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219 lines
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219 lines
10 KiB
Markdown
[#]: collector: (lujun9972)
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[#]: translator: (MjSeven)
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[#]: reviewer: (wxy)
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[#]: publisher: (wxy)
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[#]: url: (https://linux.cn/article-11158-1.html)
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[#]: subject: (Getting help for Linux shell built-ins)
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[#]: via: (https://www.networkworld.com/article/3410350/getting-help-for-linux-shell-built-ins.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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获取有关 Linux shell 内置命令的帮助
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======
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> Linux 内置命令属于用户 shell 的一部分，本文将告诉你如何识别它们并获取使用它们的帮助。
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Linux 内置命令是内置于 shell 中的命令，很像内置于墙中的书架。与标准 Linux 命令存储在 `/usr/bin` 中的方式不同，你不会找到它们的独立文件，你可能使用过相当多的内置命令，但你不会感觉到它们与 `ls` 和 `pwd` 等命令有何不同。
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内置命令与其他 Linux 命令一样使用，它们可能要比不属于 shell 的类似命令运行得快一些。Bash 内置命令包括 `alias`、`export` 和 `bg` 等。
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正如你担心的那样，因为内置命令是特定于 shell 的，所以它们不会提供手册页。使用 `man` 来查看 `bg`，你会看到这样的东西：
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```
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$ man bg
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No manual entry for bg
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```
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判断内置命令的另一个提示是当你使用 `which` 命令来识别命令的来源时，Bash 不会响应，表示没有与内置命令关联的文件：
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```
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$ which bg
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$
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```
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另一方面，如果你的 shell 是 `/bin/zsh`，你可能会得到一个更有启发性的响应：
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```
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% which bg
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bg: shell built-in command
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```
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bash 提供了额外的帮助信息，但它是通过使用 `help` 命令实现的：
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```
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$ help bg
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bg: bg [job_spec ...]
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Move jobs to the background.
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Place the jobs identified by each JOB_SPEC in the background, as if they
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had been started with `&'. If JOB_SPEC is not present, the shell's notion
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of the current job is used.
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Exit Status:
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Returns success unless job control is not enabled or an error occurs.
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```
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如果你想要查看 bash 提供的所有内置命令的列表，使用 `compgen -b` 命令。通过管道将命令输出到列中，以获得较好格式的清单。
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```
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$ compgen -b | column
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. compgen exit let return typeset
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: complete export local set ulimit
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[ compopt false logout shift umask
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alias continue fc mapfile shopt unalias
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bg declare fg popd source unset
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bind dirs getopts printf suspend wait
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break disown hash pushd test
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builtin echo help pwd times
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caller enable history read trap
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cd eval jobs readarray true
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command exec kill readonly type
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```
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如果你使用 `help` 命令，你将看到一个内置命令列表以及简短描述。但是，这个列表被截断了（以 `help` 命令结尾）：
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```
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$ help
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GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
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These shell commands are defined internally. Type `help' to see this list.
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Type `help name' to find out more about the function `name'.
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Use `info bash' to find out more about the shell in general.
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Use `man -k' or `info' to find out more about commands not in this list.
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A star (*) next to a name means that the command is disabled.
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job_spec [&] history [-c] [-d offset] [n] or histo>
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(( expression )) if COMMANDS; then COMMANDS; [ elif CO>
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. filename [arguments] jobs [-lnprs] [jobspec ...] or jobs ->
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: kill [-s sigspec | -n signum | -sigsp>
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[ arg... ] let arg [arg ...]
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[[ expression ]] local [option] name[=value] ...
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alias [-p] [name[=value] ... ] logout [n]
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bg [job_spec ...] mapfile [-d delim] [-n count] [-O ori>
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bind [-lpsvPSVX] [-m keymap] [-f filen> popd [-n] [+N | -N]
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break [n] printf [-v var] format [arguments]
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builtin [shell-builtin [arg ...]] pushd [-n] [+N | -N | dir]
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caller [expr] pwd [-LP]
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case WORD in [PATTERN [| PATTERN]...) > read [-ers] [-a array] [-d delim] [-i>
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cd [-L|[-P [-e]] [-@]] [dir] readarray [-d delim] [-n count] [-O o>
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command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or>
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compgen [-abcdefgjksuv] [-o option] [-> return [n]
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complete [-abcdefgjksuv] [-pr] [-DEI] > select NAME [in WORDS ... ;] do COMMA>
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compopt [-o|+o option] [-DEI] [name ..> set [-abefhkmnptuvxBCHP] [-o option-n>
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continue [n] shift [n]
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coproc [NAME] command [redirections] shopt [-pqsu] [-o] [optname ...]
