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UNIX for Dummies Questions & Answers
X Window system - concept
Post 302555274 by pludi on Wednesday 14th of September 2011 10:04:35 AM
09-14-2011
There's a pretty good overview at Wikipedia.
The tl;dr version:
The X Window System can be seen as 3 components interacting.
First, there's the server component, whose job is to (basically) tell the graphics card what to draw on the monitor (screen), and reads keyboard and mouse input. Each computer can run multiple server instances, where each uses a distinctive display (defined by the server it belongs to, the size, resolution, color depth, and network port [more on that later]). The server is usually started by the xinit program, which in turn is usually started by the startx shell script, which does some preliminary setup.
The second part is the client, which is the application the user wants to interact with. It can, but doesn't have to, run on the same machine as the server. Window managers and desktops like Xfce, Gnome, KDE, or twm are all clients here, with the main job of providing easier access.
Last but not least is the network protocol, which is the way the client and server communicate with each other. Keyboard and mouse input received by the server is sent to the client, which acts on it (or not
), and as a result sends new drawing instructions back to the server, which then tells the graphics card what to draw. Rinse, repeat.
Modern Linux distributions mostly install the server and some client programs by default, and the server is automatically started at boot as a daemon, with the login manager (XDM, GDM, KDM, ...) as the first client, which then handles user logins, etc. Configuration files for this and other things reside in /etc/X11, or sometimes just in /etc, or some other subdirectories, which is distribution specific.
But in some cases there's no need to install and run a graphical user interface, for example with servers.
But some servers do have the libraries and programs installed that are required to communicate with a remote X11 server. For example, it's possible to install an X11 server on a Windows machine, connect to a remote machine using PuTTY and have a feature called "X11 forwardning" enabled. Here the network communication is tunneled through the SSH connection, and client programs like xeyes are displayed as if running on the Windows machine. With this feature the SSH daemon prepares the environment so that clients can easily connect to the server without any additional setup required by the user.
The tl;dr version:
The X Window System can be seen as 3 components interacting.
First, there's the server component, whose job is to (basically) tell the graphics card what to draw on the monitor (screen), and reads keyboard and mouse input. Each computer can run multiple server instances, where each uses a distinctive display (defined by the server it belongs to, the size, resolution, color depth, and network port [more on that later]). The server is usually started by the xinit program, which in turn is usually started by the startx shell script, which does some preliminary setup.
The second part is the client, which is the application the user wants to interact with. It can, but doesn't have to, run on the same machine as the server. Window managers and desktops like Xfce, Gnome, KDE, or twm are all clients here, with the main job of providing easier access.
Last but not least is the network protocol, which is the way the client and server communicate with each other. Keyboard and mouse input received by the server is sent to the client, which acts on it (or not
), and as a result sends new drawing instructions back to the server, which then tells the graphics card what to draw. Rinse, repeat.Modern Linux distributions mostly install the server and some client programs by default, and the server is automatically started at boot as a daemon, with the login manager (XDM, GDM, KDM, ...) as the first client, which then handles user logins, etc. Configuration files for this and other things reside in /etc/X11, or sometimes just in /etc, or some other subdirectories, which is distribution specific.
But in some cases there's no need to install and run a graphical user interface, for example with servers.
But some servers do have the libraries and programs installed that are required to communicate with a remote X11 server. For example, it's possible to install an X11 server on a Windows machine, connect to a remote machine using PuTTY and have a feature called "X11 forwardning" enabled. Here the network communication is tunneled through the SSH connection, and client programs like xeyes are displayed as if running on the Windows machine. With this feature the SSH daemon prepares the environment so that clients can easily connect to the server without any additional setup required by the user.
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LEARN ABOUT REDHAT
grpck
GRPCK(1) General Commands Manual GRPCK(1) NAME
grpck - verify integrity of group files SYNOPSIS
grpck [-r] [group shadow] DESCRIPTION
grpck verifies the integrity of the system authentication information. All entries in the /etc/group and /etc/gshadow are checked to see that the entry has the proper format and valid data in each field. The user is prompted to delete entries that are improperly formatted or which have other incorrectable errors. Checks are made to verify that each entry has - the correct number of fields - a unique group name - a valid list of members and administrators The checks for correct number of fields and unique group name are fatal. If the entry has the wrong number of fields, the user will be prompted to delete the entire line. If the user does not answer affirmatively, all further checks are bypassed. An entry with a dupli- cated group name is prompted for deletion, but the remaining checks will still be made. All other errors are warnings and the user is encouraged to run the groupmod command to correct the error. The commands which operate on the /etc/group file are not able to alter corrupted or duplicated entries. grpck should be used in those circumstances to remove the offending entry. OPTIONS
By default, grpck operates on the files /etc/group and /etc/gshadow. The user may select alternate files with the group and shadow parame- ters. Additionally, the user may execute the command in read-only mode by specifying the -r flag. This causes all questions regarding changes to be answered no without user intervention. grpck can also sort entries in /etc/group and /etc/gshadow by GID. To run it in sort mode pass it -s flag. No checks are performed then, it just sorts. FILES
/etc/group - group account information /etc/gshadow - encrypted passwords and group administrator information /etc/passwd - user information SEE ALSO
group(5), passwd(5), shadow(5), groupmod(8) DIAGNOSTICS
The grpck command exits with the following values: 0 Success 1 Syntax Error 2 One or more bad group entries 3 Cannot open group files 4 Cannot lock group files 5 Cannot update group files AUTHOR
Julianne Frances Haugh (jockgrrl@ix.netcom.com) GRPCK(1)