linux系统编程：自己动手写一个who命令

who命令的作用用于显示当前有哪些用户登录到系统。

这个命令执行的原理是读取了系统上utmp文件中记录的所有登录信息，直接显示出来的

utmp文件在哪里呢？

man who的时候，在手册下面有这么一段说明：意思就是不指定文件参数，那么读取的就是/var/run/utmp，到底是不是，验证下

If FILE is not specified, use /var/run/utmp.  /var/log/wtmp as FILE  is
       common.   If  ARG1  ARG2  given, -m presumed: 'am i' or 'mom likes' are
       usual.

当我指定file参数为/var/run/utmp或者省略这个参数的时候，结果都是一样， 当我用一个错误的文件时，没有任何结果，从这里可以推断，who命令确实从/var/run/utmp中读取用户登录的信息

ghostwu@ubuntu:~$ who
ghostwu  tty7         -- : (:)
ghostwu  pts/       -- : (:)
ghostwu  pts/       -- : (:)
ghostwu  pts/       -- : (:)
ghostwu@ubuntu:~$ who -b
         system boot  -- :
ghostwu@ubuntu:~$ who -b /var/run/utmp
         system boot  -- :
ghostwu@ubuntu:~$ who -b /var/run/utmp2
ghostwu@ubuntu:~$ who -b /var/run/utmp3

那么utmp到底在哪里？

利用man -k utmp 查找所有的可能：    推断--->  utmp (5) - login records  这里的可能性比较大，描述说，这里是记录登录信息的

ghostwu@ubuntu:~$ man -k utmp
endutent ()         - access utmp file entries
endutxent ()        - access utmp file entries
getutent ()         - access utmp file entries
getutent_r ()       - access utmp file entries
getutid ()          - access utmp file entries
getutid_r ()        - access utmp file entries
getutline ()        - access utmp file entries
getutline_r ()      - access utmp file entries
getutmp ()          - copy utmp structure to utmpx, and vice versa
getutmpx ()         - copy utmp structure to utmpx, and vice versa
getutxent ()        - access utmp file entries
getutxid ()         - access utmp file entries
getutxline ()       - access utmp file entries
login ()            - write utmp and wtmp entries
logout ()           - write utmp and wtmp entries
pututline ()        - access utmp file entries
pututxline ()       - access utmp file entries
sessreg ()          - manage utmpx/wtmpx entries for non-init clients
setutent ()         - access utmp file entries
setutxent ()        - access utmp file entries
systemd-update-utmp () - Write audit and utmp updates at bootup, runlevel ch...
systemd-update-utmp-runlevel.service () - Write audit and utmp updates at bo...
systemd-update-utmp.service () - Write audit and utmp updates at bootup, run...
utmp ()             - login records
utmpdump ()         - dump UTMP and WTMP files in raw format
utmpname ()         - access utmp file entries
utmpx ()            - login records
utmpxname ()        - access utmp file entries

接下来，我们去 man 5 utmp 看下，会发现有这么一段提示：

The file is a sequence of utmp structures, declared as follows in <utmp.h> (note that this
       is only one of several definitions around; details depend on the version of libc):

意思是utmp文件的信息是一系列utmp结构体数据, 这个结构体定义在utmp.h文件中，  每个linux发行版可能不一样.

接下来，我用强大的find命令查找到了2个目标：

ghostwu@ubuntu:~$ find /usr/include -name "utmp.h"
/usr/include/x86_64-linux-gnu/bits/utmp.h
/usr/include/utmp.h

结构体的定义就在这个文件中( /usr/include/x86_64-linux-gnu/bits/utmp.h  )

这里有两个宏要注意下( ut_time和UTMP_FILE ), 下面的程序会用到

#ifndef _NO_UT_TIME
/* We have a problem here: `ut_time' is also used otherwise.  Define
   _NO_UT_TIME if the compiler complains.  */
# define ut_time    ut_tv.tv_sec
#endif

ghostwu@ubuntu:~$ grep "UTMP_FILE" /usr/include/utmp.h
#define UTMP_FILE    _PATH_UTMP
#define UTMP_FILENAME    _PATH_UTMP
ghostwu@ubuntu:~$ grep "_PATH_UTMP" /usr/include/utmp.h
#define UTMP_FILE    _PATH_UTMP
#define UTMP_FILENAME    _PATH_UTMP
ghostwu@ubuntu:~$ grep "_PATH_UTMP" /usr/include/x86_64-linux-gnu/bits/utmp.h
ghostwu@ubuntu:~$ grep "_PATH_UTMP" /usr/include/*.h
/usr/include/paths.h:#define    _PATH_UTMP    "/var/run/utmp"
/usr/include/utmp.h:#define UTMP_FILE    _PATH_UTMP
/usr/include/utmp.h:#define UTMP_FILENAME    _PATH_UTMP
/usr/include/utmpx.h:# define UTMPX_FILE    _PATH_UTMPX
/usr/include/utmpx.h:# define UTMPX_FILENAME    _PATH_UTMPX
ghostwu@ubuntu:~$

