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。
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。
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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$ man bg No manual entry for bg
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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$ which bg $
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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% which bg bg: shell built-in command
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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$ help bg bg: bg [job_spec ...] Move jobs to the background.
Place the jobs identified by each JOB_SPEC in the background, as if they had been started with `&'. If JOB_SPEC is not present, the shell's notion of the current job is used.
Exit Status: Returns success unless job control is not enabled or an error occurs.
$ help GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu) These shell commands are defined internally. Type `help' to see this list. Type `help name' to find out more about the function `name'. Use `info bash' to find out more about the shell in general. Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
job_spec [&] history [-c] [-d offset] [n] or histo> (( expression )) if COMMANDS; then COMMANDS; [ elif CO> . filename [arguments] jobs [-lnprs] [jobspec ...] or jobs -> : kill [-s sigspec | -n signum | -sigsp> [ arg... ] let arg [arg ...] [[ expression ]] local [option] name[=value] ... alias [-p] [name[=value] ... ] logout [n] bg [job_spec ...] mapfile [-d delim] [-n count] [-O ori> bind [-lpsvPSVX] [-m keymap] [-f filen> popd [-n] [+N | -N] break [n] printf [-v var] format [arguments] builtin [shell-builtin [arg ...]] pushd [-n] [+N | -N | dir] caller [expr] pwd [-LP] case WORD in [PATTERN [| PATTERN]...) > read [-ers] [-a array] [-d delim] [-i> cd [-L|[-P [-e]] [-@]] [dir] readarray [-d delim] [-n count] [-O o> command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or> compgen [-abcdefgjksuv] [-o option] [-> return [n] complete [-abcdefgjksuv] [-pr] [-DEI] > select NAME [in WORDS ... ;] do COMMA> compopt [-o|+o option] [-DEI] [name ..> set [-abefhkmnptuvxBCHP] [-o option-n> continue [n] shift [n] coproc [NAME] command [redirections] shopt [-pqsu] [-o] [optname ...] declare [-aAfFgilnrtux] [-p] [name[=va> source filename [arguments] dirs [-clpv] [+N] [-N] suspend [-f] disown [-h] [-ar] [jobspec ... | pid . <p>'> test [expr] echo [-neE] [arg ...] time [-p] pipeline enable [-a] [-dnps] [-f filename] [nam> times eval [arg ...] trap [-lp] [[arg] signal_spec ...] exec [-cl] [-a name] [command [argumen> true exit [n] type [-afptP] name [name ...] export [-fn] [name[=value] ...] or exp> typeset [-aAfFgilnrtux] [-p] name[=va> false ulimit [-SHabcdefiklmnpqrstuvxPT] [li> fc [-e ename] [-lnr] [first] [last] or> umask [-p] [-S] [mode] fg [job_spec] unalias [-a] name [name ...] for NAME [in WORDS ... ] ; do COMMANDS> unset [-f] [-v] [-n] [name ...] for (( exp1; exp2; exp3 )); do COMMAND> until COMMANDS; do COMMANDS; done function name { COMMANDS ; } or name (> variables - Names and meanings of som> getopts optstring name [arg] wait [-fn] [id ...] hash [-lr] [-p pathname] [-dt] [name .> while COMMANDS; do COMMANDS; done help [-dms] [pattern ...] { COMMANDS ; }
从上面的清单中可以看出，help 命令本身就是内置的。
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：
Display the list of currently remembered directories. Directories find their way onto the list with the `pushd' command; you can get back up through the list with the `popd' command.
Options: -c clear the directory stack by deleting all of the elements -l do not print tilde-prefixed versions of directories relative to your home directory -p print the directory stack with one entry per line -v print the directory stack with one entry per line prefixed with its position in the stack
Arguments: +N Displays the Nth entry counting from the left of the list shown by dirs when invoked without options, starting with zero.
-N Displays the Nth entry counting from the right of the list shown by dirs when invoked without options, starting with zero.
Exit Status: Returns success unless an invalid option is supplied or an error occurs.
ZSHBUILTINS(1) General Commands Manual ZSHBUILTINS(1)
NAME zshbuiltins - zsh built-in commands
SHELL BUILTIN COMMANDS Some shell builtin commands take options as described in individual en‐ tries; these are often referred to in the list below as `flags' to avoid confusion with shell options, which may also have an effect on the behav‐ iour of builtin commands. In this introductory section, `option' always has the meaning of an option to a command that should be familiar to most command line users. …
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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bg [ job ... ] job ... & Put each specified job in the background, or the current job if none is specified.
bindkey See the section `Zle Builtins' in zshzle(1).
break [ n ] Exit from an enclosing for, while, until, select or repeat loop. If an arithmetic expression n is specified, then break n levels instead of just one.
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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。
