Make sure we redirect stdin from /dev/null for RUN and shell()

man/remind.1.in

@@ -758,11 +758,12 @@ are treated as the beginnings of new paragraphs, and the \fB$FirstIndent\fR
 indentation is used for the next line.  You can use two consecutive
 newlines to have spaced paragraphs emitted from a single reminder body.
 .PP
-A \fBRUN\fR-type
-reminder also passes the \fIbody\fR through the substitution filter, but
-then executes the result as a system command.  A \fBCAL\fR-type reminder
-is used only to place entries in the calendar produced when \fBRemind\fR
-is run with the \fB\-c\fR, \fB\-s\fR or \fB\-p\fR options.
+A \fBRUN\fR-type reminder also passes the \fIbody\fR through the
+substitution filter, but then executes the result as a system command.
+A \fBCAL\fR-type reminder is used only to place entries in the
+calendar produced when \fBRemind\fR is run with the \fB\-c\fR,
+\fB\-s\fR or \fB\-p\fR options.  When \fBRemind\fR runs a command, it
+sets the command's standard input to come from /dev/null.
 .PP
 A \fBPS\fR or \fBPSFILE\fR-type reminder is used to pass PostScript code
 directly to the printer when producing PostScript calendars.  This can
@@ -4621,6 +4622,9 @@ been used, \fBshell()\fR will result in an error, and \fIcmd\fR will
 not be executed.
 .RS
 .PP
+When \fBshell\fR runs \fIcmd\fR, it arranges for \fIcmd\fR's standard
+input file descriptor to be connected to /dev/null.
+.PP
 If \fImaxlen\fR is specified, then \fBshell()\fR returns the first
 \fImaxlen\fR characters of output (rather than the first 511).  If
 \fImaxlen\fR is specified as a negative number, then it defaults to

src/funcs.c

@@ -2412,6 +2412,10 @@ static int FShell(func_info *info)
        used a static buffer for reading results from shell() command */
     int maxlen = 511;
 
+    /* Redirect stdin to /dev/null */
+    int stdin_dup;
+    int devnull = -1;
+
     DBufInit(&buf);
     if (RunDisabled) return E_RUN_DISABLED;
     ASSERT_TYPE(0, STR_TYPE);
@@ -2428,8 +2432,25 @@ static int FShell(func_info *info)
         }
     }
 
+    stdin_dup = dup(STDIN_FILENO);
+    if (stdin_dup >= 0) {
+        devnull = open("/dev/null", O_RDONLY);
+        if (devnull >= 0) {
+            if (dup2(devnull, STDIN_FILENO) >= 0) {
+                set_cloexec(stdin_dup);
+            }
+            (void) close(devnull);
+        }
+    }
+
     fp = popen(ARGSTR(0), "r");
-    if (!fp) return E_IO_ERR;
+    if (!fp) {
+        if (stdin_dup >= 0) {
+            (void) dup2(stdin_dup, STDIN_FILENO);
+            (void) close(stdin_dup);
+        }
+        return E_IO_ERR;
+    }
     while (1) {
         ch = getc(fp);
         if (ch == EOF) {
@@ -2438,6 +2459,10 @@ static int FShell(func_info *info)
         if (isspace(ch)) ch = ' ';
         if (DBufPutc(&buf, (char) ch) != OK) {
             pclose(fp);
+            if (stdin_dup >= 0) {
+                (void) dup2(stdin_dup, STDIN_FILENO);
+                (void) close(stdin_dup);
+            }
             DBufFree(&buf);
             return E_NO_MEM;
         }
@@ -2454,6 +2479,10 @@ static int FShell(func_info *info)
     /* XXX Should we consume remaining output from cmd? */
 
     pclose(fp);
+    if (stdin_dup >= 0) {
+        (void) dup2(stdin_dup, STDIN_FILENO);
+        (void) close(stdin_dup);
+    }
     r = RetStrVal(DBufValue(&buf), info);
     DBufFree(&buf);
     return r;

tests/test-rem

@@ -848,6 +848,10 @@ SET $WarningLevel today()
 SET $WarningLevel "3.1.4"
 EOF
 
+# Make sure shell() and RUN redirect stdin to /dev/null
+(echo 'set a shell("cat")'; sleep 1; echo 'rem msg devnull test [a]') | $REMIND - 2025-10-01 >> ../tests/test.out 2>&1
+(echo 'REM RUN cat'; sleep 1; echo 'rem msg devnull test b') | $REMIND - 2025-10-01 >> ../tests/test.out 2>&1
+
 cmp -s ../tests/test.out ../tests/test.cmp
 if [ "$?" = "0" ]; then
    echo "Remind:  Acceptance test PASSED"

tests/test.cmp

@@ -39907,3 +39907,11 @@ SET $WarningLevel today()
 SET $WarningLevel "3.1.4"
 -stdin-(8): Ill-formed number
 
+Reminders for Wednesday, 1st October, 2025:
+
+devnull test 
+
+Reminders for Wednesday, 1st October, 2025:
+
+devnull test b
+