8.4 Multiuser Session

These commands allow other users to gain access to one single screen session. When attaching to a multiuser screen the sessionname is specified as username/sessionname to the -S command line option. Screen must be compiled with multiuser support to enable features described here.

8.4.1 Multiuser

Command: multiuser state

(none)
Switch between single-user and multi-user mode. Standard screen operation is single-user. In multi-user mode the commands acladd, aclchg and acldel can be used to enable (and disable) other users accessing this screen.

8.4.2 Acladd

Command: acladd usernames

Command: addacl usernames

(none)
Enable users to fully access this screen session. Usernames can be one user or a comma separated list of users. This command enables to attach to the screen session and performs the equivalent of aclchg usernames +rwx "#?". To add a user with restricted access, use the aclchg command below. Addacl is a synonym to acladd. Multi-user mode only.

8.4.3 Aclchg

Command: aclchg usernames permbits list

Command: chacl usernames permbits list

(none)
Change permissions for a comma separated list of users. Permission bits are represented as `r', `w' and `x'. Prefixing `+' grants the permission, `-' removes it. The third parameter is a comma separated list of commands or windows (specified either by number or title). The special list `#' refers to all windows, `?' to all commands. If usernames consists of a single `*', all known users are affected. A command can be executed when the user has the `x' bit for it. The user can type input to a window when he has its `w' bit set and no other user obtains a writelock for this window. Other bits are currently ignored. To withdraw the writelock from another user in e.g. window 2: `aclchg username -w+w 2'. To allow read-only access to the session: `aclchg username -w "#"'. As soon as a user's name is known to screen, he can attach to the session and (per default) has full permissions for all command and windows. Execution permission for the acl commands, at and others should also be removed or the user may be able to regain write permission. Chacl is a synonym to aclchg. Multi-user mode only.

8.4.4 Acldel

Command: acldel username

(none)
Remove a user from screen's access control list. If currently attached, all the user's displays are detached from the session. He cannot attach again. Multi-user mode only.

8.4.5 Aclgrp

Command: aclgrp username [groupname]

(none)
Creates groups of users that share common access rights. The name of the group is the username of the group leader. Each member of the group inherits the permissions that are granted to the group leader. That means, if a user fails an access check, another check is made for the group leader. A user is removed from all groups the special value `none' is used for groupname. If the second parameter is omitted all groups the user is in are listed.

8.4.6 Displays

Command: displays

(C-a *)
Shows a tabular listing of all currently connected user front-ends (displays). This is most useful for multiuser sessions.

8.4.7 aclumask

Command: aclumask [users]+/-bits ...

Command: umask [users]+/-bits ...

(none)
This specifies the access other users have to windows that will be created by the caller of the command. Users may be no, one or a comma separated list of known usernames. If no users are specified, a list of all currently known users is assumed. Bits is any combination of access control bits allowed defined with the aclchg command. The special username `?' predefines the access that not yet known users will be granted to any window initially. The special username `??' predefines the access that not yet known users are granted to any command. Rights of the special username nobody cannot be changed (see the su command). Umask is a synonym to aclumask.

8.4.8 Wall

Command: wall message

(none)
Write a message to all displays. The message will appear in the terminal's status line.

8.4.9 Writelock

Command: writelock on|off|auto

(none)
In addition to access control lists, not all users may be able to write to the same window at once. Per default, writelock is in `auto' mode and grants exclusive input permission to the user who is the first to switch to the particular window. When he leaves the window, other users may obtain the writelock (automatically). The writelock of the current window is disabled by the command writelock off. If the user issues the command writelock on he keeps the exclusive write permission while switching to other windows.

Command: defwritelock on|off|auto

(none)
Sets the default writelock behavior for new windows. Initially all windows will be created with no writelocks.

8.4.10 Su

Command: su [username [password [password2]]]

(none)
Substitute the user of a display. The command prompts for all parameters that are omitted. If passwords are specified as parameters, they have to be specified un-crypted. The first password is matched against the systems passwd database, the second password is matched against the screen password as set with the commands acladd or password. Su may be useful for the screen administrator to test multiuser setups. When the identification fails, the user has access to the commands available for user `nobody'. These are detach, license, version, help and displays.

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