add basic cat implementation in NodeJS

Author: Pezhman-Azizi

implement-shell-tools/cat/cat.js

@@ -0,0 +1,135 @@
+#!/usr/bin/env node
+
+// A simple reimplementation of `cat` in NodeJS.
+// Supports:
+//   node cat.js sample-files/1.txt
+//   node cat.js -n sample-files/1.txt
+//   node cat.js -b sample-files/3.txt
+//   node cat.js sample-files/*.txt
+//
+// -n : number all lines
+// -b : number non-empty lines
+
+const fs = require("fs");
+const path = require("path");
+
+// -------- argument parsing --------
+const args = process.argv.slice(2);
+
+if (args.length === 0) {
+  console.error("Usage: node cat.js [-n | -b] <file> [<file> ...]");
+  process.exit(1);
+}
+
+let numberAll = false;
+let numberNonBlank = false;
+let fileArgs = [];
+
+// Very small argument parser:
+// We only care about -n or -b as the FIRST arg (like the coursework examples).
+if (args[0] === "-n") {
+  numberAll = true;
+  fileArgs = args.slice(1);
+} else if (args[0] === "-b") {
+  numberNonBlank = true;
+  fileArgs = args.slice(1);
+} else {
+  fileArgs = args;
+}
+
+if (fileArgs.length === 0) {
+  console.error("cat.js: no input files");
+  process.exit(1);
+}
+
+// -------- helper functions --------
+
+/**
+ * Format a single line with the correct line number, if needed.
+ *
+ * @param {string} line - line text (without the trailing newline)
+ * @param {number} currentLineNumber - global line counter
+ * @param {boolean} numberAll - true if -n
+ * @param {boolean} numberNonBlank - true if -b
+ * @returns {{ text: string, nextLineNumber: number }}
+ */
+function formatLine(line, currentLineNumber, numberAll, numberNonBlank) {
+  let output = "";
+  let nextLineNumber = currentLineNumber;
+
+  const isBlank = line === "";
+
+  if (numberAll) {
+    // Always number every line
+    output =
+      currentLineNumber.toString().padStart(6, " ") + "\t" + line;
+    nextLineNumber++;
+  } else if (numberNonBlank) {
+    // Number only non-blank lines
+    if (isBlank) {
+      output = line; // No number, just the blank line
+    } else {
+      output =
+        currentLineNumber.toString().padStart(6, " ") + "\t" + line;
+      nextLineNumber++;
+    }
+  } else {
+    // No numbering
+    output = line;
+  }
+
+  return { text: output, nextLineNumber };
+}
+
+/**
+ * Print a file to stdout according to the options.
+ *
+ * @param {string} filePath
+ * @param {number} startLineNumber
+ * @returns {number} next line number
+ */
+function printFile(filePath, startLineNumber) {
+  let content;
+
+  try {
+    content = fs.readFileSync(filePath, "utf8");
+  } catch (err) {
+    console.error(`cat.js: ${filePath}: ${err.message}`);
+    return startLineNumber;
+  }
+
+  const lines = content.split("\n");
+  let lineNumber = startLineNumber;
+
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+    const { text, nextLineNumber } = formatLine(
+      line,
+      lineNumber,
+      numberAll,
+      numberNonBlank
+    );
+    lineNumber = nextLineNumber;
+
+    // Re-add the newline we lost when splitting
+    process.stdout.write(text);
+
+    // Avoid adding an extra newline at very end if the file
+    // doesn't end with \n — Node's split keeps the last segment.
+    if (i < lines.length - 1 || content.endsWith("\n")) {
+      process.stdout.write("\n");
+    }
+  }
+
+  return lineNumber;
+}
+
+// -------- main execution --------
+
+let globalLineNumber = 1;
+
+for (const file of fileArgs) {
+  // Shell expands *.txt, so here we just get each file path.
+  const resolvedPath = path.resolve(file);
+  globalLineNumber = printFile(resolvedPath, globalLineNumber);
+}