cd (command)

From Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2008)

cd, sometimes also available as chdir (change directory), is a command line command to change the current working directory in operating systems such as Unix, DOS, OS/2 and Windows. It is also available for use in shell scripts and batch files.

chdir(2) is the system call which changes the working directory, as defined by POSIX. CHDIR() is also a Visual Basic function which changes the working directory.

[edit] Usage

A directory is a logical section of a file system used to hold files. Directories may also contain other directories. The cd command can be used to change into a subdirectory, move back into the parent directory, move all the way back to the root directory or move to any given directory.

Consider the following subsection of a Unix filesystem, which shows a user's home directory (represented as "~") with a file ("text.txt") and three subdirectories.

A user's view of the file system in Unix-like systems begins with the home directory (often abbreviated to ~). From there, the tree can spread into more subdirectories and/or files.

If the user's current working directory is the home directory ("~"), then entering the command ls followed by cd games might produce the following transcript:

  me@host:~$ ls
  workreports games encyclopedia text.txt
  me@host:~$ cd games
  me@host:~/games$

The user is now in the "games" directory.

A similar session in DOS (though the concept of a "home directory" may not apply, depending on the specific version) would look like this:

  C:\> dir
  workreports        <DIR>       Wed Oct 9th   9:01
  games              <DIR>       Tue Oct 8th  14:32
  encyclopedia       <DIR>       Mon Oct 1st  10:05
  text        txt           1903 Thu Oct10th  12:43
  C:\> cd games
  C:\games>

Note that cd has different effects on different operating systems if given with no arguments. For example, if executed without arguments in DOS, OS/2, or Windows, the current working directory is printed. If executed without arguments in Unix, then the user is returned to the home directory. The effect of executing the cd command within a script or batch file also varies. In DOS, the caller's current directory can be directly altered by the batch file's use of this command. In Unix the caller's current directory is not altered by the script's invocation of the cd command. This is because in Unix, the script is usually executed within a subshell.

[edit] Options (version specific)

• -p Print the final directory stack, just like dirs.
• -l, '~' or '~name' in the output is expanded explicitly to home or the pathname of the home directory for user name.
• -n Entries are wrapped before they reach the edge of the screen.
• -v entries are printed one per line, preceded by their stack positions.

[edit] How it works

cd is frequently included built directly into the command line interpreter. This is the case in most of the Unix shells (Bourne shell, tcsh, bash, etc.), cmd.exe and Windows PowerShell on Windows and COMMAND.COM on DOS.

Command line shells on Windows usually use the Windows API to change the current working directory, whereas on Unix systems cd calls the chdir() POSIX C function. This means that when the command is executed, no new process is created to migrate to the other directory as is the case with other commands such as ls. Instead, the shell itself executes this command. This is because, when a new process is created, child process inherits the directory in which the parent process was created. If the cd command inherits the parent process' directory, then the objective of the command cd will never be achieved.

Windows PowerShell, Microsoft's new object-oriented command line shell and scripting language executes the cd command (cmdlet) within the shell's process. However, since PowerShell is based on the .NET Framework and has a different architecture than previous shells, all of PowerShell's cmdlets like ls, rm etc. run in the shell's process. Of course, this is not true for legacy commands which still run in a separate process.

[edit] References

• Microsoft TechNet Chdir (Cd) article
• cd: change the working directory – Commands & Utilities Reference, The Single UNIX® Specification, Issue 6 from The Open Group

[edit] See also