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Linux一切皆文件,如果你没做到这一步,那这就是句话而已

前言

相信当你接触Linux的时候,身边的人就向你灌输一个概念,或者说你就总是听到一句话:Linux系统中一切皆文件

Linux下“一切皆文件”是Unix/Linux的基本哲学之一。不信,看下面这张图

Linux一切皆文件,如果你没做到这一步,那这就是句话而已

 

普通文件、目录、字符设备、块设备和网络设备(套接字)等在Unix/Linux都被当做文件来对待。虽然他们的类型不同,但是linux系统为它们提供了一套统一的操作接口。

Linux的一切皆文件是指,Linux世界中的所有、任意、一切东西都可以通过文件的方式访问、管理。

反过来说,是Linux和GNU世界定的规范,任何东西都挂在文件系统之上,即使它们不是文件,也以文件的形式来呈现。

比如我们经常会讲的进程(/proc)、设备(/dev)、Socket等等,实际上都不是文件,但是你可以以文件系统的规范来访问它,修改属主和属性。

那你对这句话的理解是不是仅仅止步于此呢?那我想,下面的这些内容你真的要看一下了

 

实际操作认证

Linux下有lsof命令,可以查看所有已经打开的文件,你使用lsof -p [pid]的方式就可以查看对应的进程都打开了什么文件,而其中的type字段就是表明它是什么类型,通过man losf 命令你可以查看到它有下面这么多种。

TYPE       is the type of the node associated with the file - e.g., GDIR, GREG, VDIR, VREG, etc.
      or ``IPv4'' for an IPv4 socket;
      or ``IPv6'' for an open IPv6 network file - even if its address is IPv4, mapped  in  an IPv6 address;
      or ``ax25'' for a Linux AX.25 socket;
      or ``inet'' for an Internet domain socket;
      or ``lla'' for a HP-UX link level access file;
      or ``rte'' for an AF_ROUTE socket;
      or ``sock'' for a socket of unknown domain;
      or ``unix'' for a UNIX domain socket;
      or ``x.25'' for an HP-UX x.25 socket;
      or ``BLK'' for a block special file;
      or ``CHR'' for a character special file;
      or ``DEL'' for a Linux map file that has been deleted;
      or ``DIR'' for a directory;
      or ``DOOR'' for a VDOOR file;
      or ``FIFO'' for a FIFO special file;
      or ``KQUEUE'' for a BSD style kernel event queue file;
      or ``LINK'' for a symbolic link file;
      or ``MPB'' for a multiplexed block file;
      or ``MPC'' for a multiplexed character file;
      or  ``NOFD''  for a Linux /proc/<PID>/fd directory that can't be opened -- the directory      path appears in the NAME column, followed by an error message;
      or ``PAS'' for a /proc/as file;
      or ``PAXV'' for a /proc/auxv file;
      or ``PCRE'' for a /proc/cred file;
      or ``PCTL'' for a /proc control file;
      or ``PCUR'' for the current /proc process;
      or ``PCWD'' for a /proc current working directory;
      or ``PDIR'' for a /proc directory;
      or ``PETY'' for a /proc executable type (etype);
      or ``PFD'' for a /proc file descriptor;
      or ``PFDR'' for a /proc file descriptor directory;
      or ``PFIL'' for an executable /proc file;
      or ``PFPR'' for a /proc FP register set;
      or ``PGD'' for a /proc/pagedata file;
      or ``PGID'' for a /proc group notifier file;
      or ``PIPE'' for pipes;
      or ``PLC'' for a /proc/lwpctl file;
      or ``PLDR'' for a /proc/lpw directory;
      or ``PLDT'' for a /proc/ldt file;
      or ``PLPI'' for a /proc/lpsinfo file;
      or ``PLST'' for a /proc/lstatus file;
      or ``PLU'' for a /proc/lusage file;
      or ``PLWG'' for a /proc/gwindows file;
      or ``PLWI'' for a /proc/lwpsinfo file;
      or ``PLWS'' for a /proc/lwpstatus file;
      or ``PLWU'' for a /proc/lwpusage file;
      or ``PLWX'' for a /proc/xregs file;
      or ``PMAP'' for a /proc map file (map);
      or ``PMEM'' for a /proc memory image file;
      or ``PNTF'' for a /proc process notifier file;
      or ``POBJ'' for a /proc/object file;
      or ``PODR'' for a /proc/object directory;
      or ``POLP'' for an old format /proc light weight process file;
      or ``POPF'' for an old format /proc PID file;
      or ``POPG'' for an old format /proc page data file;
      or ``PORT'' for a SYSV named pipe;
      or ``PREG'' for a /proc register file;
      or ``PRMP'' for a /proc/rmap file;
      or ``PRTD'' for a /proc root directory;
      or ``PSGA'' for a /proc/sigact file;
      or ``PSIN'' for a /proc/psinfo file;
      or ``PSTA'' for a /proc status file;
      or ``PSXSEM'' for a POSIX semaphore file;
      or ``PSXSHM'' for a POSIX shared memory file;
      or ``PTS'' for a /dev/pts file;
      or ``PUSG'' for a /proc/usage file;
      or ``PW'' for a /proc/watch file;
      or ``PXMP'' for a /proc/xmap file;
      or ``REG'' for a regular file;
      or ``SMT'' for a shared memory transport file;
      or ``STSO'' for a stream socket;
      or ``UNNM'' for an unnamed type file;
      or ``XNAM'' for an OpenServer Xenix special file of unknown type;
      or ``XSEM'' for an OpenServer Xenix semaphore file;
      or ``XSD'' for an OpenServer Xenix shared data file;
      or the four type number octets if the corresponding name isn't known.

