screen can allow multiple users to access the same session. This can be useful for all sorts of information sharing, from helpdesk applications to extreme programming. By default, multiuser mode is disabled, for security reasons.

To start using multiuser mode, use the command multiuser on in either your .screenrc or on screen's command line. Then use acladd or aclchg to tell screen about the users that you want to connect. screen will not allow connections from any user it hasn't been explicitly told about. Once the permissions are set up, the other users run screen -r <your username>/ to attach. (If you want multiple people (including you) to be attached at the same time, use -x instead of -r. If you have multiple sessions, put the session name after the slash.)

screen can require a password before the user connects. To enable this, add a crypted password as a parameter to acladd. The easiest way to get a crypted password is to run the screen command password. It will prompt you twice for a password. If the passwords match, it will put the crypted version in the paste buffer. From there, you can paste it into the config file or onto the command line.

To simplify permission management, screen supports groups, to a degree. Each group is named after the group owner, and shares that owner's permissions. If a member of the group fails a permission check, the check is repeated against the group owner's permissions.

screen has a relatively flexible representation of permissions. The permissions that can be set are read, write, and execute. Read does nothing. Write applies to windows and allows the user to type into the windows. Execute applies to commands and allows the user to run the specified commands. “#” can be used to apply to all windows and “?” to all commands.

When experimenting with permissions, the su command is sometimes useful. It allows you to change the effective user for the current display (just like the Unix su command allows you to operate as a different Unix user).

Somewhat relatedly, only one user can be typing in a given window at once. That user has the window's writelock. Normally, writelocks are in auto mode, meaning that whoever types into the window first gets the lock. The lock is relinquished when the user leaves that window. An acquired writelock can also be removed by using the command writelock off. If the user uses writelock on, he will keep the writelock even after leaving the window.

screen can allow programs to send commands to it via the escape sequence ESC ] 83 ; cmd ^G; in order for this to work, the pseudo-user :window: must exist and have the appropriate permissions to execute the supplied command.

• acladd - Adds users with full permission to all windows.
• aclchg - Adds users with more flexible permissions or changes the permissions on an existing user.
• acldel - Removes a user from screen's knowledge.
• aclgrp - Adds a user to a group or just describes user's group membership.
• aclumask - Sets default permissions for windows not yet created.
• defescape - Like escape, but sets the command character for all users.
• defwritelock - Sets the default writelock setting for new windows.
• multiuser - Enables or disables multiuser mode.
• su - Operate as a different user.
• writelock - Sets writelock mode for current window.

Add user phil with password moo, gives him full permission to everything:

acladd phil QSsUHy/lmL5CM

Remove all of phil's permissions (but he can still connect to the session and view all windows):

aclchg phil -rwx "#?"

Allow phil to write to windows 1, 2, and 7. Also allow him to run the commands select, next, and prev. Add a new user, bob, and give him the same permissions:

aclchg phil,bob +rwx 1,2,7,select,next,prev

Remove user bob:

acldel bob

Add user jeff to group phil:

aclgrp jeff phil

Show what groups (if any) phil belongs to:

aclgrp phil

* If you get a “chmod /dev/pts/xx: Operation not permitted” error, it may be because you have su on a tty you do not own. This does not work because you have to own the tty for screen to work.

* If you have a /tmp/uscreen file instead of a /tmp/screen/S-xxxxxxxx file, put multiuser on in your .screenrc file, and try again. I also had to forcibly remove /tmp/uscreen before restarting screen.

* You may also need to set-uid on the screen executable. Do this on your screen executable as root: chmod u+s /usr/bin/screen. This is in man screen towards the end.