hush: handle empty execs

Sometimes variable expansions yield empty strings, and if they happen to
be a command someone wants to run like `$foo`, then hush currently
segfaults.  So handle `` and $().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

3 files changed, 36 insertions, 0 deletions

diff --git a/shell/hush.c b/shell/hush.c
index 2d333d731..1d2826d9a 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -3895,6 +3895,12 @@ static NOINLINE int run_pipe(struct pipe *pi)
                         argv_expanded = expand_strvec_to_strvec(argv + command->assignment_cnt);
                 }
 
+                /* if someone gives us an empty string: ``, $(), ... */
+                if (!argv_expanded[0]) {
+                        debug_leave();
+                        return 0;
+                }
+
                 x = find_builtin(argv_expanded[0]);
 #if ENABLE_HUSH_FUNCTIONS
                 funcp = NULL;
diff --git a/shell/hush_test/hush-psubst/emptytick.right b/shell/hush_test/hush-psubst/emptytick.right
new file mode 100644
index 000000000..d4b70c58a
--- /dev/null
+++ b/shell/hush_test/hush-psubst/emptytick.right
@@ -0,0 +1,14 @@
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
+0
diff --git a/shell/hush_test/hush-psubst/emptytick.tests b/shell/hush_test/hush-psubst/emptytick.tests
new file mode 100755
index 000000000..af3a1836c
--- /dev/null
+++ b/shell/hush_test/hush-psubst/emptytick.tests
@@ -0,0 +1,16 @@
+true;  ``; echo $?
+false; ``; echo $?
+true;  `""`; echo $?
+false; `""`; echo $?
+true;  `     `; echo $?
+false; `     `; echo $?
+
+true;  $(); echo $?
+false; $(); echo $?
+true;  $(""); echo $?
+false; $(""); echo $?
+true;  $(     ); echo $?
+false; $(     ); echo $?
+
+true;  exec ''; echo $?
+false; exec ''; echo $?