 

这里我也添加一些实例给大家更好地展示一下

Linux一切皆文件,如果你没做到这一步,那这就是句话而已

 

# 进到proc目录
[root@k8s ~]# cd /proc
[root@k8s proc]# ls
1     1063  1077  11    12    1504  19    2175  24   280  297  301  317  39   407  413  49   519  726  737        bus       devices      filesystems  kallsyms    kpageflags  modules       partitions   softirqs       timer_list   vmstat
10    1066  1078  1100  13    16    2     2176  25   281  298  302  36   391  408  414  495  52   729  743        cgroups   diskstats    fs           kcore       loadavg     mounts        sched_debug  stat           timer_stats  zoneinfo
102   1068  1083  1102  14    1746  20    2177  26   282  299  303  37   392  409  415  5    53   733  8          cmdline   dma          interrupts   keys        locks       mpt           schedstat    swaps          tty
1038  1071  1085  1110  15    1748  2005  2178  27   29   3    304  38   4    410  416  50   6    734  9          consoles  driver       iomem        key-users   mdstat      mtrr          scsi         sys            uptime
1061  1072  1097  1113  1502  1749  2007  22    279  294  30   305  380  405  411  47   51   66   735  acpi       cpuinfo   execdomains  ioports      kmsg        meminfo     net           self         sysrq-trigger  version
1062  1073  1099  1164  1503  18    21    23    28   296  300  31   381  406  412  48   511  7    736  buddyinfo  crypto    fb           irq          kpagecount  misc        pagetypeinfo  slabinfo     sysvipc        vmallocinfo

# 可以看到上面有很多以数字为名称的目录,而这些目录就对应每一个进程
# 使用ps -ef|grep nginx可以找到Nginx主进程的PID是2175
[root@k8s proc]# ps -ef|grep nginx
root       2175      1  0 10:23 ?        00:00:00 nginx: master process /usr/sbin/nginx
nginx      2176   2175  0 10:23 ?        00:00:00 nginx: worker process
nginx      2177   2175  0 10:23 ?        00:00:00 nginx: worker process
root       2182   2007  0 10:24 pts/0    00:00:00 grep --color=auto nginx

# 进入/proc/2175目录,可以看到这个进程相关的目录
[root@k8s 2175]# ls
attr       cgroup      comm             cwd      fd       io        map_files  mountinfo   net        oom_adj        pagemap      projid_map  schedstat  smaps  statm    task     wchan
autogroup  clear_refs  coredump_filter  environ  fdinfo   limits    maps       mounts      ns         oom_score      patch_state  root        sessionid  stack  status   timers
auxv       cmdline     cpuset           exe      gid_map  loginuid  mem        mountstats  numa_maps  oom_score_adj  personality  sched       setgroups  stat   syscall  uid_map
# 这里面的内容就是2175进程的全部内容了,非常多的命令实际上都是从这个目录读取的信息,比如上面的ps命令

# 上面的文件我举几个例子解释一下
# 1. fd目录,里面是此进程打开的文件的情况,每个均链接至实际的文件或设备
[root@k8s 2175]# ll fd
总用量 0
lrwx------ 1 root root 64 9  29 10:53 0 -> /dev/null
lrwx------ 1 root root 64 9  29 10:53 1 -> /dev/null
lrwx------ 1 root root 64 9  29 10:53 10 -> socket:[26530]
l-wx------ 1 root root 64 9  29 10:53 2 -> /var/log/nginx/error.log
lrwx------ 1 root root 64 9  29 10:53 3 -> socket:[26527]
l-wx------ 1 root root 64 9  29 10:53 4 -> /var/log/nginx/error.log
l-wx------ 1 root root 64 9  29 10:53 5 -> /var/log/nginx/access.log
lrwx------ 1 root root 64 9  29 10:53 6 -> socket:[27700]
lrwx------ 1 root root 64 9  29 10:53 7 -> socket:[27701]
lrwx------ 1 root root 64 9  29 10:53 8 -> socket:[26528]
lrwx------ 1 root root 64 9  29 10:53 9 -> socket:[26529]

