5.26.8 Remote files

Since one of the intentions for file handlers is to allow remote files to be accessed, a common method of providing new methods of doing this has been implemented, in the remote.jl Lisp library.

Accessing a file name matching the regular expression:

^/(([a-zA-Z0-9._-]+)@)?([a-zA-Z0-9._-]+):

for example /john@host.com:file refers to a file called ‘file’ owned by the user ‘john’, on the system ‘host.com’.

If no username is specified explicitly, two variables are used to select the user:

Variable: remote-host-user-alist

An alist mapping host regexps to the default user name to use for remote file connections to that host.

Variable: remote-default-user

User name to use for remote file connections when otherwise unspecified. By default the current user name on the local system.

Two variables control how individual hosts are matched to methods of accessing files.

Variable: remote-auto-backend-alist

An alist of (host-regexp . backend-type) mapping host names to methods of accessing remote files.

Variable: remote-default-backend

A symbol defining the method to use for otherwise unselected hosts.

A method of accessing files, or a backend is a symbol whose remote-backend property names a function to call when files need to be accessed. For example the ftp backend is initialised as:

(put 'ftp 'remote-backend remote-ftp-handler)

The backend function is called as (function split-name operation args). The split-name is a three-element list, (user-or-nil host file) defining the file to be accessed. The other options are as usual. Further details can be found in the remote.jl, remote-ftp.jl and remote-rcp.jl Lisp source files.

The ftp backend is currently the most well-developed, several functions and variables may be used to customise its behaviour.

Function: remote-ftp-add-passwd user host passwd

Add the string passwd as the password for the FTP session connecting to user@host.

Variable: remote-ftp-show-messages

When true (the default), messages are displayed as FTP commands are executed.

Variable: remote-ftp-display-progress

When true (the default) display progress of FTP transfers.

Variable: remote-ftp-anon-users

A regular expression matching the user names for “anonymous” FTP sessions.

Variable: remote-ftp-anon-passwd

The string to send as the passwd of an anonymous FTP session. By default the current uses email address.

There is a problem with the ftp backend however; due to limitations in the FTP protocol, not all Librep file operations are supported, with the most obvious exception being the make-symlink function.

When this is a problem it may be possible to use rep’s custom file transfer protocol. If it is possible to use rsh to connect to the remote host in question, then the rep backend may be used.

The rep-remote program distributed with Librep must exist on the remote host, this is executed via rsh and provides a protocol for executing all of Librep’s file operations on that host. See the lisp/remote-rep.jl file in the distribution for more details.