UTMP_FILE的查找思路： 首先grep两个目录下面的文件utmp.h，在/usr/include/utmp.h找到一个宏定义 _PATH_UTMP，下一步就是确定 _PATH_UTMP到底是什么，利用grep "_PATH_UTMP" /usr/include/*.h

最终在paths.h头文件中，发现了他的真面目

who命令书写思路：

1）从/var/run/utmp读取文件，每次读取一个struct utmp结构体这么大，如果长度每次都有这么大，继续读取

2）格式化4个信息：用户名，主机，地址，时间

3）只打印当前活动的用户(当前登录的用户)

4）格式化时间( 小时，分钟，秒， >10的补0, <10的原样返回 )

源代码

 /*================================================================
 *   Copyright (C) 2018 . All rights reserved.
 *
 *   文件名称：mywho.c
 *   创 建 者：ghostwu(吴华)
 *   创建日期：2018年01月08日
 *   描    述：
 *
 ================================================================*/

 #include <stdio.h>
 #include <utmp.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <time.h>
 #include <string.h>

 #ifndef UTMP_FILE
 #define UTMP_FILE "/var/run/utmp"
 #endif

 int count = ;

 //格式化时间, <10 就补0, >10　原样返回
 char* format_time( char* s, const char *time ) {
     if( strlen( time ) <  ) {
         return strcat( s, time );
     }
     return strcpy( s, time );
 }

 void show_info( struct utmp* t_utmp ) {
     if ( t_utmp->ut_type != USER_PROCESS ) //不显示 非活跃的用户信息
       return;

     printf( "%-8.8s", t_utmp->ut_user );
     printf( " " );
     printf( "%-8.8s", t_utmp->ut_line );
     printf( " " );

     //printf( " " );
     //printf( "%12.12s", ctime( (time_t*)&(t_utmp->ut_time) ) + 4 ); //+4--->去除天(day)和后面的空格

     /*测试localtime用法
     //当前时间
     time_t now;
     struct tm* pNow;
     time( &now );
     pNow = localtime( &now );
     printf( "%d-%d-%d %d:%d", pNow->tm_year + 1900, pNow->tm_mon + 1, pNow->tm_mday, pNow->tm_hour, pNow->tm_min );
     */

     struct tm* ptm;
     time_t u_time = t_utmp->ut_time;
     ptm = localtime( &u_time );
     int ihour = ptm->tm_hour;
     int imin = ptm->tm_min;

     char hour[] = "";
     char hour2[] = "";
     sprintf( hour2, "%d", ihour );
     format_time( hour, hour2 );

     char min[] = "";
     char min2[] = "";
     sprintf( min2, "%d", imin );
     format_time( min, min2 );

     //printf( "%d-%d-%d %d:%d", ptm->tm_year + 1900, ptm->tm_mon + 1, ptm->tm_mday, ihour, imin );
     printf( "%d-%d-%d %s:%s", ptm->tm_year + , ptm->tm_mon + , ptm->tm_mday, hour, min );

     printf( " " );
     printf( "%-8.8s", t_utmp->ut_host );

     printf( "\n" );
 }

 int main(int argc, char *argv[])
 {
     struct utmp myutmp;
     int fd = -;
     int reclen = sizeof( myutmp );

     fd = open( UTMP_FILE, O_RDONLY );

     if( - == fd ) {
         perror( "open utmp" );
         exit( - );
     }

     //printf( "fd = %d\n", fd );

     while( read( fd, &myutmp, reclen ) == reclen ) {
         count++;
         show_info( &myutmp );
     }
     printf( "文件读取的次数:%d\n", count );
     close( fd );

     return ;
 }

总结：

一个非常小的功能，囊括以下知识点：

1）文件读取

2）man手册与系统命令使用技巧

3）指针用法

4）字符串函数用法

5）时间函数用法

6）宏与typedef的用法