# Bash 的 cd 命令

cd [-L|-P] [DIRECTORY]

$ pwd                       # 查看当前路径
/home/user/primers
$ cd bash                   # 相对路径
$ pwd 
/home/user/primers/bash
$ cd -                      # 切换回之前的目录
$ pwd 
/home/user/primers
$ cd /etc                   # 绝对路径
$ pwd 
/etc

$ pwd                       # 查看当前路径
/home/user/primers
$ ln -s /usr/lib lib        # 创建符号链接
$ cd lib                    # 进入 lib
$ pwd 
/home/user/primers/bash/lib
$ cd ..                     # 返回上级目录
$ cd -P lib                 # 进入 lib，跟踪符号链接
$ pwd 
/usr/lib

CD(1P)                  POSIX Programmer's Manual                  CD(1P)
PROLOG         top
       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.
NAME         top
       cd — change the working directory
SYNOPSIS         top
       cd [-L|-P] [directory]

       cd -
DESCRIPTION         top
       The cd utility shall change the working directory of the current
       shell execution environment (see Section 2.12, Shell Execution
       Environment) by executing the following steps in sequence. (In the
       following steps, the symbol curpath represents an intermediate
       value used to simplify the description of the algorithm used by
       cd.  There is no requirement that curpath be made visible to the
       application.)

        1. If no directory operand is given and the HOME environment
           variable is empty or undefined, the default behavior is
           implementation-defined and no further steps shall be taken.

        2. If no directory operand is given and the HOME environment
           variable is set to a non-empty value, the cd utility shall
           behave as if the directory named in the HOME environment
           variable was specified as the directory operand.

        3. If the directory operand begins with a <slash> character, set
           curpath to the operand and proceed to step 7.

        4. If the first component of the directory operand is dot or dot-
           dot, proceed to step 6.

        5. Starting with the first pathname in the <colon>-separated
           pathnames of CDPATH (see the ENVIRONMENT VARIABLES section) if
           the pathname is non-null, test if the concatenation of that
           pathname, a <slash> character if that pathname did not end
           with a <slash> character, and the directory operand names a
           directory. If the pathname is null, test if the concatenation
           of dot, a <slash> character, and the operand names a
           directory. In either case, if the resulting string names an
           existing directory, set curpath to that string and proceed to
           step 7. Otherwise, repeat this step with the next pathname in
           CDPATH until all pathnames have been tested.

        6. Set curpath to the directory operand.

        7. If the -P option is in effect, proceed to step 10. If curpath
           does not begin with a <slash> character, set curpath to the
           string formed by the concatenation of the value of PWD, a
           <slash> character if the value of PWD did not end with a
           <slash> character, and curpath.

        8. The curpath value shall then be converted to canonical form as
           follows, considering each component from beginning to end, in
           sequence:

            a. Dot components and any <slash> characters that separate
               them from the next component shall be deleted.

            b. For each dot-dot component, if there is a preceding
               component and it is neither root nor dot-dot, then:

                i.  If the preceding component does not refer (in the
                    context of pathname resolution with symbolic links
                    followed) to a directory, then the cd utility shall
                    display an appropriate error message and no further
                    steps shall be taken.

               ii.  The preceding component, all <slash> characters
                    separating the preceding component from dot-dot, dot-
                    dot, and all <slash> characters separating dot-dot
                    from the following component (if any) shall be
                    deleted.

            c. An implementation may further simplify curpath by removing
               any trailing <slash> characters that are not also leading
               <slash> characters, replacing multiple non-leading
               consecutive <slash> characters with a single <slash>, and
               replacing three or more leading <slash> characters with a
               single <slash>.  If, as a result of this canonicalization,
               the curpath variable is null, no further steps shall be
               taken.

        9. If curpath is longer than {PATH_MAX} bytes (including the
           terminating null) and the directory operand was not longer
           than {PATH_MAX} bytes (including the terminating null), then
           curpath shall be converted from an absolute pathname to an
           equivalent relative pathname if possible. This conversion
           shall always be considered possible if the value of PWD, with
           a trailing <slash> added if it does not already have one, is
           an initial substring of curpath.  Whether or not it is
           considered possible under other circumstances is unspecified.
           Implementations may also apply this conversion if curpath is
           not longer than {PATH_MAX} bytes or the directory operand was
           longer than {PATH_MAX} bytes.

       10. The cd utility shall then perform actions equivalent to the
           chdir() function called with curpath as the path argument. If
           these actions fail for any reason, the cd utility shall
           display an appropriate error message and the remainder of this
           step shall not be executed. If the -P option is not in effect,
           the PWD environment variable shall be set to the value that
           curpath had on entry to step 9 (i.e., before conversion to a
           relative pathname). If the -P option is in effect, the PWD
           environment variable shall be set to the string that would be
           output by pwd -P.  If there is insufficient permission on the
           new directory, or on any parent of that directory, to
           determine the current working directory, the value of the PWD
           environment variable is unspecified.

       If, during the execution of the above steps, the PWD environment
       variable is set, the OLDPWD environment variable shall also be set
       to the value of the old working directory (that is the current
       working directory immediately prior to the call to cd).
OPTIONS         top
       The cd utility shall conform to the Base Definitions volume of
       POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported by the implementation:

       -L        Handle the operand dot-dot logically; symbolic link
                 components shall not be resolved before dot-dot
                 components are processed (see steps 8.  and 9. in the
                 DESCRIPTION).

