对于Unix/Linux程序员来说，"rm -rf /"一直被认为是一个极度危险的操作，因为直接把根目录给删除了，整个操作系统也就崩溃了。但实际上会是这样的吗？呵呵，请看图：
啊哈，世界并没有安静，一如既往地喧嚣。怎么回事儿？让我们来扒一扒源代码，
01 - 下载源代码(coreutils-8.30)
root# cd /tmp
root# wget https://download.fedoraproject.org/pub/fedora/linux/updates/29/Everything/SRPMS/Packages/c/coreutils-8.30-7.fc29.src.rpm
root# mkdir /tmp/coreutils
root# rpm -ivh --define '_topdir /tmp/coreutils' coreutils-8.30-7.fc29.src.rpm
root# cd /tmp/coreutils/SOURCES
root# ls -l *.xz
-rw-rw-r--. 1 root root 5359532 Jul 2 2018 coreutils-8.30.tar.xz
root# tar Jxf coreutils-8.30.tar.xz 02 - 查看rm -rf /的运行轨迹
1 root@intel-sharkbay-dh-02:/# strace rm -rf /
2 execve("/usr/bin/rm", ["rm", "-rf", "/"], 0x7ffde6de8960 /* 43 vars */) = 0
3 brk(NULL) = 0x558b0cb0f000
4 ...<snip>...
5 lstat("/", {st_mode=S_IFDIR|0555, st_size=224, ...}) = 0
6 newfstatat(AT_FDCWD, "/", {st_mode=S_IFDIR|0555, st_size=224, ...}, AT_SYMLINK_NOFOLLOW) = 0
7 openat(AT_FDCWD, "/usr/share/locale/en_US.UTF-8/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
8 ...<snip>...
9 openat(AT_FDCWD, "/usr/share/locale/en/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
10 write(2, "rm: ", 4rm: ) = 4
11 write(2, "it is dangerous to operate recur"..., 45it is dangerous to operate recursively on '/') = 45
12 write(2, "\n", 1
13 ) = 1
14 write(2, "rm: ", 4rm: ) = 4
15 write(2, "use --no-preserve-root to overri"..., 48use --no-preserve-root to override this failsafe) = 48
16 write(2, "\n", 1
17 ) = 1
18 lseek(0, 0, SEEK_CUR) = -1 ESPIPE (Illegal seek)
19 close(0) = 0
20 close(1) = 0
21 close(2) = 0
22 exit_group(1) = ?
23 +++ exited with 1 +++ 注意第5行和11行：
5 lstat("/", {st_mode=S_IFDIR|0555, st_size=224, ...}) = 0
11 write(2, "it is dangerous to operate recur"..., 45it is dangerous to operate recursively on '/') = 45 03 - 查看rm对应的源代码
03.01 main() in rm.c
/* coreutils-8.30/src/rm.c#209 */
208 int
209 main (int argc, char **argv)
210 {
211 bool preserve_root = true;
...
229 while ((c = getopt_long (argc, argv, "dfirvIR", long_opts, NULL)) != -1)
230 {
231 switch (c)
232 {
...
237 case 'f':
238 x.interactive = RMI_NEVER;
239 x.ignore_missing_files = true;
240 prompt_once = false;
241 break;
...
255 case 'r':
256 case 'R':
257 x.recursive = true;
258 break;
...
341 }
342
343 if (x.recursive && preserve_root)
344 {
345 static struct dev_ino dev_ino_buf;
346 x.root_dev_ino = get_root_dev_ino (&dev_ino_buf);
347 if (x.root_dev_ino == NULL)
348 die (EXIT_FAILURE, errno, _("failed to get attributes of %s"),
349 quoteaf ("/"));
350 }
...
370 enum RM_status status = rm (file, &x);
371 assert (VALID_STATUS (status));
372 return status == RM_ERROR ? EXIT_FAILURE : EXIT_SUCCESS;
373 } 在第346行，调用了函数get_root_dev_info(), 用以获取 root设备信息。
346 x.root_dev_ino = get_root_dev_ino (&dev_ino_buf); 其中，get_root_dev_info()实现如下：
/* coreutils-8.30/lib/root-dev-ino.c */
25 /* Call lstat to get the device and inode numbers for '/'.
26 Upon failure, return NULL. Otherwise, set the members of
27 *ROOT_D_I accordingly and return ROOT_D_I. */
28 struct dev_ino *
29 get_root_dev_ino (struct dev_ino *root_d_i)
30 {
31 struct stat statbuf;
32 if (lstat ("/", &statbuf))
33 return NULL;
34 root_d_i->st_ino = statbuf.st_ino;
35 root_d_i->st_dev = statbuf.st_dev;
36 return root_d_i;
37 } 第32行调用了lstat(), 这和我们在前面看到的strace rm -rf /的输出正好对应。
5 lstat("/", {st_mode=S_IFDIR|0555, st_size=224, ...}) = 0 rm.c的第370行调用了函数rm(), 这是我们接下来要重点分析的函数。
370 enum RM_status status = rm (file, &x); 03.02 rm() in remove.c
/* coreutils-8.30/src/remove.c#576 */
574 /* Remove FILEs, honoring options specified via X.
