帮你精通Linux：从Manual手册中获得全局掌控感

湛蓝之海

当今可能我们很多人已经不频繁使用man命令了，取而代之的是更加友好简单的tldr。

$ tldr du |head  
du 
Disk usage: estimate and summarize file and directory space usage. 
 
 - List the sizes of a directory and any subdirectories, in the given unit (B/KB/MB): 
   du -{{b|k|m}} {{path/to/directory}} 
 - List the sizes of a directory and any subdirectories, in human-readable form (i.e. auto-selecting the appropriate unit for each size): 
   du -h {{path/to/directory}}r 
 - Show the size of a single directory, in human readable units:

然而，man命令始终是我个人的最爱。每当心情低落时候，我就会中用man命令信步由缰的做各种查询。从中慢慢获得秩序感，全局的掌控感，低落的心情也慢慢疗愈。
man命令让我们与计算机对话沟通，又仿佛促使我们与计算机融为一体，人机合一。
1.man的9个章节
man可能是唯一一个可以自我陈述的命令。我们先从最直觉的使用方法开始。

man man

从结果中，我们看到Manual手册共分为9个章节(sections)。似乎有点多，我们将归类为4个部分。

前三项是commands(shell-commands, system-calls,libraries)
接着是files(special-file,file-format)
然后是Miscellaneous, Game也放在这里
最后是System(Administation,Kernel)

2.存放地址
Linux系统中一切皆为文件，我们首先感兴趣的是，这些manual手册存放在哪里。根据前文帮你精通Linux：透彻解析Linux的目录树结构可以简单推测，

配置文件在/etc/目录下，具体则为/etc/manpath.config;
变量文件在/var/目录下，具体则为/var/cache/man/index.(bt|db|dir|pag)

/etc和/var是linux系统下的最重要的两个目录。
man文件本身存储在/usr/share目录下，/usr/share/man
如何查看特定命令的存储位置呢?需要-w选项(对应长选项--where)

$ man -w time 
/usr/share/man/man1/time.1.gz

加上选-a(对应--all)则能查到time在所有章节中的位置。

$ man -aw time 
/usr/share/man/man1/time.1.gz 
/usr/share/man/man2/time.2.gz 
/usr/share/man/man7/time.7.gz

从结果中看到，在man1(shell commands), man2(system calls)和man7(Miscellaneous)中都有time这个命令。
作为补充，我们也可以用-f选项查看命令所属的sections。

$ man -f man 
man (7)        - macros to format man pages 
man (1)        - an interface to the on-line reference manuals 
 
$ man -f time 
time (7)       - overview of time and timers 
time (1)       - run programs and summarize system resource usage 
time (2)       - get time in seconds

此处的 -f 等价于--whatis
whatis (1) - display one-line manual page descriptions
3.指定查询章节
从 -w 选项中看到了命令的地址，便可以指定章节查询。

whatis (1)       - display one-line manual page descriptions

man  7 time

4.正则查询
man命令最有意思的就是正则查询，心情不好的时候，可以玩上一整天。
从正则查询中，能够获得对全局的掌控感，比如查询systemd有多少个命令：

$ man -k ^systemd |nl |tail  
   158  systemd.service (5)  - Service unit configuration 
   159  systemd.slice (5)  - Slice unit configuration 
   160  systemd.socket (5)   - Socket unit configuration 
   161  systemd.special (7)  - Special systemd units 
   162  systemd.swap (5)   - Swap unit configuration 
   163  systemd.syntax (7)   - General syntax of systemd configuration files 
   164  systemd.target (5)   - Target unit configuration 
   165  systemd.time (7)   - Time and date specifications 
   166  systemd.timer (5)  - Timer unit configuration 
   167  systemd.unit (5)   - Unit configuration

从结果中国马上获知167这个结果。
或者查询有多少与calendar相关的命令：

$ man -k calendar |nl 
   1  cal (1)        - displays a calendar and the date of Easter 
   2  calendar (1)     - reminder service 
   3  ncal (1)       - displays a calendar and the date of Easter

又或者查询全部的ls命令：

$ man -k ^ls |nl 
   1  ls (1)         - list directory contents 
   2  lsar (1)       - list archive file contents 
   3  lsattr (1)       - list file attributes on a Linux second extended file system 
   4  lsb_release (1)    - print distribution-specific information 
   5  lsblk (8)      - list block devices 
   6  lscpu (1)      - display information about the CPU architecture 
   7  lsearch (3)      - linear search of an array 
   8  lseek (2)      - reposition read/write file offset 
   9  lseek64 (3)      - reposition 64-bit read/write file offset 
  10  lsetxattr (2)    - set an extended attribute value 
  11  lshw (1)       - list hardware 
  12  lsinitramfs (8)    - list content of an initramfs image 
  13  lsipc (1)      - show information on IPC facilities currently employed in ... 
  14  lslocks (8)      - list local system locks 
  15  lslogins (1)     - display information about known users in the system 
  16  lsmem (1)      - list the ranges of available memory with their online status 
  17  lsmod (8)      - Show the status of modules in the Linux Kernel 
  18  lsns (8)       - list namespaces 
  19  lsof (8)       - list open files 
  20  lspci (8)      - list all PCI devices 
  21  lspcmcia (8)     - display extended PCMCIA debugging information 
  22  lspgpot (1)      - extracts the ownertrust values from PGP keyrings and list... 
  23  lstat (2)      - get file status 
  24  lstat64 (2)      - get file status 
  25  lsusb (8)      - list USB devices

5.全局掌控
文章开头就介绍过section 2 中是system-calls命令。我们不免心生好奇，哪些是system-calls呢?

