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：
$ 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：
$ 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：
% 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：
$ help bg
bg: bg [job_spec ...]
Move jobs to the background.
Place the jobs identified by each JOB_SPEC in the background,asif 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.
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。
$ compgen -b | column
. compgen exit let return typeset
: complete exportlocalset ulimit
[ compopt false logout shift umask
aliascontinue fc mapfile shopt unalias
bg declare fg popd source unset
bind dirs getopts printf suspend wait
break disown hash pushd test
builtin echo help pwd times
caller enable history read trap
cdeval jobs readarray true
command execkill readonly type
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）：
$ 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 notinthislist.
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：
$ help dirs
dirs: dirs [-clpv][+N][-N]
Display directory stack.
Display the list of currently remembered directories.Directories
find their way onto the listwith 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.