Make '$?' and '$$' variables more consistent in Eshell

Previously, '$?' (last exit code) was only useful for external
commands, and '$$' (last result) was only useful for Lisp commands.

* lisp/eshell/esh-cmd.el (eshell-lisp-form-nil-is-failure): New
option.
(eshell-lisp-command): Set last exit code to 1 when the command
signals an error, and 2 if it returns nil (for Lisp forms only).

* lisp/eshell/esh-proc.el (eshell-sentinel): Set last result to t if
the command succeeded.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/while-loop-lisp-form, esh-cmd-test/until-loop-lisp-form)
(esh-cmd-test/if-else-statement-lisp-form)
(esh-cmd-test/if-else-statement-lisp-form-2)
(esh-cmd-test/unless-else-statement-lisp-form): New tests.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/last-status-var-lisp-command)
(esh-var-test/last-status-var-lisp-form)
(esh-var-test/last-status-var-lisp-form-2)
(esh-var-test/last-status-var-ext-cmd)
(esh-var-test/last-status-var-ext-cmd): New tests.
(esh-var-test/last-result-var2): Rename from this...
( esh-var-test/last-result-var-twice): ... to this.

* doc/misc/eshell.texi (Variables): Update documentation about '$?'
and '$$'.
(Control Flow): Mention that '(lisp forms)' can be used as
conditionals.

* etc/NEWS: Announce this change (bug#57129).

2 files changed, 109 insertions, 1 deletions

diff --git a/test/lisp/eshell/esh-cmd-tests.el b/test/lisp/eshell/esh-cmd-tests.el
index b31159a1a8f..e86985ec717 100644
--- a/test/lisp/eshell/esh-cmd-tests.el
+++ b/test/lisp/eshell/esh-cmd-tests.el
@@ -139,6 +139,15 @@ e.g. \"{(+ 1 2)} 3\" => 3"
               "{ setq eshell-test-value (cdr eshell-test-value) }")
       "(1 2)\n(2)\n"))))
 
+(ert-deftest esh-cmd-test/while-loop-lisp-form ()
+  "Test invocation of a while loop using a Lisp form."
+  (with-temp-eshell
+   (let ((eshell-test-value 0))
+     (eshell-command-result-p
+      (concat "while (/= eshell-test-value 3) "
+              "{ setq eshell-test-value (1+ eshell-test-value) }")
+      "1\n2\n3\n"))))
+
 (ert-deftest esh-cmd-test/while-loop-ext-cmd ()
   "Test invocation of a while loop using an external command."
   (skip-unless (executable-find "["))
@@ -158,6 +167,16 @@ e.g. \"{(+ 1 2)} 3\" => 3"
               "{ setq eshell-test-value t }")
       "t\n"))))
 
+(ert-deftest esh-cmd-test/until-loop-lisp-form ()
+  "Test invocation of an until loop using a Lisp form."
+  (skip-unless (executable-find "["))
+  (with-temp-eshell
+   (let ((eshell-test-value 0))
+     (eshell-command-result-p
+      (concat "until (= eshell-test-value 3) "
+              "{ setq eshell-test-value (1+ eshell-test-value) }")
+      "1\n2\n3\n"))))
+
 (ert-deftest esh-cmd-test/until-loop-ext-cmd ()
   "Test invocation of an until loop using an external command."
   (skip-unless (executable-find "["))
@@ -188,6 +207,30 @@ e.g. \"{(+ 1 2)} 3\" => 3"
      (eshell-command-result-p "if $eshell-test-value {echo yes} {echo no}"
                               "no\n"))))
 
+(ert-deftest esh-cmd-test/if-else-statement-lisp-form ()
+  "Test invocation of an if/else statement using a Lisp form."
+  (with-temp-eshell
+   (eshell-command-result-p "if (zerop 0) {echo yes} {echo no}"
+                            "yes\n")
+   (eshell-command-result-p "if (zerop 1) {echo yes} {echo no}"
+                            "no\n")
+   (let ((debug-on-error nil))
+     (eshell-command-result-p "if (zerop \"foo\") {echo yes} {echo no}"
+                              "no\n"))))
+
+(ert-deftest esh-cmd-test/if-else-statement-lisp-form-2 ()
+  "Test invocation of an if/else statement using a Lisp form.
+This tests when `eshell-lisp-form-nil-is-failure' is nil."
+  (let ((eshell-lisp-form-nil-is-failure nil))
+    (with-temp-eshell
+     (eshell-command-result-p "if (zerop 0) {echo yes} {echo no}"
+                              "yes\n")
+     (eshell-command-result-p "if (zerop 1) {echo yes} {echo no}"
+                              "yes\n")
+     (let ((debug-on-error nil))
+       (eshell-command-result-p "if (zerop \"foo\") {echo yes} {echo no}"
+                                "no\n")))))
+
 (ert-deftest esh-cmd-test/if-else-statement-ext-cmd ()
   "Test invocation of an if/else statement using an external command."
   (skip-unless (executable-find "["))
@@ -217,6 +260,17 @@ e.g. \"{(+ 1 2)} 3\" => 3"
      (eshell-command-result-p "unless $eshell-test-value {echo no} {echo yes}"
                               "no\n"))))
 
