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 n, k, nfiles;
  20   char *ptr;
  21
  22   /* Check for flag.  Only flag is -n, to say how many lines to print. */
  23   k = 1;
  24   ptr = argv[1];
  25   n = DEFAULT;
  26   if (argc > 1 && *ptr++ == '-') {
  27         k++;
  28         n = atoi(ptr);
  29         if (n <= 0) usage();
  30   }
  31   nfiles = argc - k;
  32
  33   if (nfiles == 0) {
  34         /* Print standard input only. */
  35         do_file(n, stdin);
  36         exit(0);
  37   }
  38
  39   /* One or more files have been listed explicitly. */
  40   while (k < argc) {
  41         if (nfiles > 1) printf("==> %s <==\n", argv[k]);
  42         if ((f = fopen(argv[k], "r")) == NULL)
  43                 fprintf(stderr, "%s: cannot open %s: %s\n",
  44                         argv[0], argv[k], strerror(errno));
  45         else {
  46                 do_file(n, f);
  47                 fclose(f);
  48         }
  49         k++;
  50         if (k < argc) printf("\n");
  51   }
  52   return(0);
  53 }
  54
  55
  56
  57 void do_file(n, f)
  58 int n;
  59 FILE *f;
  60 {
  61   int c;
  62
  63   /* Print the first 'n' lines of a file. */
  64   while (n) switch (c = getc(f)) {
  65             case EOF:
  66                 return;
  67             case '\n':
  68                 --n;
  69             default:    putc((char) c, stdout);
  70         }
  71 }
  72
  73
  74 void usage()
  75 {
  76   fprintf(stderr, "Usage: head [-n] [file ...]\n");
  77   exit(1);
  78 }