关于Linux命令底层那点事

不背锅运维 2022-05-29

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Linux操作系统有N多系统内置的命令，用途之广，功能之强悍。但在实际工作中，用来用去的命令无非就那几十个，当对陌生的命令语法、参数不熟悉的时候，可以通过help帮助进行查看，或者进行百度、谷歌。

本篇并不打算去讲那些百度便可查到的各种命令的使用方法、技巧，因为已经烂大街的各种命令就没必要去讲了。既然都烂大街了，还讲锤子呢？干运维的朋友都知道，笔者就不说废话了。本篇讲点别的！

开篇之前，先引蛇出洞

linux命令是存在于文件系统上的一个可执行程序文件，在众多命令中，比如突然出现在笔者脑海里的sed命令，如下可以看到：

[root@mysql8db ~]# which sed
/usr/bin/sed

[root@mysql8db ~]# ls -l /usr/bin/sed
-rwxr-xr-x 1 root root 76072 Apr  1  2020 /usr/bin/sed
[root@mysql8db ~]# 

[root@mysql8db ~]# od /usr/bin/sed | less
0000000 042577 043114 000402 000001 000000 000000 000000 000000
0000020 000002 000076 000001 000000 023422 000100 000000 000000
0000040 000100 000000 000000 000000 020750 000001 000000 000000
0000060 000000 000000 000100 000070 000011 000100 000035 000034
0000100 000006 000000 000005 000000 000100 000000 000000 000000
0000120 000100 000100 000000 000000 000100 000100 000000 000000
0000140 000770 000000 000000 000000 000770 000000 000000 000000
...
...

这条蛇引的好，居然拿sed命令这条蛇先开刀了。

跳过前戏，直奔主题

在这之前先提出一个问题，sed命令的功能，它是怎么起到作用的？

简单剖析

sed命令是Linux操作系统内置、且处于用户空间的一个系统命令。一般来说，系统命令，都是使用C语言编写（Linux操作系统也都是用C语言编写的）。用C语言编写的程序代码区可以看成一个函数的集合，系统命令通过执行一个个函数完成任务。也就是说，sed命令的功能之所以能做到对文件进行各种操作，在sed命令程序代码里，是有调用某些函数的（或者说API）。

进一步剖析

sed命令（也可以说是sed命令的程序），在执行之前其实已经拥有了一个函数列表，它们存在于内核，但需要通过系统调用来完成。系统调用是什么鬼？比如用户想要请求系统打开文件、关闭文件或读写文件，比如笔者在本篇要讲的sed命令程序，它就可以通过文件系统相关的系统调用来做到。再比如，用户想要获得系统时间等，就可以通过时钟相关的系统调用来做到，系统调用是用户级程序访问内核最基本的方法。比如做C开发的程序员，在实际C语言开发中一般是不会直接进行系统调用的，而是通过一些用户编程接口（API）来调用。这些API主要是通过C库（libc）实现，在调用这些函数（API）时，函数内部实际上是在进行系统调用。

上图笔者亲自所画，终于不用特别声明侵网上的版权了。哈哈！

Linux操作系统有300多个以上的系统调用，经过笔者的不断搜索和整理，列出了Linux系统调用表，笔者参考的是下面2个连接：

https://filippo.io/linux-syscall-table/
https://www.tutorialspoint.com/unix_system_calls/index.htm

