Userland: Add the utility "nl" (number line) (#693)

I wrote a version of nl for Serenity with a lot but not all of the 
options in POSIX nl. It includes line count type (-b), increment (-i),
delimiter (-s), start number (-v), and width (-w).

Userland/nl.cpp

@@ -0,0 +1,114 @@
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibCore/CArgsParser.h>
+
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char** argv)
+{
+    CArgsParser args_parser("nl");
+    args_parser.add_arg("b", "type", "Line count type. \n\tt counts non-empty lines. \n\ta counts all lines. \n\tn counts no lines.");
+    args_parser.add_arg("i", "incr", "Set line count increment.");
+    args_parser.add_arg("s", "delim", "Set buffer between the line numbers and text. 1-63 bytes");
+    args_parser.add_arg("v", "startnum", "Initial value used to number logical page lines.");
+    args_parser.add_arg("w", "width", "The number of characters used for the line number.");
+    args_parser.add_single_value("file");
+    CArgsParserResult args = args_parser.parse(argc, argv);
+
+    bool all_lines_flag = false;
+    bool line_numbers_flag = true;
+    String value_of_b;
+    if (args.is_present("b")) {
+        value_of_b = args.get("b");
+        if (value_of_b == "a")
+            all_lines_flag = true;
+        else if (value_of_b == "t")
+            all_lines_flag = false;
+        else if (value_of_b == "n")
+            line_numbers_flag = false;
+        else {
+            args_parser.print_usage();
+            return 1;
+        }
+    }
+
+    long line_number_increment = 1;
+    String value_of_i;
+    if (args.is_present("i")) {
+        value_of_i = args.get("i");
+        line_number_increment = atol(value_of_i.characters());
+        if (!line_number_increment) {
+            args_parser.print_usage();
+            return 1;
+        }
+    }
+
+    bool delimiter_flag = false;
+    String value_of_s;
+    if (args.is_present("s")) {
+        value_of_s = args.get("s");
+        if (value_of_s.length() > 0 && value_of_s.length() < 64)
+            delimiter_flag = true;
+        else {
+            args_parser.print_usage();
+            return 1;
+        }
+    }
+    char delimiter[64];
+    strcpy(delimiter, delimiter_flag ? value_of_s.characters() : "  ");
+
+    long line_number = 1;
+    String value_of_v;
+    if (args.is_present("v")) {
+        value_of_v = args.get("v");
+        line_number = atol(value_of_v.characters());
+    }
+
+    String value_of_w;
+    unsigned int line_number_width = 6;
+    if (args.is_present("w")) {
+        value_of_w = args.get("w");
+        line_number_width = atol(value_of_w.characters());
+    }
+
+    Vector<String> files = args.get_single_values();
+    Vector<FILE*> file_pointers;
+    if (files.size() > 0) {
+        for (auto& file : files) {
+            FILE* file_pointer;
+            if ((file_pointer = fopen(file.characters(), "r")) == NULL) {
+                fprintf(stderr, "unable to open %s\n", file.characters());
+                continue;
+            }
+            file_pointers.append(file_pointer);
+        }
+    } else {
+        file_pointers.append(stdin);
+    }
+
+    line_number -= line_number_increment; // so the line number can start at 1 when added below
+    for (auto& file_pointer : file_pointers) {
+        int previous_character = 0;
+        int next_character = 0;
+        while ((next_character = fgetc(file_pointer)) != EOF) {
+            if (previous_character == 0 || previous_character == '\n') {
+                if (!all_lines_flag && next_character == '\n') {
+                    // skips printing line count on empty lines if all_lines_flags is false
+                    printf("\n");
+                    continue;
+                }
+                if (line_numbers_flag)
+                    printf("%*lu%s", line_number_width, (line_number += line_number_increment), delimiter);
+                else
+                    printf("%*s", line_number_width, "");
+            }
+            putchar(next_character);
+            previous_character = next_character;
+        }
+        fclose(file_pointer);
+        if (previous_character != '\n')
+            printf("\n"); // for cases where files have no trailing newline
+    }
+    return 0;
+}