From ec4caeaaf4ba3d7869263ce9da5409ae79ea2237 Mon Sep 17 00:00:00 2001
From: Michael Mikonos <127171689+mknos@users.noreply.github.com>
Date: Fri, 20 Oct 2023 19:58:12 +0800
Subject: [PATCH] tail -n: 0 versus +0 versus -0

* I wrote a test harness for tail-zero-lines:
   for opt in '-0' '+0' '-n 0' '-n -0' '-n +0' ; do echo "tail $opt"; perl tail $opt in; done;
* Some cases result in the error: The number cannot be null
* "tail -n 0" is accepted but "tail -n +0" and "tail -n -0" are not
* When debugging I found that check_number() is not called for "tail -n 0" (but it should be)
* check_number() was only called if $opt_b/$opt_c/$opt_n were true
* NB: GNU tail allows -n 0, but raising error in this version is consistent internally, and with "head -n 0"
---
 bin/tail | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/bin/tail b/bin/tail
index 752dd0e9..692e73f2 100755
--- a/bin/tail
+++ b/bin/tail
@@ -90,9 +90,18 @@ sub parse_args()
     usage 1, "-f and -r cannot be used together"
         if $opt_f and $opt_r;
 
-    $point = check_number($opt_b), $type = "b" if $opt_b;
-    $point = check_number($opt_c), $type = "c" if $opt_c;
-    $point = check_number($opt_n), $type = "n" if $opt_n;
+    if (defined $opt_b) {
+        $point = check_number($opt_b);
+        $type = 'b';
+    }
+    if (defined $opt_c) {
+        $point = check_number($opt_c);
+        $type = 'c';
+    }
+    if (defined $opt_n) {
+        $point = check_number($opt_n);
+        $type = 'n';
+    }
 
     for (@ARGV) {
         $point = check_number($1), shift @ARGV, last