commands/simple/head.c

   1 /* head - print the first few lines of a file   Author: Andy Tanenbaum */
   2
   3 #include <errno.h>
   4 #include <stdlib.h>
   5 #include <stdio.h>
   6 #include <string.h>
   7
   8 #define DEFAULT 10
   9
  10 _PROTOTYPE(int main, (int argc, char **argv));
  11 _PROTOTYPE(void do_file, (int n, FILE *f));
  12 _PROTOTYPE(void usage, (void));
  13
  14 int main(argc, argv)
  15 int argc;
  16 char *argv[];
  17 {
  18   FILE *f;
  19   int legacy, n, k, nfiles;
  20   char *ptr;
  21
  22   /* Check for flags.  One can only specify how many lines to print. */
  23   k = 1;
  24   n = DEFAULT;
  25   legacy = 0;
  26   for (k = 1; k < argc && argv[k][0] == '-'; k++) {
  27         ptr = &argv[k][1];
  28         if (ptr[0] == 'n' && ptr[1] == 0) {
  29                 k++;
  30                 if (k >= argc) usage();
  31                 ptr = argv[k];
  32         }
  33         else if (ptr[0] == '-' && ptr[1] == 0) {
  34                 k++;
  35                 break;
  36         }
  37         else if (++legacy > 1) usage();
  38         n = atoi(ptr);
  39         if (n <= 0) usage();
  40   }
  41   nfiles = argc - k;
  42
  43   if (nfiles == 0) {
  44         /* Print standard input only. */
  45         do_file(n, stdin);
  46         exit(0);
  47   }
  48
  49   /* One or more files have been listed explicitly. */
  50   while (k < argc) {
  51         if (nfiles > 1) printf("==> %s <==\n", argv[k]);
  52         if ((f = fopen(argv[k], "r")) == NULL)
  53                 fprintf(stderr, "%s: cannot open %s: %s\n",
  54                         argv[0], argv[k], strerror(errno));
  55         else {
  56                 do_file(n, f);
  57                 fclose(f);
  58         }
  59         k++;
  60         if (k < argc) printf("\n");
  61   }
  62   return(0);
  63 }
  64
  65
  66
  67 void do_file(n, f)
  68 int n;
  69 FILE *f;
  70 {
  71   int c;
  72
  73   /* Print the first 'n' lines of a file. */
  74   while (n) switch (c = getc(f)) {
  75             case EOF:
  76                 return;
  77             case '\n':
  78                 --n;
  79             default:    putc((char) c, stdout);
  80         }
  81 }
  82
  83
  84 void usage()
  85 {
  86   fprintf(stderr, "Usage: head [-lines | -n lines] [file ...]\n");
  87   exit(1);
  88 }