Added support for -n +NUM in tail.js (with sign) (shelljs#1027)

A-725-K · web-flow · commit 42754c14ec5f · 2022-01-15T13:44:54.000-08:00
diff --git a/src/tail.js b/src/tail.js
@@ -32,12 +32,20 @@ function _tail(options, files) {
   if (!files && !pipe) common.error('no paths given');
 
   var idx = 1;
+  var plusOption = false;
   if (options.numLines === true) {
     idx = 2;
+    if (arguments[1][0] === '+') {
+      plusOption = true;
+    }
     options.numLines = Number(arguments[1]);
   } else if (options.numLines === false) {
     options.numLines = 10;
   }
+  // arguments[0] is a json object
+  if (arguments[0].numLines[0] === '+') {
+    plusOption = true;
+  }
   options.numLines = -1 * Math.abs(options.numLines);
   files = [].slice.call(arguments, idx);
 
@@ -69,12 +77,14 @@ function _tail(options, files) {
       shouldAppendNewline = false;
     }
 
-    tail = tail.concat(lines.slice(options.numLines));
+    tail = tail.concat(plusOption ? lines.slice(-options.numLines - 1) : lines.slice(options.numLines));
   });
 
   if (shouldAppendNewline) {
     tail.push(''); // to add a trailing newline once we join
   }
+
   return tail.join('\n');
 }
+
 module.exports = _tail;
diff --git a/test/tail.js b/test/tail.js
@@ -90,7 +90,7 @@ test('reading more lines than are in the file (with trailing newline)', t => {
   t.is(result.toString(), 'short2\nshort1\n'); // these files only have one line (with \n)
 });
 
-test('Globbed file', t => {
+test('globbed file', t => {
   const result = shell.tail('test/resources/head/file?.txt');
   t.falsy(shell.error());
   t.is(result.code, 0);
@@ -102,7 +102,7 @@ test('Globbed file', t => {
       .join('\n') + '\n');
 });
 
-test('With `\'-n\' <num>` option', t => {
+test('with `\'-n\' <num>` option', t => {
   const result = shell.tail('-n', 4, 'test/resources/head/file2.txt',
     'test/resources/head/file1.txt');
   t.falsy(shell.error());
@@ -115,7 +115,20 @@ test('With `\'-n\' <num>` option', t => {
       .join('\n') + '\n');
 });
 
-test('With `{\'-n\': <num>}` option', t => {
+test('with `\'-n\' +<num>` option', t => {
+  const result = shell.tail('-n', '+48', 'test/resources/head/file2.txt',
+    'test/resources/head/file1.txt');
+  t.falsy(shell.error());
+  t.is(result.code, 0);
+  t.is(result.toString(),
+    bottomOfFile2
+      .slice(0, 3)
+      .reverse()
+      .concat(bottomOfFile1.slice(0, 3).reverse())
+      .join('\n') + '\n');
+});
+
+test('with `{\'-n\': <num>}` option', t => {
   const result = shell.tail({ '-n': 4 }, 'test/resources/head/file2.txt',
     'test/resources/head/file1.txt');
   t.falsy(shell.error());
@@ -128,6 +141,19 @@ test('With `{\'-n\': <num>}` option', t => {
       .join('\n') + '\n');
 });
 
+test('with `{\'-n\': +<num>}` option', t => {
+  const result = shell.tail({ '-n': '+48' }, 'test/resources/head/file2.txt',
+    'test/resources/head/file1.txt');
+  t.falsy(shell.error());
+  t.is(result.code, 0);
+  t.is(result.toString(),
+    bottomOfFile2
+      .slice(0, 3)
+      .reverse()
+      .concat(bottomOfFile1.slice(0, 3).reverse())
+      .join('\n') + '\n');
+});
+
 test('negative values are the same as positive values', t => {
   const result = shell.tail('-n', -4, 'test/resources/head/file2.txt',
     'test/resources/head/file1.txt');
