Local transports handle deliveries to files and pipes. (The autoreply transport can be thought of as similar to a pipe.) Exim always runs transports in subprocesses, under specified uids and gids. Typical deliveries to local mailboxes run under the uid and gid of the local user.
Exim also sets a specific current directory while running the transport; for some transports a home directory setting is also relevant. The pipe transport is the only one which sets up environment variables; see section 28.3 for details.
The values used for the uid, gid, and the directories may come from several different places. In many cases, the router that handles the address associates settings with that address as a result of its check_local_user, group, or user options. However, values may also be given in the transport's own configuration, and these override anything that comes with the address.
All transports have the options group and user. If group is set, it overrides any group that may be set in the address, even if user is not set. This makes it possible, for example, to run local mail delivery under the uid of the recipient, but in a special group. For example:
group_delivery: driver = appendfile file = /var/spool/mail/$local_part group = mail
If user is set for a transport, its value overrides what is set in the address. If user is non-numeric and group is not set, the gid associated with the user is used. If user is numeric, group must be set.
When the uid is taken from the transport's configuration, the initgroups() function is called for the groups associated with that uid if the initgroups option is set for the transport. When the uid is not specified by the transport, but is associated with the address by a router, the option for calling initgroups() is taken from the router configuration.
The pipe transport contains the special option pipe_as_creator. If this is set and user is not set, the uid of the process that called Exim to receive the message is used, and if group is not set, the corresponding original gid is also used.
Routers may set current and home directories for local transports by means of the transport_current_directory and transport_home_directory options. When check_local_user is set for a router, the home directory value defaults to the user's home directory. However, transports can override the values set by the router by means of the current_directory and home_directory options.
If neither the router nor the transport sets a current directory, Exim uses the value of the home directory, if it is set. Otherwise it sets the current directory to / before running a local transport.
Normally a local delivery is handling a single address, and in that case the variables such as $domain and $local_part are set during local deliveries. However, in some circumstances more than one address may be handled at once (for example, while writing batch SMTP for onward transmission by some other means). In this case, the variables associated with the local part are never set, $domain is set only if all the addresses have the same domain, and $original_domain is never set.