+(ert-deftest esh-cmd-test/unless-else-statement-lisp-form ()
+  "Test invocation of an unless/else statement using a Lisp form."
+  (with-temp-eshell
+   (eshell-command-result-p "unless (zerop 0) {echo no} {echo yes}"
+                            "yes\n")
+   (eshell-command-result-p "unless (zerop 1) {echo no} {echo yes}"
+                            "no\n")
+   (let ((debug-on-error nil))
+     (eshell-command-result-p "unless (zerop \"foo\") {echo no} {echo yes}"
+                              "no\n"))))
+
 (ert-deftest esh-cmd-test/unless-else-statement-ext-cmd ()
   "Test invocation of an unless/else statement using an external command."
   (skip-unless (executable-find "["))
diff --git a/test/lisp/eshell/esh-var-tests.el b/test/lisp/eshell/esh-var-tests.el
index 54e701a6aab..66dabd424bd 100644
--- a/test/lisp/eshell/esh-var-tests.el
+++ b/test/lisp/eshell/esh-var-tests.el
@@ -500,18 +500,72 @@ inside double-quotes"
    (eshell-command-result-p "echo $INSIDE_EMACS[, 1]"
                             "eshell")))
 
+(ert-deftest esh-var-test/last-status-var-lisp-command ()
+  "Test using the \"last exit status\" ($?) variable with a Lisp command"
+  (with-temp-eshell
+   (eshell-command-result-p "zerop 0; echo $?"
+                            "t\n0\n")
+   (eshell-command-result-p "zerop 1; echo $?"
+                            "0\n")
+   (let ((debug-on-error nil))
+     (eshell-command-result-p "zerop foo; echo $?"
+                              "1\n"))))
+
+(ert-deftest esh-var-test/last-status-var-lisp-form ()
+  "Test using the \"last exit status\" ($?) variable with a Lisp form"
+  (let ((eshell-lisp-form-nil-is-failure t))
+  (with-temp-eshell
+   (eshell-command-result-p "(zerop 0); echo $?"
+                            "t\n0\n")
+   (eshell-command-result-p "(zerop 1); echo $?"
+                            "2\n")
+   (let ((debug-on-error nil))
+     (eshell-command-result-p "(zerop \"foo\"); echo $?"
+                              "1\n")))))
+
+(ert-deftest esh-var-test/last-status-var-lisp-form-2 ()
+  "Test using the \"last exit status\" ($?) variable with a Lisp form.
+This tests when `eshell-lisp-form-nil-is-failure' is nil."
+  (let ((eshell-lisp-form-nil-is-failure nil))
+    (with-temp-eshell
+     (eshell-command-result-p "(zerop 0); echo $?"
+                              "0\n")
+     (eshell-command-result-p "(zerop 0); echo $?"
+                              "0\n")
+     (let ((debug-on-error nil))
+       (eshell-command-result-p "(zerop \"foo\"); echo $?"
+                                "1\n")))))
+
+(ert-deftest esh-var-test/last-status-var-ext-cmd ()
+  "Test using the \"last exit status\" ($?) variable with an external command"
+  (skip-unless (executable-find "["))
+  (with-temp-eshell
+   (eshell-command-result-p "[ foo = foo ]; echo $?"
+                            "0\n")
+   (eshell-command-result-p "[ foo = bar ]; echo $?"
+                            "1\n")))
+
 (ert-deftest esh-var-test/last-result-var ()
   "Test using the \"last result\" ($$) variable"
   (with-temp-eshell
    (eshell-command-result-p "+ 1 2; + $$ 2"
                             "3\n5\n")))
 
-(ert-deftest esh-var-test/last-result-var2 ()
+(ert-deftest esh-var-test/last-result-var-twice ()
   "Test using the \"last result\" ($$) variable twice"
   (with-temp-eshell
    (eshell-command-result-p "+ 1 2; + $$ $$"
                             "3\n6\n")))
 
+(ert-deftest esh-var-test/last-result-var-ext-cmd ()
+  "Test using the \"last result\" ($$) variable with an external command"
+  (skip-unless (executable-find "["))
+  (with-temp-eshell
+   (eshell-command-result-p "[ foo = foo ]; format \"%s\" $$"
+                            "t\n")
+   (eshell-command-result-p "[ foo = bar ]; format \"%s\" $$"
+                            "nil\n")))
+
 (ert-deftest esh-var-test/last-result-var-split-indices ()
   "Test using the \"last result\" ($$) variable with split indices"
   (with-temp-eshell