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1 前言

  linux命令非常多，少用的命令往往易忘记，甚至常用的语法较为复杂的命令也不好记住。当然有些太复杂的命令也不需要死记硬背，我们往往会借助man命令去查找命令的具有用法。man功能着实强大，但有的时候显得繁琐累赘；我们只需要关键的使用语法、例子，man给出了一堆详细信息，影响我们提取关键信息，特别是比较陌生的命令。

acuity@ubuntu:~$ man tar
TAR(1)                                                                                              BSD General Commands Manual                                                                                              TAR(1)

NAME
     tar — The GNU version of the tar archiving utility

SYNOPSIS
     tar [-] A --catenate --concatenate | c --create | d --diff --compare | --delete | r --append | t --list | --test-label | u --update | x --extract --get [options] [pathname ...]

DESCRIPTION
     Tar stores and extracts files from a tape or disk archive.

     The first argument to tar should be a function; either one of the letters Acdrtux, or one of the long function names.  A function letter need not be prefixed with ``-'', and may be combined with other single-letter
     options.  A long function name must be prefixed with --.  Some options take a parameter; with the single-letter form these must be given as separate arguments.  With the long form, they may be given by appending =value to
     the option.

FUNCTION LETTERS
     Main operation mode:

     -A, --catenate, --concatenate
           append tar files to an archive

     -c, --create
           create a new archive

     -d, --diff, --compare
           find differences between archive and file system

     --delete
           delete from the archive (not on mag tapes!)

     -r, --append
           append files to the end of an archive

     -t, --list
           list the contents of an archive

     --test-label
           test the archive volume label and exit

  总会有一些高效的工具出现，去改变一些东西，比如tldr工具，一个使用非常方便、高效的linux命令工具查询器，而且还是开源的。

2 tldr

  tldr，作者给出的含义是Too Long; Didn’t Read，直译即是“太长，不值得一读”，严重怀疑在明嘲暗讽man。tldr简化了烦琐的man的输出帮助信息，只列出关键的语法信息、例子，方便用户使用。

  tldr开源地址：https://github.com/tldr-pages/tldr

3 安装

  tldr安装比较简单，主机联网状态下，可以直接在线安装。x64 ubuntu16 安装步骤如下：

• 第一步， 安装 npm

sudo apt-get install npm -y

  为了快速下载，我们先更新一下npm 的包镜像源：

sudo npm config set registry https://registry.npm.taobao.org

  接着升级到最新版本的npm：

sudo npm install npm@latest -g

• 第二步，安装 nodejs

sudo apt install nodejs

  可能会提示如下错误：

acuity@ubuntu:~$ sudo apt install nodejs
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  gyp libjs-inherits libjs-node-uuid libjs-underscore libuv1 libuv1-dev node-abbrev node-ansi node-ansi-color-table node-archy node-async node-block-stream node-combined-stream node-cookie-jar node-delayed-stream node-forever-agent
  node-form-data node-fstream node-fstream-ignore node-github-url-from-git node-glob node-graceful-fs node-gyp node-inherits node-ini node-json-stringify-safe node-lockfile node-lru-cache node-mime node-minimatch node-mkdirp
  node-mute-stream node-node-uuid node-nopt node-normalize-package-data node-npmlog node-once node-osenv node-qs node-read node-read-package-json node-request node-retry node-rimraf node-semver node-sha node-sigmund node-slide
  node-tar node-tunnel-agent node-underscore node-which
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  nodejs-dev nodejs-legacy npm
The following packages will be upgraded:
  nodejs
1 upgraded, 0 newly installed, 3 to remove and 89 not upgraded.
Need to get 18.1 MB of archives.
After this operation, 68.5 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 https://deb.nodesource.com/node_12.x xenial/main amd64 nodejs amd64 12.20.1-1nodesource1 [18.1 MB]
Err:1 https://deb.nodesource.com/node_12.x xenial/main amd64 nodejs amd64 12.20.1-1nodesource1                                                                                                                                    Operation too slow. Less than 10 bytes/sec transferred the last 120 seconds
E: Failed to fetch https://deb.nodesource.com/node_12.x/pool/main/n/nodejs/nodejs_12.20.1-1nodesource1_amd64.deb  Operation too slow. Less than 10 bytes/sec transferred the last 120 seconds
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

  如果出现该错误，说明安装源过于陈旧，根据以下步骤更新安装源：

【1】配置域名解析文件resolv.conf ，增加“nameserver 8.8.8.8”内容

sudo vim /etc/resolv.conf   

【2】更新安装源

sudo apt-get update

  更新完成，再执行sudo apt install nodejs即可成功安装nodejs。

• 第三步，安装tldr

sudo npm install -g tldr

• 第四步，使用前建议先更新下

sudo  tldr --update 

4 使用

  执行tldr会提示使用方法，还有例子，很是人性化。

acuity@ubuntu:~$ tldr 
Usage: tldr command [options]

Simplified and community-driven man pages

Options:
  -V, --version            output the version number
  -l, --list               List all commands for the chosen platform in the cache
  -a, --list-all           List all commands in the cache
  -1, --single-column      List single command per line (use with options -l or -a)
  -r, --random             Show a random command
  -e, --random-example     Show a random example
  -f, --render [file]      Render a specific markdown [file]
  -m, --markdown           Output in markdown format
  -o, --os [type]          Override the operating system [linux, osx, sunos]
  --linux                  Override the operating system with Linux
  --osx                    Override the operating system with OSX
  --sunos                  Override the operating system with SunOS
  -t, --theme [theme]      Color theme (simple, base16, ocean)
  -s, --search [keywords]  Search pages using keywords
  -u, --update             Update the local cache
  -c, --clear-cache        Clear the local cache
  -h, --help               display help for command

  Examples:

    $ tldr tar
    $ tldr du --os=linux
    $ tldr --search "create symbolic link to file"
    $ tldr --list
    $ tldr --list-all
    $ tldr --random
    $ tldr --random-example

  To control the cache:

    $ tldr --update
    $ tldr --clear-cache

  To render a local file (for testing):

    $ tldr --render /path/to/file.md

  查询tar命令的使用方法，与前面的man的查询结果相比，tldr的结果显得通俗、精炼，方便用户抓住关键信息。

acuity@ubuntu:~$ tldr tar
✔ Page not found. Updating cache...
✔ Creating index...

  tar

  Archiving utility.
  Often combined with a compression method, such as gzip or bzip.
  More information: https://www.gnu.org/software/tar.

  - [c]reate an archive from [f]iles:
    tar cf target.tar file1 file2 file3

  - [c]reate a g[z]ipped archive from [f]iles:
    tar czf target.tar.gz file1 file2 file3

  - [c]reate a g[z]ipped archive from a directory using relative paths:
    tar czf target.tar.gz --directory=path/to/directory .

  - E[x]tract a (compressed) archive [f]ile into the current directory:
    tar xf source.tar[.gz|.bz2|.xz]

  - E[x]tract a (compressed) archive [f]ile into the target directory:
    tar xf source.tar[.gz|.bz2|.xz] --directory=directory

  - [c]reate a compressed archive from [f]iles, using [a]rchive suffix to determine the compression program:
    tar caf target.tar.xz file1 file2 file3

  - Lis[t] the contents of a tar [f]ile [v]erbosely:
    tar tvf source.tar

  - E[x]tract [f]iles matching a pattern:
    tar xf source.tar --wildcards "*.html"