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declare [-aAfFgilnrtux] [-p] [name[=va> source filename [arguments]
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dirs [-clpv] [+N] [-N] suspend [-f]
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disown [-h] [-ar] [jobspec ... | pid . <p>'> test [expr]
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echo [-neE] [arg ...] time [-p] pipeline
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enable [-a] [-dnps] [-f filename] [nam> times
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eval [arg ...] trap [-lp] [[arg] signal_spec ...]
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exec [-cl] [-a name] [command [argumen> true
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exit [n] type [-afptP] name [name ...]
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export [-fn] [name[=value] ...] or exp> typeset [-aAfFgilnrtux] [-p] name[=va>
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false ulimit [-SHabcdefiklmnpqrstuvxPT] [li>
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fc [-e ename] [-lnr] [first] [last] or> umask [-p] [-S] [mode]
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fg [job_spec] unalias [-a] name [name ...]
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for NAME [in WORDS ... ] ; do COMMANDS> unset [-f] [-v] [-n] [name ...]
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for (( exp1; exp2; exp3 )); do COMMAND> until COMMANDS; do COMMANDS; done
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function name { COMMANDS ; } or name (> variables - Names and meanings of som>
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getopts optstring name [arg] wait [-fn] [id ...]
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hash [-lr] [-p pathname] [-dt] [name .> while COMMANDS; do COMMANDS; done
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help [-dms] [pattern ...] { COMMANDS ; }
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```
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从上面的清单中可以看出，`help` 命令本身就是内置的。
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你可以通过向 `help` 命令提供你感兴趣的内置命令名称来获取关于它们的更多信息，例如 `help dirs`：
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```
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$ help dirs
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dirs: dirs [-clpv] [+N] [-N]
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Display directory stack.
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Display the list of currently remembered directories. Directories
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find their way onto the list with the `pushd' command; you can get
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back up through the list with the `popd' command.
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Options:
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-c clear the directory stack by deleting all of the elements
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-l do not print tilde-prefixed versions of directories relative
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to your home directory
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-p print the directory stack with one entry per line
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-v print the directory stack with one entry per line prefixed
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with its position in the stack
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Arguments:
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+N Displays the Nth entry counting from the left of the list
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shown by dirs when invoked without options, starting with
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zero.
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-N Displays the Nth entry counting from the right of the list
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shown by dirs when invoked without options, starting with
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zero.
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Exit Status:
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Returns success unless an invalid option is supplied or an error occurs.
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```
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内置命令提供了每个 shell 的大部分功能。你使用的任何 shell 都有一些内置命令，但是如何获取这些内置命令的信息可能因 shell 而异。例如，对于 `zsh`，你可以使用 `man zshbuiltins` 命令获得其内置命令的描述。
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```
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$ man zshbuiltins
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ZSHBUILTINS(1) General Commands Manual ZSHBUILTINS(1)
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NAME
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zshbuiltins - zsh built-in commands
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SHELL BUILTIN COMMANDS
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Some shell builtin commands take options as described in individual en‐
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tries; these are often referred to in the list below as `flags' to avoid
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confusion with shell options, which may also have an effect on the behav‐
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iour of builtin commands. In this introductory section, `option' always
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has the meaning of an option to a command that should be familiar to most
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command line users.
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…
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```
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在这个冗长的手册页中，你将找到一个内置命令列表，其中包含有用的描述，如下摘录中所示：
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```
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bg [ job ... ]
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job ... &
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Put each specified job in the background, or the current job if
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none is specified.
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bindkey
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See the section `Zle Builtins' in zshzle(1).
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break [ n ]
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Exit from an enclosing for, while, until, select or repeat loop.
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If an arithmetic expression n is specified, then break n levels
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instead of just one.
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```
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### 最后
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Linux 内置命令对于每个 shell 都很重要，它的操作类似特定于 shell 的命令一样。如果你经常使用不同的 shell，并注意到你经常使用的某些命令似乎不存在或者不能按预期工作，那么它可能是你使用的其他 shell 之一中的内置命令。
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3410350/getting-help-for-linux-shell-built-ins.html
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[MjSeven](https://github.com/MjSeven)
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校对：[wxy](https://github.com/wxy)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://images.idgesg.net/images/article/2019/07/linux_penguin-100802549-large.jpg
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[2]: https://www.youtube.com/playlist?list=PL7D2RMSmRO9J8OTpjFECi8DJiTQdd4hua
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[3]: https://www.networkworld.com/article/3242170/linux/invaluable-tips-and-tricks-for-troubleshooting-linux.html
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[4]: https://www.facebook.com/NetworkWorld/
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[5]: https://www.linkedin.com/company/network-world
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