No man page for the cd command
Ubuntu Linux 15.10 — I just noticed that there is no man page for cd This seems a bit strange. I tried:
which is super-handy for flipping between the last dir and the current dir and cd — which seems to be an alias for
3 Answers 3
cd is not a command, it’s built into your shell. This is necessary because your current working directory is controlled by the PWD environment variable named after the pwd or «print working directory» command.
The environment variables of a parent process cannot be changed by a child process. So if your shell ran /bin/cd which changed PWD it would only affect /bin/cd and anything it ran. It would not change the shell’s PWD .
Some systems, like OS X and CentOS, map the cd man page to builtin which lists all the shell built ins and lets you know you should look at your shell’s man page.
You can check what shell you have with echo $SHELL , it’s probably bash .
I don’t get what your explanation about environment variables has to do with cd not being listed on man . May I request further explanation put more easily? 🙂
@SmartHumanism A cd program cannot change your directory, so it has to be built into the shell, so it’s not a program. man is set up for programs, so cd doesn’t get a man page. Even if we decided it should anyway, every shell has a slightly different cd . Which one gets man cd ?
cd is a builtin shell command.
$ type cd cd is a shell builtin You can open a help page for cd on Bash with
Which currently shows (Ubuntu 16.04):
$ help cd cd: cd [-L|[-P [-e]] [-@]] [dir] Change the shell working directory. Change the current directory to DIR. The default DIR is the value of the HOME shell variable. The variable CDPATH defines the search path for the directory containing DIR. Alternative directory names in CDPATH are separated by a colon (:). A null directory name is the same as the current directory. If DIR begins with a slash (/), then CDPATH is not used. If the directory is not found, and the shell option `cdable_vars' is set, the word is assumed to be a variable name. If that variable has a value, its value is used for DIR. Options: -L force symbolic links to be followed: resolve symbolic links in DIR after processing instances of `..' -P use the physical directory structure without following symbolic links: resolve symbolic links in DIR before processing instances of `..' -e if the -P option is supplied, and the current working directory cannot be determined successfully, exit with a non-zero status -@ on systems that support it, present a file with extended attributes as a directory containing the file attributes The default is to follow symbolic links, as if `-L' were specified. `..' is processed by removing the immediately previous pathname component back to a slash or the beginning of DIR. Exit Status: Returns 0 if the directory is changed, and if $PWD is set successfully when -P is used; non-zero otherwise. Unfortunately, it does not answer your questions. There is documentation that does, however.
It opens many pages of help with less , my default viewer. I can find the help for cd by pressing the / key, then typing cd , then Enter , and pressing n twice gets me to the third instance of the substring, and the help, which reads:
cd [-L|[-P [-e]] [-@]] [dir] Change the current directory to dir. if dir is not supplied, the value of the HOME shell variable is the default. Any addi‐ tional arguments following dir are ignored. The variable CDPATH defines the search path for the directory containing dir: each directory name in CDPATH is searched for dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH is not used. The -P option causes cd to use the physical directory structure by resolving symbolic links while traversing dir and before processing instances of .. in dir (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed by resolving the link after processing instances of .. in dir. If .. appears in dir, it is processed by removing the immediately previous pathname component from dir, back to a slash or the beginning of dir. If the -e option is supplied with -P, and the current working directory cannot be success‐ fully determined after a successful directory change, cd will return an unsuccessful status. On systems that support it, the -@ option presents the extended attributes associated with a file as a directory. An argument of - is converted to $OLDPWD before the directory change is attempted. If a non-empty direc‐ tory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return value is true if the directory was successfully changed; false otherwise. Look for the — argument about the seventh line from the end:
An argument of — is converted to $OLDPWD before the directory change is attempted.
Note that there is no — argument — which seems to mean that it actually ignores it.
Нет man-страницы для команды cd
Я что-то упускаю здесь очень очевидное или должна присутствовать справочная страница?
3 ответа
cd это не команда, она встроена в вашу оболочку. Это необходимо, потому что ваш текущий рабочий каталог контролируется PWD переменная среды, названная в честь pwd или команда «печать рабочего каталога».
Переменные среды родительского процесса не могут быть изменены дочерним процессом. Так что если ваша оболочка побежала /bin/cd который изменился PWD это повлияет только на /bin/cd и все, что он бежал. Это не изменит оболочки PWD ,
Некоторые системы, такие как OS X и CentOS, отображают cd страница руководства для builtin который перечисляет все встроенные модули оболочки и дает вам знать, что вы должны взглянуть на справочную страницу вашей оболочки.
Вы можете проверить, какая оболочка у вас есть echo $SHELL , это, вероятно bash ,
cd это встроенная команда оболочки
Вы можете открыть страницу справки для cd на Баш с
Который сейчас показывает (Ubuntu 16.04):
$ help cd cd: cd [-L|[-P [-e]] [-@]] [dir] Change the shell working directory. Change the current directory to DIR. The default DIR is the value of the HOME shell variable. The variable CDPATH defines the search path for the directory containing DIR. Alternative directory names in CDPATH are separated by a colon (:). A null directory name is the same as the current directory. If DIR begins with a slash (/), then CDPATH is not used. If the directory is not found, and the shell option `cdable_vars' is set, the word is assumed to be a variable name. If that variable has a value, its value is used for DIR. Options: -L force symbolic links to be followed: resolve symbolic links in DIR after processing instances of `..' -P use the physical directory structure without following symbolic links: resolve symbolic links in DIR before processing instances of `..' -e if the -P option is supplied, and the current working directory cannot be determined successfully, exit with a non-zero status -@ on systems that support it, present a file with extended attributes as a directory containing the file attributes The default is to follow symbolic links, as if `-L' were specified. `..' is processed by removing the immediately previous pathname component back to a slash or the beginning of DIR. Exit Status: Returns 0 if the directory is changed, and if $PWD is set successfully when -P is used; non-zero otherwise. К сожалению, он не отвечает на ваши вопросы. Есть документация, которая делает, однако.
Вы можете добраться до него с
Это открывает много страниц помощи с less Мой зритель по умолчанию. Я могу найти помощь для CD, нажав / затем введите cd , затем Enter и нажав n дважды возвращает меня к третьему экземпляру подстроки и справке, которая гласит:
cd [-L|[-P [-e]] [-@]] [dir] Change the current directory to dir. if dir is not supplied, the value of the HOME shell variable is the default. Any addi‐ tional arguments following dir are ignored. The variable CDPATH defines the search path for the directory containing dir: each directory name in CDPATH is searched for dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH is not used. The -P option causes cd to use the physical directory structure by resolving symbolic links while traversing dir and before processing instances of .. in dir (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed by resolving the link after processing instances of .. in dir. If .. appears in dir, it is processed by removing the immediately previous pathname component from dir, back to a slash or the beginning of dir. If the -e option is supplied with -P, and the current working directory cannot be success‐ fully determined after a successful directory change, cd will return an unsuccessful status. On systems that support it, the -@ option presents the extended attributes associated with a file as a directory. An argument of - is converted to $OLDPWD before the directory change is attempted. If a non-empty direc‐ tory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return value is true if the directory was successfully changed; false otherwise. Ищите — спор о седьмой строчке с конца.
Обратите внимание, что нет — аргумент — который, кажется, означает, что он фактически игнорирует это.