The server keeps track of a name prefix for the client. The name prefix is a pathname; it always begins with a slash.
The name prefix is initially a server-defined function of the username. It may be changed by CWD and CDUP requests.
The name prefix allows the client to abbreviate encoded pathnames in parameters for various requests.
The CWD command -- Change Working Directory
A CWD request has a nonempty parameter giving an encoded pathname. It asks the server to set the name prefix to this pathname, or to another pathname that will have the same effect as this pathname if the filesystem does not change.
The server may accept a CWD request using code 200 or 250:
> CWD /public < 250 Okay.
The server may reject a CWD request using code 550:
> CWD /pubilc < 550 /pubilc: No such file or directory.
Most servers reject CWD requests unless the pathname refers to an accessible directory. Some servers accept all CWD requests, without regard for what directories are actually accessible.
The PWD command -- Print Working Directory
A PWD request asks the server to print the name prefix:
> PWD < 257 "/home/joe"
If the server accepts the PWD request (required code 257), its response consists of exactly one line, with text in the following format:
Many servers fail to check for double quotes in the name prefix. There is no reliable way for the client to compensate for this.
Servers are permitted to reject PWD requests with code 550, but high-quality servers will try to accept all PWD requests.
The CDUP command
A CDUP request asks the server to remove the last slash, and everything following it, from the name prefix.If this produces an empty name prefix, the new name prefix is a single slash. CDUP parameters are prohibited.
The server may accept a CDUP request using code 200 or 250. (RFC 959 says that code 200 is required;but it also says that CDUP uses the same codes as CWD.) The server may reject a CDUP request using code 550.
RFC 959 envisioned CDUP as a means of undoing a previous CWD, to simplify namespace navigation:
> PWD < 257 "/home/joe" > CWD tex < 250 Okay. > PWD < 257 "/home/joe/tex" > CDUP < 200 Okay. > PWD < 257 "/home/joe"
However, this is horribly unreliable in practice. Here is a typical example from a UNIX
server where there is a ``symbolic link'' from
> PWD < 257 "/" is current directory. > CWD /bin < 250 CWD command successful. > PWD < 257 "/usr/bin" is current directory. > CDUP < 250 CWD command successful. > PWD < 257 "/usr" is current directory.
It is strongly recommend that clients avoid CDUP.