Userland: Improve head program

* allow specifying files as arguments, e.g. `head a b c`
* support multiple files
* print a filename header when multiple files are being printed
* allow suppression or forcing of filename header via flags

This change drops support for the legacy `-123` syntax in favour of the
more widely-supported `-n 123` form.

fixes #105

Userland/head.cpp

@@ -1,26 +1,94 @@
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <LibCore/CArgsParser.h>
+
+int head(const String& filename, bool print_filename, int line_count);
 
 int main(int argc, char** argv)
 {
-    // FIXME: Allow setting via command-line argument.
-    int line_count = 10;
+    CArgsParser args_parser("head");
 
-    if (argc > 1) {
-        for (int i = 1; i < argc; ++i) {
-            if (argv[i][0] == '-') {
-                line_count = atoi(&argv[i][1]);
-            }
+    args_parser.add_arg("n", "lines", "Number of lines to print (default 10)");
+    args_parser.add_arg("q", "Never print filenames");
+    args_parser.add_arg("v", "Always print filenames");
+
+    CArgsParserResult args = args_parser.parse(argc, (const char**)argv);
+
+    int line_count = 10;
+    if (args.is_present("n")) {
+        line_count = strtol(args.get("n").characters(), NULL, 10);
+        if (errno) {
+            args_parser.print_usage();
+            return -1;
+        }
+    }
+
+    Vector<String> files = args.get_single_values();
+
+    bool print_filenames = files.size() > 1;
+
+    if (args.is_present("v")) {
+        print_filenames = true;
+    } else if (args.is_present("q")) {
+        print_filenames = false;
+    }
+
+    if (files.is_empty()) {
+        return head("", print_filenames, line_count);
+    }
+
+    int rc = 0;
+
+    for (auto &file : files) {
+        if (head(file, print_filenames, line_count) != 0) {
+            rc = 1;
+        }
+    }
+
+    return rc;
+}
+
+int head(const String& filename, bool print_filename, int line_count)
+{
+    bool is_stdin = false;
+    FILE* fp = nullptr;
+
+    if (filename == "" || filename == "-") {
+        fp = stdin;
+        is_stdin = true;
+    } else {
+        fp = fopen(filename.characters(), "r");
+        if (!fp) {
+            fprintf(stderr, "can't open %s for reading: %s\n", filename.characters(), strerror(errno));
+            return 1;
+        }
+    }
+
+    if (print_filename) {
+        if (is_stdin) {
+            puts("==> standard input <==");
+        } else {
+            printf("==> %s <==\n", filename.characters());
         }
     }
 
     for (int line = 0; line < line_count; ++line) {
         char buffer[BUFSIZ];
-        auto* str = fgets(buffer, sizeof(buffer), stdin);
+        auto* str = fgets(buffer, sizeof(buffer), fp);
         if (!str)
             break;
+
+        // specifically use fputs rather than puts, because fputs doesn't add
+        // its own newline.
         fputs(str, stdout);
     }
 
+    fclose(fp);
+
+    if (print_filename) {
+        puts("");
+    }
+
     return 0;
 }