希望可以做到抛砖引玉，对其有兴趣的朋友可以通过以上两个链接进行更深入的剖析。

“抄袭”的Linux系统调用表

系统调用号	函数名	入口点	源码文件
0	read	sys_read	fs/read_write.c
1	write	sys_write	fs/read_write.c
2	open	sys_open	fs/open.c
3	close	sys_close	fs/open.c
4	stat	sys_newstat	fs/stat.c
5	fstat	sys_newfstat	fs/stat.c
6	lstat	sys_newlstat	fs/stat.c
7	poll	sys_poll	fs/select.c
8	lseek	sys_lseek	fs/read_write.c
9	mmap	sys_mmap	arch/x86/kernel/sys_x86_64.c
10	mprotect	sys_mprotect	mm/mprotect.c
11	munmap	sys_munmap	mm/mmap.c
12	brk	sys_brk	mm/mmap.c
13	rt_sigaction	sys_rt_sigaction	kernel/signal.c
14	rt_sigprocmask	sys_rt_sigprocmask	kernel/signal.c
15	rt_sigreturn	stub_rt_sigreturn	arch/x86/kernel/signal.c
16	ioctl	sys_ioctl	fs/ioctl.c
17	pread64	sys_pread64	fs/read_write.c
18	pwrite64	sys_pwrite64	fs/read_write.c
19	readv	sys_readv	fs/read_write.c
20	writev	sys_writev	fs/read_write.c
21	access	sys_access	fs/open.c
22	pipe	sys_pipe	fs/pipe.c
23	select	sys_select	fs/select.c
24	sched_yield	sys_sched_yield	kernel/sched/core.c
25	mremap	sys_mremap	mm/mmap.c
26	msync	sys_msync	mm/msync.c
27	mincore	sys_mincore	mm/mincore.c
28	madvise	sys_madvise	mm/madvise.c
29	shmget	sys_shmget	ipc/shm.c
30	shmat	sys_shmat	ipc/shm.c
31	shmctl	sys_shmctl	ipc/shm.c
32	dup	sys_dup	fs/file.c
33	dup2	sys_dup2	fs/file.c
34	pause	sys_pause	kernel/signal.c
35	nanosleep	sys_nanosleep	kernel/hrtimer.c
36	getitimer	sys_getitimer	kernel/itimer.c
37	alarm	sys_alarm	kernel/timer.c
38	setitimer	sys_setitimer	kernel/itimer.c
39	getpid	sys_getpid	kernel/sys.c
40	sendfile	sys_sendfile64	fs/read_write.c
41	socket	sys_socket	net/socket.c
42	connect	sys_connect	net/socket.c
43	accept	sys_accept	net/socket.c
44	sendto	sys_sendto	net/socket.c
45	recvfrom	sys_recvfrom	net/socket.c
46	sendmsg	sys_sendmsg	net/socket.c
47	recvmsg	sys_recvmsg	net/socket.c
48	shutdown	sys_shutdown	net/socket.c
49	bind	sys_bind	net/socket.c
50	listen	sys_listen	net/socket.c
51	getsockname	sys_getsockname	net/socket.c
52	getpeername	sys_getpeername	net/socket.c
53	socketpair	sys_socketpair	net/socket.c
54	setsockopt	sys_setsockopt	net/socket.c
55	getsockopt	sys_getsockopt	net/socket.c
56	clone	stub_clone	kernel/fork.c
57	fork	stub_fork	kernel/fork.c
58	vfork	stub_vfork	kernel/fork.c
59	execve	stub_execve	fs/exec.c
60	exit	sys_exit	kernel/exit.c
61	wait4	sys_wait4	kernel/exit.c
62	kill	sys_kill	kernel/signal.c
63	uname	sys_newuname	kernel/sys.c
64	semget	sys_semget	ipc/sem.c
65	semop	sys_semop	ipc/sem.c
66	semctl	sys_semctl	ipc/sem.c
67	shmdt	sys_shmdt	ipc/shm.c
68	msgget	sys_msgget	ipc/msg.c
69	msgsnd	sys_msgsnd	ipc/msg.c
70	msgrcv	sys_msgrcv	ipc/msg.c
71	msgctl	sys_msgctl	ipc/msg.c
72	fcntl	sys_fcntl	fs/fcntl.c
