CentOS6 切换启动时的图形/文本界面的分析-白红宇

CentOS6 切换启动时的图形/文本界面的分析

阅读量：5299 次

发布时间：2019-06-14

本文共 3697 字，大约阅读时间需要 12 分钟。

虚拟机分配的内存较小，需要关闭图形界面。通过分析发现，kernel启动时，执行kernel_init()函数（init/main.c）：先新建一个initrd（init Ram Disk，内存中的文件系统，挂载到/）,然后执行init_post()。

1 static int __init kernel_init(void * unused) 2 { 3     lock_kernel(); 4  5     /* 6      * init can allocate pages on any node 7      */ 8     set_mems_allowed(node_possible_map); 9     /*10      * init can run on any cpu.11      */12     set_cpus_allowed_ptr(current, cpu_all_mask);13     /*14      * Tell the world that we're going to be the grim15      * reaper of innocent orphaned children.16      *17      * We don't want people to have to make incorrect18      * assumptions about where in the task array this19      * can be found.20      */21     init_pid_ns.child_reaper = current;22 23     cad_pid = task_pid(current);24 25     smp_prepare_cpus(setup_max_cpus);26 27     do_pre_smp_initcalls();28     start_boot_trace();29 30     smp_init();31     sched_init_smp();32 33     do_basic_setup();34 35     /*36      * check if there is an early userspace init.  If yes, let it do all37      * the work38      */39 40     if (!ramdisk_execute_command)41         ramdisk_execute_command = "/init";42 43     if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {44         ramdisk_execute_command = NULL;45         prepare_namespace();46     }47 48     /*49      * Ok, we have completed the initial bootup, and50      * we're essentially up and running. Get rid of the51      * initmem segments and start the user-mode stuff..52      */53 54     init_post();55     return 0;56 }

而在init_post()中，试图从多个位置查找init可执行文件，最后运行shell兜底（37~40行）：

1 static noinline int init_post(void) 2     __releases(kernel_lock) 3 { 4     /* need to finish all async __init code before freeing the memory */ 5     async_synchronize_full(); 6     free_initmem(); 7     unlock_kernel(); 8     mark_rodata_ro(); 9     system_state = SYSTEM_RUNNING;10     numa_default_policy();11 12     if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)13         printk(KERN_WARNING "Warning: unable to open an initial console.\n");14 15     (void) sys_dup(0);16     (void) sys_dup(0);17 18     current->signal->flags |= SIGNAL_UNKILLABLE;19 20     if (ramdisk_execute_command) {21         run_init_process(ramdisk_execute_command);22         printk(KERN_WARNING "Failed to execute %s\n",23                 ramdisk_execute_command);24     }25 26     /*27      * We try each of these until one succeeds.28      *29      * The Bourne shell can be used instead of init if we are30      * trying to recover a really broken machine.31      */32     if (execute_command) {33         run_init_process(execute_command);34         printk(KERN_WARNING "Failed to execute %s.  Attempting "35                     "defaults...\n", execute_command);36     }37     run_init_process("/sbin/init");38     run_init_process("/etc/init");39     run_init_process("/bin/init");40     run_init_process("/bin/sh");41 42     panic("No init found.  Try passing init= option to kernel.");43 }

在CentOS6中，/sbin/init属于upstart这个包，它是ubuntu对传统System V init daemon的一项重大改进，CentOS直接拿来主义了。它最大的优点：支持事件和热插拔。参考这里：

/sbin/init

|__/etc/init/*.conf 或*.override 系统级
|__/etc/dbus-1/system.d/Upstart.conf 用户级

而/etc/init/rc.conf、/etc/init/rcS.conf是/sbin/init执行的2个普通任务，

/etc/init/rc.conf

|__ /etc/rc.d/rc

它遍历：/etc/rc$runlevel.d/K*后执行initctl starting JOB=.... 来启动各种服务

/etc/init/rcS.conf

|__ /etc/rc.d/rc.sysinit

|__ /etc/inittab：【这里获取要进入的runlevel，id:5:initdefault:，5表示GUI界面】

|__ exec telinit $runlevel 【注意这里是重点，直接设置当前runlevel】

chkconfig 改变的是/etc/rcXX.d 链接

【注】CentOS7 用 /lib/systemd/system/default.target来取代/etc/inittab。

所以，只需要修改/etc/inittab： id:3:initdefault: 即可每次重启到控制台界面。

转载于:https://www.cnblogs.com/blogzcan/p/8288335.html

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