Linux下write命令同意用户跟其它终端上的用户对话。用c语言实现shell命令write。代码例如以下：

#include	<stdio.h>

#include	<fcntl.h>

#include	<unistd.h>

#include	<utmp.h>

#include	<pwd.h>

#include	<sys/types.h>

#include	<stdlib.h>

#include	<sys/stat.h>

#include	<time.h>

char * get_terminal_name(char *);

char * get_terminal_name_by_user_name(char *);

void hello();

int main(int argc, char * argv[]) {

	int fd;

	char buffer[1024];

	char * terminal_name;

	if (argc != 2) {

		printf("write usage: write [ttyname|username]");

		return 1;

	}

	terminal_name = get_terminal_name(argv[1]);

	if(terminal_name == NULL)

	{

		printf("get terminal name error\n");

		return 1;

	}

	//printf("terminate_name:%s", terminal_name);

	fd = open(terminal_name,O_WRONLY);

	hello(fd);

	while(fgets(buffer,1024,stdin) != EOF)

	{

		write(fd,buffer,strlen(buffer));

	}

	close(fd);

	return 0;

}

void hello(int fd) {

	char greeting[1024];

	struct passwd * passwd_pointer;

	time_t now;

	char host[255];

	gethostname(host, 255);

	time(&now);

	passwd_pointer = getpwuid(getuid());

	sprintf(greeting, "Message from %s@%s on %s at %5.5s ...\n",

			passwd_pointer->pw_name, host, ttyname(0), ctime(&now) + 11);

	write(fd,greeting,strlen(greeting));

}

char * get_terminal_name(char * user_input) {

	char terminal_name[255];

	struct stat stat_buffer;

	if (lstat(user_input, &stat_buffer) != -1) {

		if (S_ISCHR(stat_buffer.st_mode)) {

			return user_input;

		}

	}

	return get_terminal_name_by_user_name(user_input);

}

char * get_terminal_name_by_user_name(char * user_name) {

	struct utmp * utmp_pointer;

	setutent();

	int number = 0;

	char * result = (char *) malloc(sizeof(char) * 255);

	char *mytty = ttyname(0); /* begins "/dev/" */

	char *ttydev = mytty + strlen("/dev/");

	printf("%s\n", mytty);

	while ((utmp_pointer = getutent()) != NULL) {

		if (strcmp(user_name, utmp_pointer->ut_user) == 0

				&& utmp_pointer->ut_type == USER_PROCESS

				&& strcmp(utmp_pointer->ut_line, ttydev) != 0

				&& utmp_pointer->ut_line[0] != ':') {

			number++;

			//printf("%s\n", utmp_pointer->ut_line);

			if (number == 1) {

				strcpy(result, "/dev/");

				strcat(result, utmp_pointer->ut_line);

			}

		}

	}

	endutent();

	if (number > 1)

		printf("warning:%s is logged in %d different terminals,using %s\n",

				user_name, number, result);

	if(number == 0)

		return NULL;

	return result;

}