73	flock	sys_flock	fs/locks.c
74	fsync	sys_fsync	fs/sync.c
75	fdatasync	sys_fdatasync	fs/sync.c
76	truncate	sys_truncate	fs/open.c
77	ftruncate	sys_ftruncate	fs/open.c
78	getdents	sys_getdents	fs/readdir.c
79	getcwd	sys_getcwd	fs/dcache.c
80	chdir	sys_chdir	fs/open.c
81	fchdir	sys_fchdir	fs/open.c
82	rename	sys_rename	fs/namei.c
83	mkdir	sys_mkdir	fs/namei.c
84	rmdir	sys_rmdir	fs/namei.c
85	creat	sys_creat	fs/open.c
86	link	sys_link	fs/namei.c
87	unlink	sys_unlink	fs/namei.c
88	symlink	sys_symlink	fs/namei.c
89	readlink	sys_readlink	fs/stat.c
90	chmod	sys_chmod	fs/open.c
91	fchmod	sys_fchmod	fs/open.c
92	chown	sys_chown	fs/open.c
93	fchown	sys_fchown	fs/open.c
94	lchown	sys_lchown	fs/open.c
95	umask	sys_umask	kernel/sys.c
96	gettimeofday	sys_gettimeofday	kernel/time.c
97	getrlimit	sys_getrlimit	kernel/sys.c
98	getrusage	sys_getrusage	kernel/sys.c
99	sysinfo	sys_sysinfo	kernel/sys.c
100	times	sys_times	kernel/sys.c
101	ptrace	sys_ptrace	kernel/ptrace.c
102	getuid	sys_getuid	kernel/sys.c
103	syslog	sys_syslog	kernel/printk/printk.c
104	getgid	sys_getgid	kernel/sys.c
105	setuid	sys_setuid	kernel/sys.c
106	setgid	sys_setgid	kernel/sys.c
107	geteuid	sys_geteuid	kernel/sys.c
108	getegid	sys_getegid	kernel/sys.c
109	setpgid	sys_setpgid	kernel/sys.c
110	getppid	sys_getppid	kernel/sys.c
111	getpgrp	sys_getpgrp	kernel/sys.c
112	setsid	sys_setsid	kernel/sys.c
113	setreuid	sys_setreuid	kernel/sys.c
114	setregid	sys_setregid	kernel/sys.c
115	getgroups	sys_getgroups	kernel/groups.c
116	setgroups	sys_setgroups	kernel/groups.c
117	setresuid	sys_setresuid	kernel/sys.c
118	getresuid	sys_getresuid	kernel/sys.c
119	setresgid	sys_setresgid	kernel/sys.c
120	getresgid	sys_getresgid	kernel/sys.c
121	getpgid	sys_getpgid	kernel/sys.c
122	setfsuid	sys_setfsuid	kernel/sys.c
123	setfsgid	sys_setfsgid	kernel/sys.c
124	getsid	sys_getsid	kernel/sys.c
125	capget	sys_capget	kernel/capability.c
126	capset	sys_capset	kernel/capability.c
127	rt_sigpending	sys_rt_sigpending	kernel/signal.c
128	rt_sigtimedwait	sys_rt_sigtimedwait	kernel/signal.c
129	rt_sigqueueinfo	sys_rt_sigqueueinfo	kernel/signal.c
130	rt_sigsuspend	sys_rt_sigsuspend	kernel/signal.c
131	sigaltstack	sys_sigaltstack	kernel/signal.c
132	utime	sys_utime	fs/utimes.c
133	mknod	sys_mknod	fs/namei.c
134	uselib		fs/exec.c
135	personality	sys_personality	kernel/exec_domain.c
136	ustat	sys_ustat	fs/statfs.c
137	statfs	sys_statfs	fs/statfs.c
138	fstatfs	sys_fstatfs	fs/statfs.c
139	sysfs	sys_sysfs	fs/filesystems.c
140	getpriority	sys_getpriority	kernel/sys.c
141	setpriority	sys_setpriority	kernel/sys.c
142	sched_setparam	sys_sched_setparam	kernel/sched/core.c
