* lisp/eshell/esh-cmd.el (eshell-execute-pipeline): Use 'make-symbol'
for headproc and tailproc.
(eshell-do-pipelines, eshell-do-pipelines-synchronously): Adapt to the
above.

* test/lisp/eshell/eshell-tests.el (eshell-test/pipe-subcommand)
(eshell-test/pipe-subcommand-with-pipe): New test.

* doc/misc/eshell.texi (Bugs and ideas): Remove item about piping to
process from loop; this commit fixes it (bug#55590).

Previously, concatenating a list to a string would first convert the
list to a string.  Now, the string is concatenated with the last
element of the list.

* lisp/eshell/esh-util.el (eshell-to-flat-string): Make obsolete.

* lisp/eshell/esh-arg.el (eshell-concat, eshell-concat-1): New
functions.
(eshell-resolve-current-argument): Use 'eshell-concat'.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/interp-concat-cmd):
Add check for concatenation of multiline output of subcommands.
(esh-var-test/quoted-interp-concat-cmd): New test.

* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-test-13): Use
'eshell-concat'.

* doc/misc/eshell.texi (Expansion): Document this behavior.

* etc/NEWS: Announce the change (bug#55236).

* lisp/eshell/esh-util.el (eshell-convertible-to-number-p)
(eshell-convert-to-number): New functions...
(eshell-convert): ... use them.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/interp-convert-cmd-string-newline): Add checks for
numeric output.

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

* etc/NEWS: Announce the change (bug#55236).

This is closer in behavior to regular shells, and gives Eshell users
greater flexibility in how variables are expanded.

* lisp/eshell/esh-util.el (eshell-convert): Add TO-STRING argument.

* lisp/eshell/esh-var.el (eshell-parse-variable-ref): Add MODIFIER-P
argument and adjust how 'eshell-convert' and 'eshell-apply-indices'
are called.
(eshell-get-variable, eshell-apply-indices): Add QUOTED argument.

* test/lisp/eshell/esh-var-tests.el (eshell-test-value): New defvar.
(esh-var-test/interp-convert-var-number)
(esh-var-test/interp-convert-var-split-indices)
(esh-var-test/interp-convert-quoted-var-number)
(esh-var-test/interp-convert-quoted-var-split-indices)
(esh-var-test/interp-convert-cmd-string-newline)
(esh-var-test/interp-convert-cmd-multiline)
(esh-var-test/interp-convert-cmd-number)
(esh-var-test/interp-convert-cmd-split-indices)
(esh-var-test/quoted-interp-convert-var-number)
(esh-var-test/quoted-interp-convert-var-split-indices)
(esh-var-test/quoted-interp-convert-quoted-var-number)
(esh-var-test/quoted-interp-convert-quoted-var-split-indices)
(esh-var-test/quoted-interp-convert-cmd-string-newline)
(esh-var-test/quoted-interp-convert-cmd-multiline)
(esh-var-test/quoted-interp-convert-cmd-number)
(esh-var-test/quoted-interp-convert-cmd-split-indices): New tests.

* doc/misc/eshell.texi (Arguments): Expand this section, and document
the new behavior.
(Dollars Expansion): Provide more detail about '$(lisp)' and
'${command}' forms.

* etc/NEWS (Eshell): Announce this change (bug#55236).

* doc/misc/eshell.texi (Argument Predication and Modification):
Fix whitespace.

* lisp/eshell/em-pred.el (eshell-pred-delimiter-pairs): New variable.
(eshell-get-comparison-modifier-argument)
(eshell-get-numeric-modifier-argument)
(eshell-get-delimited-modifier-argument): New functions...
(eshell-pred-user-or-group, eshell-pred-file-time)
(eshell-pred-file-links, eshell-pred-file-size)
(eshell-pred-substitute, eshell-join-memebers, eshell-split-members):
... and use them here.
(eshell-include-members): Pass 'mod-char' and use
'eshell-get-delimited-modifier-argument'.
(eshell-pred-file-type, eshell-pred-file-mode): Use 'when-let'.
(eshell-modifier-alist): Pass modifier char to
'eshell-include-members'.

* test/lisp/eshell/em-pred-tests.el
(em-pred-test/predicate-delimiters): New test.
(em-pred-test/predicate-uid, em-pred-test/predicate-gid,
em-pred-test/modifier-include, em-pred-test/modifier-exclude): Remove
cases covered by 'em-pred-test/predicate-delimiters'.
(em-pred-test/modifier-substitute): Add test cases for new delimiter
styles.

* doc/misc/eshell.texi (Argument Predication and Modification):
Explain how string parameters are delimited.
(Argument Modifiers): Document some special delimiter behavior with
the 's/PATTERN/REPLACE/' modifier (bug#55204).

* etc/NEWS: Announce this change, and move the
'eshell-eval-using-options' entry to the Eshell section.

* lisp/eshell/em-elecslash.el: New file.
* etc/NEWS:
* doc/misc/eshell.texi (Electric forward slash): Document the module.
(Make / electric): Retitle to "Make / more electric", update, add
"@noindent", and standardize terminology.

* lisp/eshell/em-pred.el (eshell-predicate-alist): Add 'G' predicate.
(eshell-predicate-help-string): Document it.  (Bug#54470)

* test/lisp/eshell/em-pred-tests.el
(em-pred-test/predicate-effective-gid): New test.

* doc/misc/eshell.text (Argument Predication): Document 'G' predicate.

* lisp/eshell/esh-cmd.el (eshell-eval-argument): New function.
* lisp/eshell/esh-util.el (eshell-file-attributes): Pass original
value of FILE to 'file-attributes'.
* lisp/eshell/em-pred.el (eshell-predicate-alist): Change socket char
to '=', since 's' conflicts with setuid.
(eshell-modifier-alist): Fix 'E' (eval) modifier by using
'eshell-eval-argument'.  Also improve performance of 'O' (reversed
sort) modifier.
(eshell-modifier-help-string): Fix documentation of global
substitution modifier.
(eshell-pred-substitute): Fix infinite loop in some global
substitutions.
(eshell-join-members): Fix joining with implicit " " delimiter.
(Bug#54470)

* test/lisp/eshell/em-pred-tests.el: New file.

* doc/misc/eshell.texi (Argument Predication): New section.