command-line utility to convert numbers to different formats (hexidecimal, octal, binary, decimal, base-N)

base.c

/* base.c : command-line utility to convert numbers to different formats.
 * March 10, 2016 - PUBLIC DOMAIN */
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <limits.h>

int main(int argc, char **argv)
{
	int i;
	unsigned outbase = 10;
	for (i = 1; i < argc; i++) {
		const char *s = argv[i];
		if (!strcmp(s, "-h")) {
			fprintf(stderr,
				"usage: %s [-o] [-b] [-x] [-d] <numbers....>\n"
				"  -b    output in binary (base-2)\n"
				"  -o    output in octal (base-8)\n"
				"  -d    output in decimal (base-10)\n"
				"  -b    output in hexidecimal (base-16)\n"
				"numbers may be in the following formats:\n"
				"  [1-9][0-9]*           decimal input\n"
				"  0x[0-9A-F][0-9A-F]*   hexidecimal input\n"
				"  0b[01][01]*           binary input\n"
				"  0[0-7]*               octal input\n"
				"  [0-9][0-9]'[0-9a-z]   base NN format (2-36)\n",
				argv[0]);
			return EXIT_FAILURE;
		} else if (!strcmp(s, "-o")) {
			outbase = 8;
			continue;
		} else if (!strcmp(s, "-b")) {
			outbase = 2;
			continue;
		} else if (!strcmp(s, "-x")) {
			outbase = 16;
			continue;
		} else if (!strcmp(s, "-d")) {
			outbase = 10;
			continue;
		}

		char *endptr;
		unsigned b;
		const char *quote = strchr(s, '\'');
		if (quote) {
			errno = 0;
			b = strtol(s, &endptr, 10);
			if (errno) {
				perror("strtol()");
				return EXIT_FAILURE;
			}
			if (*endptr != '\'' || b < 2 || b > 36) {
				errno = EINVAL;
				perror("base must be from 2 to 36");
				return EXIT_FAILURE;

			}
			s = quote + 1;
		} else {
			if (!strncmp(s, "0b", 2)) { /* binary */
				s += 2;
				b = 2;
			} else {
				b = 0;
			}
		}
		errno = 0;
		long v = strtol(s, &endptr, b);
		if (errno) {
			perror("strtol()");
			return EXIT_FAILURE;
		}
		if (endptr == s) {
			errno = EINVAL;
			perror("empty string");
			return EXIT_FAILURE;
		}
		if (*endptr) {
			errno = EINVAL;
			perror("trailing garbage");
			return EXIT_FAILURE;
		}
		switch (outbase) {
		case 2: {
			unsigned long m = LONG_MIN;
			while (m > (unsigned long)v)
				m >>= 1;
			putchar(' ');
			do {
				putchar("10"[!(m & v)]);
				m >>= 1;
			} while (m);
			break;
		}
		case 8:
			printf(" %#lo", v);
			break;
		case 16:
			printf(" %#lx", v);
			break;
		default:
			printf(" %ld", v);
		}
	}
	printf("\n");
	return 0;
}