143	sched_getparam	sys_sched_getparam	kernel/sched/core.c
144	sched_setscheduler	sys_sched_setscheduler	kernel/sched/core.c
145	sched_getscheduler	sys_sched_getscheduler	kernel/sched/core.c
146	sched_get_priority_max	sys_sched_get_priority_max	kernel/sched/core.c
147	sched_get_priority_min	sys_sched_get_priority_min	kernel/sched/core.c
148	sched_rr_get_interval	sys_sched_rr_get_interval	kernel/sched/core.c
149	mlock	sys_mlock	mm/mlock.c
150	munlock	sys_munlock	mm/mlock.c
151	mlockall	sys_mlockall	mm/mlock.c
152	munlockall	sys_munlockall	mm/mlock.c
153	vhangup	sys_vhangup	fs/open.c
154	modify_ldt	sys_modify_ldt	arch/x86/um/ldt.c
155	pivot_root	sys_pivot_root	fs/namespace.c
156	_sysctl	sys_sysctl	kernel/sysctl_binary.c
157	prctl	sys_prctl	kernel/sys.c
158	arch_prctl	sys_arch_prctl	arch/x86/um/syscalls_64.c
159	adjtimex	sys_adjtimex	kernel/time.c
160	setrlimit	sys_setrlimit	kernel/sys.c
161	chroot	sys_chroot	fs/open.c
162	sync	sys_sync	fs/sync.c
163	acct	sys_acct	kernel/acct.c
164	settimeofday	sys_settimeofday	kernel/time.c
165	mount	sys_mount	fs/namespace.c
166	umount2	sys_umount	fs/namespace.c
167	swapon	sys_swapon	mm/swapfile.c
168	swapoff	sys_swapoff	mm/swapfile.c
169	reboot	sys_reboot	kernel/reboot.c
170	sethostname	sys_sethostname	kernel/sys.c
171	setdomainname	sys_setdomainname	kernel/sys.c
172	iopl	stub_iopl	arch/x86/kernel/ioport.c
173	ioperm	sys_ioperm	arch/x86/kernel/ioport.c
174	create_module		NOT IMPLEMENTED
175	init_module	sys_init_module	kernel/module.c
176	delete_module	sys_delete_module	kernel/module.c
177	get_kernel_syms		NOT IMPLEMENTED
178	query_module		NOT IMPLEMENTED
179	quotactl	sys_quotactl	fs/quota/quota.c
180	nfsservctl		NOT IMPLEMENTED
181	getpmsg		NOT IMPLEMENTED
182	putpmsg		NOT IMPLEMENTED
183	afs_syscall		NOT IMPLEMENTED
184	tuxcall		NOT IMPLEMENTED
185	security		NOT IMPLEMENTED
186	gettid	sys_gettid	kernel/sys.c
187	readahead	sys_readahead	mm/readahead.c
188	setxattr	sys_setxattr	fs/xattr.c
189	lsetxattr	sys_lsetxattr	fs/xattr.c
190	fsetxattr	sys_fsetxattr	fs/xattr.c
191	getxattr	sys_getxattr	fs/xattr.c
192	lgetxattr	sys_lgetxattr	fs/xattr.c
193	fgetxattr	sys_fgetxattr	fs/xattr.c
194	listxattr	sys_listxattr	fs/xattr.c
195	llistxattr	sys_llistxattr	fs/xattr.c
196	flistxattr	sys_flistxattr	fs/xattr.c
197	removexattr	sys_removexattr	fs/xattr.c
198	lremovexattr	sys_lremovexattr	fs/xattr.c
199	fremovexattr	sys_fremovexattr	fs/xattr.c
200	tkill	sys_tkill	kernel/signal.c
201	time	sys_time	kernel/time.c
202	futex	sys_futex	kernel/futex.c
203	sched_setaffinity	sys_sched_setaffinity	kernel/sched/core.c
204	sched_getaffinity	sys_sched_getaffinity	kernel/sched/core.c
205	set_thread_area		arch/x86/kernel/tls.c
