獲取有關 Linux shell 內建命令的幫助

Linux 內建命令屬於使用者 shell 的一部分，本文將告訴你如何識別它們並獲取使用它們的幫助。

Linux 內建命令是內建於 shell 中的命令，很像內建於牆中的書架。與標準 Linux 命令儲存在 /usr/bin 中的方式不同，你不會找到它們的獨立檔案，你可能使用過相當多的內建命令，但你不會感覺到它們與 ls 和 pwd 等命令有何不同。

內建命令與其他 Linux 命令一樣使用，它們可能要比不屬於 shell 的類似命令執行得快一些。Bash 內建命令包括 alias、export 和 bg 等。

正如你擔心的那樣，因為內建命令是特定於 shell 的，所以它們不會提供手冊頁。使用 man 來檢視 bg，你會看到這樣的東西：

1. $ man bg
2. No manual entry for bg

判斷內建命令的另一個提示是當你使用 which 命令來識別命令的來源時，Bash 不會響應，表示沒有與內建命令關聯的檔案：

1. $ which bg
2. $

另一方面，如果你的 shell 是 /bin/zsh，你可能會得到一個更有啟發性的響應：

1. % which bg
2. bg: shell built-in command

bash 提供了額外的幫助資訊，但它是通過使用 help 命令實現的：

1. $ help bg
2. bg: bg [job_spec ...]
3. Move jobs to the background.
4. Place the jobs identified by each JOB_SPEC in the background,asif they
5. had been started with`&'. If JOB_SPEC is not present, the shell's notion
6. of the current job is used.
7. Exit Status:
8. Returns success unless job control is not enabled or an error occurs.

如果你想要檢視 bash 提供的所有內建命令的列表，使用 compgen -b 命令。通過管道將命令輸出到列中，以獲得較好格式的清單。

1. $ compgen -b | column
2. . compgen exit let return typeset
3. : complete exportlocalset ulimit
4. [ compopt false logout shift umask
5. aliascontinue fc mapfile shopt unalias
6. bg declare fg popd source unset
7. bind dirs getopts printf suspend wait
8. break disown hash pushd test
9. builtin echo help pwd times
10. caller enable history read trap
11. cdeval jobs readarray true
12. command execkill readonly type

如果你使用 help 命令，你將看到一個內建命令列表以及簡短描述。但是，這個列表被截斷了（以 help 命令結尾）：

1. $ help
2. GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
3. These shell commands are defined internally.Type`help' to see this list.
4. Type `help name' to find out more about the function `name'.
5. Use`info bash' to find out more about the shell in general.
6. Use `man-k' or `info' to find out more about commands notinthislist.
7. A star (*)next to a name means that the command is disabled.
8. job_spec [&] history [-c][-d offset][n]or histo>
9. (( expression ))if COMMANDS;then COMMANDS;[elif CO>
10. . filename [arguments] jobs [-lnprs][jobspec ...]or jobs ->
11. :kill[-s sigspec |-n signum |-sigsp>
12. [ arg...] let arg [arg ...]
13. [[ expression ]]local[option] name[=value]...
14. alias[-p][name[=value]...] logout [n]
15. bg [job_spec ...] mapfile [-d delim][-n count][-O ori>
16. bind [-lpsvPSVX][-m keymap][-f filen> popd [-n][+N |-N]
17. break[n]printf[-v var] format [arguments]
18. builtin [shell-builtin [arg ...]] pushd [-n][+N |-N |dir]
19. caller[expr]pwd[-LP]
20. case WORD in[PATTERN [| PATTERN]...)> read [-ers][-a array][-d delim][-i>
21. cd[-L|[-P [-e]][-@]][dir] readarray [-d delim][-n count][-O o>
22. command [-pVv] command [arg ...] readonly [-aAf][name[=value]...]or>
23. compgen [-abcdefgjksuv][-o option][->return[n]
24. complete [-abcdefgjksuv][-pr][-DEI]> select NAME [in WORDS ...;]do COMMA>
25. compopt [-o|+o option][-DEI][name ..>set[-abefhkmnptuvxBCHP][-o option-n>
26. continue[n] shift [n]
27. coproc [NAME] command [redirections] shopt [-pqsu][-o][optname ...]
28. declare[-aAfFgilnrtux][-p][name[=va> source filename [arguments]
29. dirs [-clpv][+N][-N] suspend [-f]
30. disown [-h][-ar][jobspec ...| pid .<p&gt'> test [expr]
31. echo [-neE] [arg ...] time [-p] pipeline
32. enable [-a] [-dnps] [-f filename] [nam> times
33. eval [arg ...] trap [-lp] [[arg] signal_spec ...]
34. exec [-cl] [-a name] [command [argumen> true
35. exit [n] type [-afptP] name [name ...]
36. export [-fn] [name[=value] ...] or exp> typeset [-aAfFgilnrtux] [-p] name[=va>
37. false ulimit [-SHabcdefiklmnpqrstuvxPT] [li>
38. fc [-e ename] [-lnr] [first] [last] or> umask [-p] [-S] [mode]
39. fg [job_spec] unalias [-a] name [name ...]
40. for NAME [in WORDS ... ] ; do COMMANDS> unset [-f] [-v] [-n] [name ...]
41. for (( exp1; exp2; exp3 )); do COMMAND> until COMMANDS; do COMMANDS; done
42. function name { COMMANDS ; } or name (> variables - Names and meanings of som>
43. getopts optstring name [arg] wait [-fn] [id ...]
44. hash [-lr] [-p pathname] [-dt] [name .> while COMMANDS; do COMMANDS; done
45. help [-dms] [pattern ...] { COMMANDS ; }

