who命令的作用用於顯示當前有哪些用戶登錄到系統。
這個命令執行的原理是讀取了系統上utmp文件中記錄的所有登錄信息，直接顯示出來的
utmp文件在哪里呢？
man who的時候，在手冊下面有這么一段說明：意思就是不指定文件參數，那么讀取的就是/var/run/utmp，到底是不是，驗證下
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.
當我指定file參數為/var/run/utmp或者省略這個參數的時候，結果都是一樣， 當我用一個錯誤的文件時，沒有任何結果，從這里可以推斷，who命令確實從/var/run/utmp中讀取用戶登錄的信息
ghostwu@ubuntu:~$ who ghostwu tty7 2018-01-08 09:09 (:0) ghostwu pts/18 2018-01-08 12:59 (:0) ghostwu pts/19 2018-01-08 13:00 (:0) ghostwu pts/20 2018-01-08 13:03 (:0) ghostwu@ubuntu:~$ who -b system boot 2018-01-08 09:08 ghostwu@ubuntu:~$ who -b /var/run/utmp system boot 2018-01-08 09:08 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： 推斷---> utmp (5) - login records 這里的可能性比較大，描述說，這里是記錄登錄信息的
ghostwu@ubuntu:~$ man -k utmp endutent (3) - access utmp file entries endutxent (3) - access utmp file entries getutent (3) - access utmp file entries getutent_r (3) - access utmp file entries getutid (3) - access utmp file entries getutid_r (3) - access utmp file entries getutline (3) - access utmp file entries getutline_r (3) - access utmp file entries getutmp (3) - copy utmp structure to utmpx, and vice versa getutmpx (3) - copy utmp structure to utmpx, and vice versa getutxent (3) - access utmp file entries getutxid (3) - access utmp file entries getutxline (3) - access utmp file entries login (3) - write utmp and wtmp entries logout (3) - write utmp and wtmp entries pututline (3) - access utmp file entries pututxline (3) - access utmp file entries sessreg (1) - manage utmpx/wtmpx entries for non-init clients setutent (3) - access utmp file entries setutxent (3) - access utmp file entries systemd-update-utmp (8) - Write audit and utmp updates at bootup, runlevel ch... systemd-update-utmp-runlevel.service (8) - Write audit and utmp updates at bo... systemd-update-utmp.service (8) - Write audit and utmp updates at bootup, run... utmp (5) - login records utmpdump (1) - dump UTMP and WTMP files in raw format utmpname (3) - access utmp file entries utmpx (5) - login records utmpxname (3) - access utmp file entries
接下來，我們去 man 5 utmp 看下，會發現有這么一段提示：
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結構體數據, 這個結構體定義在utmp.h文件中， 每個linux發行版可能不一樣.
接下來，我用強大的find命令查找到了2個目標：
ghostwu@ubuntu:~$ find /usr/include -name "utmp.h" /usr/include/x86_64-linux-gnu/bits/utmp.h /usr/include/utmp.h
結構體的定義就在這個文件中( /usr/include/x86_64-linux-gnu/bits/utmp.h )
這里有兩個宏要注意下( ut_time和UTMP_FILE ), 下面的程序會用到
#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兩個目錄下面的文件utmp.h，在/usr/include/utmp.h找到一個宏定義 _PATH_UTMP，下一步就是確定 _PATH_UTMP到底是什么，利用grep "_PATH_UTMP" /usr/include/*.h
最終在paths.h頭文件中，發現了他的真面目
who命令書寫思路：
1）從/var/run/utmp讀取文件，每次讀取一個struct utmp結構體這么大，如果長度每次都有這么大，繼續讀取
2）格式化4個信息：用戶名，主機，地址，時間
3）只打印當前活動的用戶(當前登錄的用戶)
4）格式化時間( 小時，分鍾，秒， >10的補0, <10的原樣返回 )
源代碼
1 /*================================================================ 2 * Copyright (C) 2018 . All rights reserved. 3 * 4 * 文件名稱：mywho.c 5 * 創 建 者：ghostwu(吳華) 6 * 創建日期：2018年01月08日 7 * 描 述： 8 * 9 ================================================================*/ 10 11 #include <stdio.h> 12 #include <utmp.h> 13 #include <sys/types.h> 14 #include <sys/stat.h> 15 #include <fcntl.h> 16 #include <stdlib.h> 17 #include <unistd.h> 18 #include <time.h> 19 #include <string.h> 20 21 #ifndef UTMP_FILE 22 #define UTMP_FILE "/var/run/utmp" 23 #endif 24 25 int count = 0; 26 27 //格式化時間, <10 就補0, >10 原樣返回 28 char* format_time( char* s, const char *time ) { 29 if( strlen( time ) < 2 ) { 30 return strcat( s, time ); 31 } 32 return strcpy( s, time ); 33 } 34 35 void show_info( struct utmp* t_utmp ) { 36 if ( t_utmp->ut_type != USER_PROCESS ) //不顯示 非活躍的用戶信息 37 return; 38 39 printf( "%-8.8s", t_utmp->ut_user ); 40 printf( " " ); 41 printf( "%-8.8s", t_utmp->ut_line ); 42 printf( " " ); 43 44 //printf( " " ); 45 //printf( "%12.12s", ctime( (time_t*)&(t_utmp->ut_time) ) + 4 ); //+4--->去除天(day)和后面的空格 46 47 /*測試localtime用法 48 //當前時間 49 time_t now; 50 struct tm* pNow; 51 time( &now ); 52 pNow = localtime( &now ); 53 printf( "%d-%d-%d %d:%d", pNow->tm_year + 1900, pNow->tm_mon + 1, pNow->tm_mday, pNow->tm_hour, pNow->tm_min ); 54 */ 55 56 struct tm* ptm; 57 time_t u_time = t_utmp->ut_time; 58 ptm = localtime( &u_time ); 59 int ihour = ptm->tm_hour; 60 int imin = ptm->tm_min; 61 62 char hour[3] = "0"; 63 char hour2[3] = "0"; 64 sprintf( hour2, "%d", ihour ); 65 format_time( hour, hour2 ); 66 67 char min[3] = "0"; 68 char min2[3] = "0"; 69 sprintf( min2, "%d", imin ); 70 format_time( min, min2 ); 71 72 //printf( "%d-%d-%d %d:%d", ptm->tm_year + 1900, ptm->tm_mon + 1, ptm->tm_mday, ihour, imin ); 73 printf( "%d-%d-%d %s:%s", ptm->tm_year + 1900, ptm->tm_mon + 1, ptm->tm_mday, hour, min ); 74 75 printf( " " ); 76 printf( "%-8.8s", t_utmp->ut_host ); 77 78 printf( "\n" ); 79 } 80 81 int main(int argc, char *argv[]) 82 { 83 struct utmp myutmp; 84 int fd = -1; 85 int reclen = sizeof( myutmp ); 86 87 fd = open( UTMP_FILE, O_RDONLY ); 88 89 if( -1 == fd ) { 90 perror( "open utmp" ); 91 exit( -1 ); 92 } 93 94 //printf( "fd = %d\n", fd ); 95 96 while( read( fd, &myutmp, reclen ) == reclen ) { 97 count++; 98 show_info( &myutmp ); 99 } 100 printf( "文件讀取的次數:%d\n", count ); 101 close( fd ); 102 103 104 return 0; 105 }
總結：
一個非常小的功能，囊括以下知識點：
1）文件讀取
2）man手冊與系統命令使用技巧
3）指針用法
4）字符串函數用法
5）時間函數用法
6）宏與typedef的用法