206	io_setup	sys_io_setup	fs/aio.c
207	io_destroy	sys_io_destroy	fs/aio.c
208	io_getevents	sys_io_getevents	fs/aio.c
209	io_submit	sys_io_submit	fs/aio.c
210	io_cancel	sys_io_cancel	fs/aio.c
211	get_thread_area		arch/x86/kernel/tls.c
212	lookup_dcookie	sys_lookup_dcookie	fs/dcookies.c
213	epoll_create	sys_epoll_create	fs/eventpoll.c
214	epoll_ctl_old		NOT IMPLEMENTED
215	epoll_wait_old		NOT IMPLEMENTED
216	remap_file_pages	sys_remap_file_pages	mm/fremap.c
217	getdents64	sys_getdents64	fs/readdir.c
218	set_tid_address	sys_set_tid_address	kernel/fork.c
219	restart_syscall	sys_restart_syscall	kernel/signal.c
220	semtimedop	sys_semtimedop	ipc/sem.c
221	fadvise64	sys_fadvise64	mm/fadvise.c
222	timer_create	sys_timer_create	kernel/posix-timers.c
223	timer_settime	sys_timer_settime	kernel/posix-timers.c
224	timer_gettime	sys_timer_gettime	kernel/posix-timers.c
225	timer_getoverrun	sys_timer_getoverrun	kernel/posix-timers.c
226	timer_delete	sys_timer_delete	kernel/posix-timers.c
227	clock_settime	sys_clock_settime	kernel/posix-timers.c
228	clock_gettime	sys_clock_gettime	kernel/posix-timers.c
229	clock_getres	sys_clock_getres	kernel/posix-timers.c
230	clock_nanosleep	sys_clock_nanosleep	kernel/posix-timers.c
231	exit_group	sys_exit_group	kernel/exit.c
232	epoll_wait	sys_epoll_wait	fs/eventpoll.c
233	epoll_ctl	sys_epoll_ctl	fs/eventpoll.c
234	tgkill	sys_tgkill	kernel/signal.c
235	utimes	sys_utimes	fs/utimes.c
236	vserver		NOT IMPLEMENTED
237	mbind	sys_mbind	mm/mempolicy.c
238	set_mempolicy	sys_set_mempolicy	mm/mempolicy.c
239	get_mempolicy	sys_get_mempolicy	mm/mempolicy.c
240	mq_open	sys_mq_open	ipc/mqueue.c
241	mq_unlink	sys_mq_unlink	ipc/mqueue.c
242	mq_timedsend	sys_mq_timedsend	ipc/mqueue.c
243	mq_timedreceive	sys_mq_timedreceive	ipc/mqueue.c
244	mq_notify	sys_mq_notify	ipc/mqueue.c
245	mq_getsetattr	sys_mq_getsetattr	ipc/mqueue.c
246	kexec_load	sys_kexec_load	kernel/kexec.c
247	waitid	sys_waitid	kernel/exit.c
248	add_key	sys_add_key	security/keys/keyctl.c
249	request_key	sys_request_key	security/keys/keyctl.c
250	keyctl	sys_keyctl	security/keys/keyctl.c
251	ioprio_set	sys_ioprio_set	fs/ioprio.c
252	ioprio_get	sys_ioprio_get	fs/ioprio.c
253	inotify_init	sys_inotify_init	fs/notify/inotify/inotify_user.c
254	inotify_add_watch	sys_inotify_add_watch	fs/notify/inotify/inotify_user.c
255	inotify_rm_watch	sys_inotify_rm_watch	fs/notify/inotify/inotify_user.c
256	migrate_pages	sys_migrate_pages	mm/mempolicy.c
257	openat	sys_openat	fs/open.c
258	mkdirat	sys_mkdirat	fs/namei.c
259	mknodat	sys_mknodat	fs/namei.c
260	fchownat	sys_fchownat	fs/open.c
261	futimesat	sys_futimesat	fs/utimes.c