從上面的清單中可以看出，help 命令本身就是內建的。

你可以通過向 help 命令提供你感興趣的內建命令名稱來獲取關於它們的更多資訊，例如 help dirs：

1. $ help dirs
2. dirs: dirs [-clpv][+N][-N]
3. Display directory stack.
4. Display the list of currently remembered directories.Directories
5. find their way onto the listwith the `pushd' command; you can get
6. back up through the list with the `popd' command.
7. Options:
8. -c clear the directory stack by deleting all of the elements
9. -l do not print tilde-prefixed versions of directories relative
10. to your home directory
11. -p print the directory stack with one entry per line
12. -v print the directory stack with one entry per line prefixed
13. with its position in the stack
14. Arguments:
15. +N Displays the Nth entry counting from the left of the list
16. shown by dirs when invoked without options, starting with
17. zero.
18. -N Displays the Nth entry counting from the right of the list
19. shown by dirs when invoked without options, starting with
20. zero.
21. Exit Status:
22. Returns success unless an invalid option is supplied or an error occurs.

內建命令提供了每個 shell 的大部分功能。你使用的任何 shell 都有一些內建命令，但是如何獲取這些內建命令的資訊可能因 shell 而異。例如，對於 zsh，你可以使用 man zshbuiltins 命令獲得其內建命令的描述。

1. $ man zshbuiltins
2. ZSHBUILTINS(1)GeneralCommandsManual ZSHBUILTINS(1)
3. NAME
4. zshbuiltins - zsh built-in commands
5. SHELL BUILTIN COMMANDS
6. Some shell builtin commands take options as described in individual en‐
7. tries; these are often referred to in the list below as`flags' to avoid
8. confusion with shell options, which may also have an effect on the behav‐
9. iour of builtin commands. In this introductory section, `option' always
10. has the meaning of an option to a command that should be familiar to most
11. command line users.
12. …

在這個冗長的手冊頁中，你將找到一個內建命令列表，其中包含有用的描述，如下摘錄中所示：

1. bg [ job ...]
2. job ...&
3. Put each specified job in the background,or the current job if
4. none is specified.
5. bindkey
6. See the section `Zle Builtins' in zshzle(1).
7. break [ n ]
8. Exit from an enclosing for, while, until, select or repeat loop.
9. If an arithmetic expression n is specified, then break n levels
10. instead of just one.

最後

Linux 內建命令對於每個 shell 都很重要，它的操作類似特定於 shell 的命令一樣。如果你經常使用不同的 shell，並注意到你經常使用的某些命令似乎不存在或者不能按預期工作，那麼它可能是你使用的其他 shell 之一中的內建命令。

via：networkworld