$ man -s 2 -k . |nl |tail 
   484  vm86old (2)      - enter virtual 8086 mode 
   485  vmsplice (2)     - splice user pages to/from a pipe 
   486  vserver (2)      - unimplemented system calls 
   487  wait (2)       - wait for process to change state 
   488  wait3 (2)      - wait for process to change state, BSD style 
   489  wait4 (2)      - wait for process to change state, BSD style 
   490  waitid (2)       - wait for process to change state 
   491  waitpid (2)      - wait for process to change state 
   492  write (2)      - write to a file descriptor 
   493  writev (2)       - read or write data into multiple buffers

由上可知，系统中共有493个system-calls。我们分析下这一行命令：

man -s 2 -k .

短选代表的是长选项--section，-k 是 keyword，等价于 apropos命令，最后的 dot 点乃是正则表达式里的任意字符的表达方式。
于是，仅仅通过上面的简单命令获得对整个文档乃至整台计算机，整个计算机科学的全局掌控感。我们迅速掠览一遍这9个sections。
Section 1 Shell Commands系统命令：

$ man -s 1 -k . |nl| tail -n 5 
  2451  zless (1)      - file perusal filter for crt viewing of compressed text 
  2452  zlib_decompress (1)  - decompress mysqlpump ZLIB-compressed output 
  2453  zmore (1)      - file perusal filter for crt viewing of compressed text 
  2454  znew (1)       - recompress .Z files to .gz files 
  2455  zsoelim (1)      - satisfy .so requests in roff input

Section 2 Syetem Calls系统调用

$ man -s 2 -k . |nl| tail -n 5 
   489  wait4 (2)      - wait for process to change state, BSD style 
   490  waitid (2)       - wait for process to change state 
   491  waitpid (2)      - wait for process to change state 
   492  write (2)      - write to a file descriptor 
   493  writev (2)       - read or write data into multiple buffers

Section 3 C LIbraries C语库

$ man -s 3 -k . |nl| tail -n 5 
  1899  y1l (3)        - Bessel functions of the second kind 
  1900  yn (3)         - Bessel functions of the second kind 
  1901  ynf (3)        - Bessel functions of the second kind 
  1902  ynl (3)        - Bessel functions of the second kind 
  1903  zlib (3)       - compression/decompression library

Section 4 Special Files 特殊文件

$ man -s 4 -k . |nl| tail -n 5 
  48  veth (4)       - Virtual Ethernet Device 
  49  vmware (4)       - VMware SVGA video driver 
  50  wacom (4)      - Wacom input driver 
  51  wavelan (4)      - AT&T GIS WaveLAN ISA device driver 
  52  zero (4)       - data sink

Secion 5 File Format and conventions 文件格式及惯例

$ man -s 5 -k . |nl| tail -n 5 
   424  xorg.conf (5)    - configuration files for Xorg X server 
   425  xorg.conf.d (5)    - configuration files for Xorg X server 
   426  Xsession (5)     - initialize X session 
   427  Xsession.options (5) - configuration options for Xsession (5) 
   428  Xwrapper.config (5)  - Xorg X server binary wrapper

Secion 6 Games 游戏

$ man -s 6 -k . |nl| tail -n 5 
   1  intro (6)      - introduction to games 
   2  kmahjongg (6)    - Mahjongg game for KDE 
   3  kmines (6)       - KDE mine sweeper game 
   4  kpat (6)       - A highly addictive card game. 
   5  ksudoku (6)      - KDE Sudoku games

Section 7 Miscellaneous (including macro packages and conventions)杂项

$ man -s 7 -k . |nl| tail -n 5 
   470  XConsortium (7)    - X Consortium information 
   471  xkeyboard-config (7) - XKB data description files 
   472  XOrgFoundation (7)   - X.Org Foundation information 
   473  Xsecurity (7)    - X display access control 
   474  XStandards (7)     - X Window System Standards and Specifications

Section 8 System administration commands (usually only for root)系统管理命令

$ man -s 8 -k . |nl| tail -n 5 
   890  xtables-nft-multi (8) - iptables using nftables kernel api 
   891  xtables-translate (8) - translation tool to migrate from iptables to nftables 
   892  zdump (8)      - timezone dumper 
   893  zic (8)        - timezone compiler 
   894  zramctl (8)      - set up and control zram devices

Section 9 Kernel routines [Non standard] 内核函数

➜  ~ man -s 9 -k . 
vmxnet (9)       - vmware kernel module

掌控全局还有另外一种方法，就是intro命令：

$ man -f intro 
intro (8)      - introduction to administration and privileged commands 
intro (7)      - introduction to overview and miscellany section 
intro (3)      - introduction to library functions 
intro (4)      - introduction to special files 
intro (1)      - introduction to user commands 
intro (5)      - introduction to file formats and filesystems 
intro (6)      - introduction to games 
intro (2)      - introduction to system calls

我们看到每个section都有一个intro。
比如我们查看section 2的intro：

6.游戏
man命令有良好的疗愈功效，不妨来看看Kubuntu自带的四个游戏。

$ man -s 6 -k . |nl 
   1  intro (6)      - introduction to games 
   2  kmahjongg (6)    - Mahjongg game for KDE 
   3   (6)       - KDE mine sweeper game 
   4  kpat (6)       - A highly addictive card game. 
   5  ksudoku (6)      - KDE Sudoku games

7.收尾总结
man命令是我们最好的陪伴，帮助满血复活对工作和生活的掌控感。
原文地址：https://www.toutiao.com/i6903830053050794507/

[Linux] 帮你精通Linux：从Manual手册中获得全局掌控感

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