crypt(3) command-line wrapper, compile with: cc crypt.c -o crypt -lcrypt

crypt.c

#include <stdio.h>
#include <crypt.h>

int main(int argc, char **argv) {
        if (argc < 3) {
                fprintf(stderr, "usage: %s key salt\n", argv[0]);
                return -1;
        }
        char *result = crypt(argv[1], argv[2]);
        if (!result)
                perror("crypt");
        else
                puts(result);
        return 0;
}