262	newfstatat	sys_newfstatat	fs/stat.c
263	unlinkat	sys_unlinkat	fs/namei.c
264	renameat	sys_renameat	fs/namei.c
265	linkat	sys_linkat	fs/namei.c
266	symlinkat	sys_symlinkat	fs/namei.c
267	readlinkat	sys_readlinkat	fs/stat.c
268	fchmodat	sys_fchmodat	fs/open.c
269	faccessat	sys_faccessat	fs/open.c
270	pselect6	sys_pselect6	fs/select.c
271	ppoll	sys_ppoll	fs/select.c
272	unshare	sys_unshare	kernel/fork.c
273	set_robust_list	sys_set_robust_list	kernel/futex.c
274	get_robust_list	sys_get_robust_list	kernel/futex.c
275	splice	sys_splice	fs/splice.c
276	tee	sys_tee	fs/splice.c
277	sync_file_range	sys_sync_file_range	fs/sync.c
278	vmsplice	sys_vmsplice	fs/splice.c
279	move_pages	sys_move_pages	mm/migrate.c
280	utimensat	sys_utimensat	fs/utimes.c
281	epoll_pwait	sys_epoll_pwait	fs/eventpoll.c
282	signalfd	sys_signalfd	fs/signalfd.c
283	timerfd_create	sys_timerfd_create	fs/timerfd.c
284	eventfd	sys_eventfd	fs/eventfd.c
285	fallocate	sys_fallocate	fs/open.c
286	timerfd_settime	sys_timerfd_settime	fs/timerfd.c
287	timerfd_gettime	sys_timerfd_gettime	fs/timerfd.c
288	accept4	sys_accept4	net/socket.c
289	signalfd4	sys_signalfd4	fs/signalfd.c
290	eventfd2	sys_eventfd2	fs/eventfd.c
291	epoll_create1	sys_epoll_create1	fs/eventpoll.c
292	dup3	sys_dup3	fs/file.c
293	pipe2	sys_pipe2	fs/pipe.c
294	inotify_init1	sys_inotify_init1	fs/notify/inotify/inotify_user.c
295	preadv	sys_preadv	fs/read_write.c
296	pwritev	sys_pwritev	fs/read_write.c
297	rt_tgsigqueueinfo	sys_rt_tgsigqueueinfo	kernel/signal.c
298	perf_event_open	sys_perf_event_open	kernel/events/core.c
299	recvmmsg	sys_recvmmsg	net/socket.c
300	fanotify_init	sys_fanotify_init	fs/notify/fanotify/fanotify_user.c
301	fanotify_mark	sys_fanotify_mark	fs/notify/fanotify/fanotify_user.c
302	prlimit64	sys_prlimit64	kernel/sys.c
303	name_to_handle_at	sys_name_to_handle_at	fs/fhandle.c
304	open_by_handle_at	sys_open_by_handle_at	fs/fhandle.c
305	clock_adjtime	sys_clock_adjtime	kernel/posix-timers.c
306	syncfs	sys_syncfs	fs/sync.c
307	sendmmsg	sys_sendmmsg	net/socket.c
308	setns	sys_setns	kernel/nsproxy.c
309	getcpu	sys_getcpu	kernel/sys.c
310	process_vm_readv	sys_process_vm_readv	mm/process_vm_access.c
311	process_vm_writev	sys_process_vm_writev	mm/process_vm_access.c
312	kcmp	sys_kcmp	kernel/kcmp.c
313	finit_module	sys_finit_module	kernel/module.c

写在最后

本篇只是拿sed命令开刀了，不仅于此，其他内置的系统命令也是一样的套路。其实，在笔者看来，各种命令的使用方法、技巧并没什么可讲，一点都不好玩。底层的实现、原理、机制才是核心，知其然也要知其所以然。这是作为技术人员所必须要追求的真理！

linux系统系统调用 inotify socket

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关于Linux命令底层那点事

开篇之前，先引蛇出洞

跳过前戏，直奔主题

“抄袭”的Linux系统调用表

写在最后

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