# 2. limits,内容是此进程的系统限制信息
[root@k8s 2175]# cat limits
Limit                     Soft Limit           Hard Limit           Units
Max cpu time              unlimited            unlimited            seconds
Max file size             unlimited            unlimited            bytes
Max data size             unlimited            unlimited            bytes
Max stack size            8388608              unlimited            bytes
Max core file size        0                    unlimited            bytes
Max resident set          unlimited            unlimited            bytes
Max processes             14889                14889                processes
Max open files            1024                 4096                 files
Max locked memory         65536                65536                bytes
Max address space         unlimited            unlimited            bytes
Max file locks            unlimited            unlimited            locks
Max pending signals       14889                14889                signals
Max msgqueue size         819200               819200               bytes
Max nice priority         0                    0
Max realtime priority     0                    0
Max realtime timeout      unlimited            unlimited            us

# 3. stack,此进程的内核调用栈信息
[root@k8s 2175]# cat stack
[<ffffffff958b5859>] sigsuspend+0x39/0x70
[<ffffffff958b58ee>] SyS_rt_sigsuspend+0x5e/0x80
[<ffffffff95f92ed2>] system_call_fastpath+0x25/0x2a
[<ffffffffffffffff>] 0xffffffffffffffff

# 这些内容都不是普通的文件,你通过文件系统看,它们的大小都是0,但你看我能输出上面的这么多信息
[root@k8s 2175]# ll
总用量 0
dr-xr-xr-x 2 root root 0 9  29 10:25 attr
-rw-r--r-- 1 root root 0 9  29 10:25 autogroup
-r-------- 1 root root 0 9  29 10:25 auxv
-r--r--r-- 1 root root 0 9  29 10:23 cgroup
--w------- 1 root root 0 9  29 10:25 clear_refs
-r--r--r-- 1 root root 0 9  29 10:24 cmdline
-rw-r--r-- 1 root root 0 9  29 10:25 comm
-rw-r--r-- 1 root root 0 9  29 10:25 coredump_filter
-r--r--r-- 1 root root 0 9  29 10:25 cpuset
...
[root@k8s 2175]# du -sh
0

另外,我们经常会使用到的/dev/null, /dev/random,都是可以在文件目录中看到,但它们都不是普通的文件,而是Linux Kernel为了实现某些功能,同时存在everything is a file的约定,才把它们体现在了文件系统上。

 

另外补一个我觉得挺有用的,且应该不少人不知道的一个小技巧

man 5 proc命令会输出/proc目录的帮助信息,它里面包含/proc/[pid]目录中每个目录和文件的说明信息

PROC(5)                                                                                                                 Linux Programmer's Manual                                                                                                                 PROC(5)

NAME
       proc - process information pseudo-file system

DESCRIPTION
       The proc file system is a pseudo-file system which is used as an interface to kernel data structures.  It is commonly mounted at /proc.  Most of it is read-only, but some files allow kernel variables to be changed.

       The following outline gives a quick tour through the /proc hierarchy.

       /proc/[pid]
              There is a numerical subdirectory for each running process; the subdirectory is named by the process ID.  Each such subdirectory contains the following pseudo-files and directories.

       /proc/[pid]/auxv (since 2.6.0-test7)
              This contains the contents of the ELF interpreter information passed to the process at exec time.  The format is one unsigned long ID plus one unsigned long value for each entry.  The last entry contains two zeros.

       /proc/[pid]/cgroup (since Linux 2.6.24)
              This file describes control groups to which the process/task belongs.  For each cgroup hierarchy there is one entry containing colon-separated fields of the form:

                  5:cpuacct,cpu,cpuset:/daemons

              The colon-separated fields are, from left to right:

                  1. hierarchy ID number

                  2. set of subsystems bound to the hierarchy

                  3. control group in the hierarchy to which the process belongs

              This file is present only if the CONFIG_CGROUPS kernel configuration option is enabled.

       /proc/[pid]/cmdline
              This  holds  the complete command line for the process, unless the process is a zombie.  In the latter case, there is nothing in this file: that is, a read on this file will return 0 characters.  The command-line arguments appear in this file as a set
              of strings separated by null bytes ('\0'), with a further null byte after the last string.

       /proc/[pid]/coredump_filter (since kernel 2.6.23)
              See core(5).

       /proc/[pid]/cpuset (since kernel 2.6.12)
              See cpuset(7).

       /proc/[pid]/cwd
              This is a symbolic link to the current working directory of the process.  To find out the current working directory of process 20, for instance, you can do this:

                  $ cd /proc/20/cwd; /bin/pwd

              Note that the pwd command is often a shell built-in, and might not work properly.  In bash(1), you may use pwd -P.

              In a multithreaded process, the contents of this symbolic link are not available if the main thread has already terminated (typically by calling pthread_exit(3)).

 

其实,这就是Linux最让人着迷的地方,总是感觉很多东西都很简单,一个命令行就可以解决问题,但是深入去研究一下,你又会发现,真的有很多更神秘的地方再等待着你去探索,越深入学习越觉得兴奋,起码我个人是这样觉得

加油吧,程序员

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