       -P        Handle the operand dot-dot physically; symbolic link
                 components shall be resolved before dot-dot components
                 are processed (see step 7. in the DESCRIPTION).

       If both -L and -P options are specified, the last of these options
       shall be used and all others ignored. If neither -L nor -P is
       specified, the operand shall be handled dot-dot logically; see the
       DESCRIPTION.
OPERANDS         top
       The following operands shall be supported:

       directory An absolute or relative pathname of the directory that
                 shall become the new working directory. The
                 interpretation of a relative pathname by cd depends on
                 the -L option and the CDPATH and PWD environment
                 variables. If directory is an empty string, the results
                 are unspecified.

       -         When a <hyphen-minus> is used as the operand, this shall
                 be equivalent to the command:

                     cd "$OLDPWD" && pwd

                 which changes to the previous working directory and then
                 writes its name.
STDIN         top
       Not used.
INPUT FILES         top
       None.
ENVIRONMENT VARIABLES         top
       The following environment variables shall affect the execution of
       cd:

       CDPATH    A <colon>-separated list of pathnames that refer to
                 directories. The cd utility shall use this list in its
                 attempt to change the directory, as described in the
                 DESCRIPTION. An empty string in place of a directory
                 pathname represents the current directory. If CDPATH is
                 not set, it shall be treated as if it were an empty
                 string.

       HOME      The name of the directory, used when no directory
                 operand is specified.

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base
                 Definitions volume of POSIX.1‐2017, Section 8.2,
                 Internationalization Variables for the precedence of
                 internationalization variables used to determine the
                 values of locale categories.)

       LC_ALL    If set to a non-empty string value, override the values
                 of all the other internationalization variables.

       LC_CTYPE  Determine the locale for the interpretation of sequences
                 of bytes of text data as characters (for example,
                 single-byte as opposed to multi-byte characters in
                 arguments).

       LC_MESSAGES
                 Determine the locale that should be used to affect the
                 format and contents of diagnostic messages written to
                 standard error.

       NLSPATH   Determine the location of message catalogs for the
                 processing of LC_MESSAGES.

       OLDPWD    A pathname of the previous working directory, used by cd
                 -.

       PWD       This variable shall be set as specified in the
                 DESCRIPTION. If an application sets or unsets the value
                 of PWD, the behavior of cd is unspecified.
ASYNCHRONOUS EVENTS         top
       Default.
STDOUT         top
       If a non-empty directory name from CDPATH is used, or if cd - is
       used, an absolute pathname of the new working directory shall be
       written to the standard output as follows:

           "%s\n", <new directory>

       Otherwise, there shall be no output.
STDERR         top
       The standard error shall be used only for diagnostic messages.
OUTPUT FILES         top
       None.
EXTENDED DESCRIPTION         top
       None.
EXIT STATUS         top
       The following exit values shall be returned:

        0    The directory was successfully changed.

       >0    An error occurred.
CONSEQUENCES OF ERRORS         top
       The working directory shall remain unchanged.

       The following sections are informative.
APPLICATION USAGE         top
       Since cd affects the current shell execution environment, it is
       always provided as a shell regular built-in. If it is called in a
       subshell or separate utility execution environment, such as one of
       the following:

           (cd /tmp)
           nohup cd
           find . -exec cd {} \;

       it does not affect the working directory of the caller's
       environment.

       The user must have execute (search) permission in directory in
       order to change to it.
EXAMPLES         top
       The following template can be used to perform processing in the
       directory specified by location and end up in the current working
       directory in use before the first cd command was issued:

           cd location
           if [ $? -ne 0 ]
           then
               print error message
               exit 1
           fi
           ... do whatever is desired as long as the OLDPWD environment variable
               is not modified
           cd -
RATIONALE         top
       The use of the CDPATH was introduced in the System V shell. Its
       use is analogous to the use of the PATH variable in the shell. The
       BSD C shell used a shell parameter cdpath for this purpose.

       A common extension when HOME is undefined is to get the login
       directory from the user database for the invoking user. This does
       not occur on System V implementations.

       Some historical shells, such as the KornShell, took special
       actions when the directory name contained a dot-dot component,
       selecting the logical parent of the directory, rather than the
       actual parent directory; that is, it moved up one level toward the
       '/' in the pathname, remembering what the user typed, rather than
       performing the equivalent of:

           chdir("..");

       In such a shell, the following commands would not necessarily
       produce equivalent output for all directories:

           cd .. && ls      ls ..

       This behavior is now the default. It is not consistent with the
       definition of dot-dot in most historical practice; that is, while
       this behavior has been optionally available in the KornShell,
       other shells have historically not supported this functionality.
       The logical pathname is stored in the PWD environment variable
       when the cd utility completes and this value is used to construct
       the next directory name if cd is invoked with the -L option.
FUTURE DIRECTIONS         top
       None.
SEE ALSO         top
       Section 2.12, Shell Execution Environment, pwd(1p)

       The Base Definitions volume of POSIX.1‐2017, Chapter 8,
       Environment Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1‐2017, chdir(3p)
COPYRIGHT         top
       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics Engineers,
       Inc and The Open Group.  In the event of any discrepancy between
       this version and the original IEEE and The Open Group Standard,
       the original IEEE and The Open Group Standard is the referee
       document. The original Standard can be obtained online at
       http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                2017                            CD(1P)