575 Return RM_OK if successful. */
576 enum RM_status
577 rm (char *const *file, struct rm_options const *x)
578 {
579 enum RM_status rm_status = RM_OK;
580
581 if (*file)
582 {
...
590 FTS *fts = xfts_open (file, bit_flags, NULL);
591
592 while (1)
593 {
...
596 ent = fts_read (fts);
...
607 enum RM_status s = rm_fts (fts, ent, x);
...
610 UPDATE_STATUS (rm_status, s);
611 }
...
618 }
619
620 return rm_status;
621 } 在第607行，函数rm()调用了函数rm_fts()。
03.03 rm_fts() in remove.c
/* coreutils-8.30/src/remove.c#418 */
417 static enum RM_status
418 rm_fts (FTS *fts, FTSENT *ent, struct rm_options const *x)
419 {
420 switch (ent->fts_info)
421 {
...
438 /* Perform checks that can apply only for command-line arguments. */
439 if (ent->fts_level == FTS_ROOTLEVEL)
440 {
...
454 /* POSIX also says:
455 If a command line argument resolves to "/" (and --preserve-root
456 is in effect -- default) diagnose and skip it. */
457 if (ROOT_DEV_INO_CHECK (x->root_dev_ino, ent->fts_statp))
458 {
459 ROOT_DEV_INO_WARN (ent->fts_path);
460 fts_skip_tree (fts, ent);
461 return RM_ERROR;
462 }
...
571 }
572 } 在第457行和459行，分别调用了宏函数。其中，L457的宏是把当前操作目录与根(/)做比较。
457 if (ROOT_DEV_INO_CHECK (x->root_dev_ino, ent->fts_statp)) 而L459是打印警告信息。
459 ROOT_DEV_INO_WARN (ent->fts_path); 03.04 ROOT_DEV_INO_WARN() in lib/root-dev-info.h
/* coreutils-8.30/lib/root-dev-ino.h#33 */
33 # define ROOT_DEV_INO_WARN(Dirname) \
34 do \
35 { \
36 if (STREQ (Dirname, "/")) \
37 error (0, 0, _("it is dangerous to operate recursively on %s"), \
38 quoteaf (Dirname)); \
39 else \
40 error (0, 0, \
41 _("it is dangerous to operate recursively on %s (same as %s)"), \
42 quoteaf_n (0, Dirname), quoteaf_n (1, "/")); \
43 error (0, 0, _("use --no-preserve-root to override this failsafe")); \
44 } \
45 while (0) 第37行和43行打印的信息，和我们执行rm -rf /之后看到的信息完全一致。
root@intel-sharkbay-dh-02:/# rm -rf /
rm: it is dangerous to operate recursively on '/'
rm: use --no-preserve-root to override this failsafe 03.05 ROOT_DEV_INO_CHECK() in lib/root-dev-info.h
/* coreutils-8.30/lib/root-dev-ino.h#30 */
30 # define ROOT_DEV_INO_CHECK(Root_dev_ino, Dir_statbuf) \
31 (Root_dev_ino && SAME_INODE (*Dir_statbuf, *Root_dev_ino)) 宏SAME_INODE的实现如下：
/* coreutils-8.30/lib/same-inode.h#42 */
42 # define SAME_INODE(a, b) \
43 ((a).st_ino == (b).st_ino \
44 && (a).st_dev == (b).st_dev) 到此为止，我们就完全看明白了为什么"rm -rf /"会被拒绝执行，因为现代版的rm命令已经已经针对野蛮的"rm -rf /"行为做了安全性检查。
结论：
- 在现代的Unix/Linux系统中，"rm -rf /"不再危险，因为rm命令本身做了相应的安全检查。也就是说，不但"rm -rf /"不危险，而且"rm -rf //"和"rm -rf /tmp/../"之类的也不危险。
- 虽然"rm -rf /"已经不再危险，但是"rm -rf /*"还是很危险滴，因为通配符'*'的存在。如好奇，请在你的虚拟机中予以尝试 。但是，本人强烈不推荐这么做! 否则，如下类似的输出就会泛洪于你的终端之上，即使你立即按CTRL+C也于事无补，至少常见的系统命令诸如ls, rm是立马不见了。。。
root@hp-moonshot-03-c31:/# id -un
root
root@hp-moonshot-03-c31:/# rm -rf /*
rm: cannot remove '/boot/efi': Device or resource busy
rm: cannot remove '/dev/mqueue': Device or resource busy
rm: cannot remove '/dev/hugepages': Device or resource busy
rm: cannot remove '/dev/pts/1': Operation not permitted
...<snip>...
rm: cannot remove '/proc/63/mountstats': Operation not permitted
rm: cannot remove '/proc/63/clear_refs': Operation not permitted
^C
root@hp-moonshot-03-c31:/#
root@hp-moonshot-03-c31:/# ls
bash: /usr/bin/ls: No such file or directory 因为
root# rm -rf /*
等价于
root# rm -rf /bin /boot /dev /etc /home /lib /lib64 /lost+found /media /misc /mnt /net /nfs /opt /proc /root /run /sbin /srv /sys /tmp /usr /var 所以
!!! NEVER TRY "rm -rf /*" !!!