): Take a COMMAND argument.  Return
':eshell-background' form for deferred background commands.
(eshell-do-eval): Remove check for 'eshell-current-subjob-p'.  This is
handled differently now.

* lisp/eshell/eshell.el (eshell-command): Wait for all processes to
exit when running synchronously.

* lisp/eshell/esh-mode.el (eshell-intercept-commands)
(eshell-watch-for-password-prompt):
* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments):
* lisp/eshell/em-smart.el (eshell-smart-display-move): Use
'eshell-foreground-command'.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/background/simple-command)
(esh-cmd-test/background/subcommand): New tests.
(esh-cmd-test/throw): Use 'eshell-foreground-command'.

* test/lisp/eshell/eshell-tests.el (eshell-test/queue-input): Use
'eshell-foreground-command'.

* test/lisp/eshell/em-script-tests.el
(em-script-test/source-script/background): Make the test script more
complex.

* test/lisp/eshell/eshell-tests.el
(eshell-test/eshell-command/pipeline-wait): New test.

* doc/misc/eshell.texi (Bugs and ideas): Remove implemented feature.

* lisp/eshell/esh-proc.el (eshell-subjob-messages): New variable...
(eshell-record-process-object)
(eshell-remove-process-entry): ... check it.

* lisp/eshell/em-script.el (eshell-source-file): Set
'eshell-subjob-messages' to nil.

* lisp/eshell/esh-cmd.el (eshell-do-subjob): Set
'eshell-subjob-messages' to t.

* test/lisp/eshell/em-script-tests.el
(em-script-test/source-script/background): New test.

In batch mode, SIGPIPEs can cause Emacs to abort (bug#66186).

* lisp/eshell/esh-io.el (eshell-output-object-to-target): Update
comment.

* test/lisp/eshell/esh-proc-tests.el
(esh-proc-test/pipeline-connection-type/middle)
(esh-proc-test/pipeline-connection-type/last): Use '(ignore)', since
that causes no output when called with no arguments, thus avoiding a
risky 'process-send-string'.