Add head util

src/head.c

@@ -0,0 +1,81 @@
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int usage(char *argv0) {
+  printf("Usage: %s [-n number] [file ...]\n", argv0);
+  return 2;
+}
+
+int main(int argc, char *argv[]) {
+  char ch;
+  int opt;
+  FILE *file = stdin;
+
+  int number = 10;
+  int line = 0;
+
+  bool multiple_files = false;
+
+  while ((opt = getopt(argc, argv, "n:")) != -1) {
+    switch (opt) {
+      case 'n':
+        if (optarg[0] == '-') {
+          optarg++;
+        }
+        for (size_t i = 0; i < strlen(optarg); i++) {
+          if (!isdigit(optarg[i])) {
+            fprintf(stderr, "%s: invalid number of lines: '%s'\n", argv[0], optarg);
+            return 1;
+          }
+        }
+        number = atoi(optarg);
+        break;
+      case '?':
+        fprintf(stderr, "%s: invalid option -- '%c'\n", argv[0], optopt);
+        return usage(argv[0]);
+    }
+  }
+
+  if (optind == argc) { // FIXME: this code is not clean
+    goto READ_STDIN;
+  }
+
+  if (argc - optind > 1) {
+    multiple_files = true;
+  }
+
+  for (; optind < argc; optind++) {
+    if (strcmp(argv[optind], "-")) {
+      file = fopen(argv[optind], "r");
+      if (file == NULL) {
+        fprintf(stderr, "%s: ", argv[0]);
+        perror(argv[optind]);
+        return -1;
+      }
+      if (multiple_files) {
+        printf("%s==> %s <==\n", (optind - 1 > argc - optind ? "\n" : ""), argv[optind]);
+      }
+    } else {
+      file = stdin;
+      if (multiple_files) {
+        printf("%s==> %s <==\n", (optind - 1 > argc - optind ? "\n" : ""), "standard input");
+      }
+    }
+READ_STDIN:
+    line = 0;
+    while ((ch = fgetc(file)) != EOF && line < number) {
+      putchar(ch);
+      if (ch == '\n') {
+        line++;
+      }
+    }
+    if (file != NULL && file != stdin) {
+      fclose(file);
+    }
+  }
+  return 0;
+}