Ubuntu Manpage: Xsession - initialize X session

Provided by: x11-common_7.7+19ubuntu14_all

NAME

       Xsession - initialize X session

SYNOPSIS

       Xsession [ session-type ]

DESCRIPTION

/etc/X11/Xsession is a Bourne shell (sh(1)) script which is run when an X Window System
session is begun by startx(1) or a display manager such as xdm(1). (Some display managers
only invoke Xsession when specifically directed to so by the user; see the documentation
for your display manager to find out more.) Administrators unfamiliar with the Bourne
shell will likely find the Xsession.options(5) configuration file easier to deal with than
Xsession itself.

Xsession is not intended to be invoked directly by the user; to be effective it needs to
run in a special environment associated with X server initialization. startx, xdm,
xinit(1), and other similar programs handle this.

By default on a Debian system, Xsession is used by both common methods of starting the X
Window System, xdm (or another X display manager) and startx. To change this for xdm,
edit the ‘DisplayManager*session’ resource in the /etc/X11/xdm/xdm-config file — for other
display managers, consult their documentation. To stop startx from using Xsession by
default, replace the contents of the /etc/X11/xinit/xinitrc file.

The Xsession script is quite flexible, and extensive customization of the X startup
procedure is possible without modifying the script itself. See “CUSTOMIZING THE STARTUP
PROCEDURE” below.

SESSION TYPES
Xsession may optionally be passed a single argument indicating the type of X session to be
started. It is up to the display manager to set the argument. To pass Xsession an
argument from startx or xinit, /etc/X11/Xsession (or /etc/X11/xinit/xinitrc) must be
called explicitly with a path, as in startx /etc/X11/Xsession failsafe. By default, three
different arguments are supported:

failsafe
invokes a session consisting solely of an x-terminal-emulator(1) (no window manager
is launched). If the x-terminal-emulator program cannot be found, the session
exits. The ‘failsafe’ argument is ignored if there is no ‘allow-failsafe’ line in
Xsession.options.

default
produces the same behavior as if no session type argument had been given at all.

program
starts program if it can be found in the $PATH. This is usually a session manager
or a very featureful window manager. If program is not found, the Xsession script
proceeds with its default behavior. This argument is ignored if there is no
‘allow-user-xsession’ line in Xsession.options. (If the administrator does not
want users writing their own .xsession files, it makes little sense to permit them
to specify the names of arbitrary programs to run.) Note that the restriction may
be easy to bypass, e.g. by using a .gnomerc file instead.

DEFAULT STARTUP PROCEDURE
Initially, Xsession performs some housekeeping. It declares a set of built-in functions
(see “BUILT-IN SHELL FUNCTIONS” below) and variables, then attempts to create a log file
for the X session, or append to an existing one. Historically this is called an ‘error’
file, but it catches all sorts of diagnostic output from various X clients run in the
user's session, not just error messages. If it is impossible to write to an error file,
the script (and thus the X session) aborts. For convenience, once the error file is
successfully opened, Xsession reports the fact that the session has started, the invoking
username, and the date to the error file. This makes it easier to discern which X session
produced a particular line of output in the file.

Xsession next confirms that its script directory, Xsession.d, exists. If it does not, the
script aborts. After the script directory is confirmed to be present, Xsession uses
run-parts(1) to identify files in that directory that should be sourced (executed) in the
shell's environment. Only files named in a certain way are sourced; see the run-parts
manual page for a description of valid characters in the filename. (This restriction
enables the administrator to move experimental or problematic files out of the way of the
script but keep them in an obvious place, for instance by renaming them with ‘.old’ or
‘.broken’ appended to the filename.)

SUPPLIED SCRIPTS
Five shell script portions are supplied by default to handle the details of the session
startup procedure.

/etc/X11/Xsession.d/20x11-common_process-args
Arguments are processed as described in “SESSION TYPES” above. The startup
program, if one is identified at this point, is merely stored for later reference,
and not immediately executed.

/etc/X11/Xsession.d/30x11-common_xresources
X resources are merged. run-parts is again used, this time to identify files in
the /etc/X11/Xresources directory that should be processed with ‘xrdb -merge’.
Next, if the line ‘allow-user-resources’ is present in Xsession.options, the user's
$HOME/.Xresources file is merged in the same way.

/etc/X11/Xsession.d/35x11-common_xhost-local
Give access to the X server to the same user on the local host. If the xhost
command is available, it will use it to allow any process of the same user running
on the local host to access the X server.

/etc/X11/Xsession.d/40x11-common_xsessionrc
Source global environment variables. This script will source anything in
$HOME/.xsessionrc if the file is present. This allows the user to set global
environment variables for their X session, such as locale information.

/etc/X11/Xsession.d/50x11-common_determine-startup
Determine startup program. The X client to launch as the controlling process (the
one that, upon exiting, causes the X server to exit as well) is determined next.
If a program or failsafe argument was given and is allowed (see above), it is used
as the controlling process. Otherwise, if the line ‘allow-user-xsession’ is
present in Xsession.options, a user-specified session program or script is used.
In the latter case, two historically popular names for user X session scripts are
searched for: $HOME/.xsession and $HOME/.Xsession (note the difference in case).
The first one found is used. If the script is not executable, it is marked to be
executed with the Bourne shell interpreter, sh. Finally, if none of the above
succeeds, the following programs are searched for: /usr/bin/x-session-manager,
/usr/bin/x-window-manager, and /usr/bin/x-terminal-emulator. The first one found
is used. If none are found, Xsession aborts with an error.

/etc/X11/Xsession.d/90x11-common_ssh-agent
Start ssh-agent(1), if needed. If the line ‘use-ssh-agent’ is present in
Xsession.options, and no SSH agent process appears to be running already, ssh-agent
is marked to be used to execute the startup program determined previously. Note:
this functionality may move to the ssh package in the future.

/etc/X11/Xsession.d/99x11-common_start
Start the X session. The startup program is executed, inside a Bourne shell if it
is not executable, and inside an ssh-agent if necessary. The shell's exec command
is used to spare a slot in the process table.

CUSTOMIZING THE STARTUP PROCEDURE
Of course, any of the existing files can be edited in place.

Because the order in which the various scripts in /etc/X11/Xsession.d are executed is
important, files to be added to this directory should have a well-formed name. The
following format is recommended:

* a two-digit number denoting sequence;

* the name of the package providing the script (or ‘custom’ for locally-created scripts);

* an underscore;

* a description of the script's basic function, using only characters allowed by
run-parts.

Here is an example of how one might write a script, named 40custom_load-xmodmap, to invoke
xmodmap(1):

SYSMODMAP="/etc/X11/Xmodmap"
USRMODMAP="$HOME/.Xmodmap"

if [ -x /usr/bin/X11/xmodmap ]; then
if [ -f "$SYSMODMAP" ]; then
xmodmap "$SYSMODMAP"
fi
fi

if [ -x /usr/bin/X11/xmodmap ]; then
if [ -f "$USRMODMAP" ]; then
xmodmap "$USRMODMAP"
fi
fi

Those writing scripts for Xsession to execute should avail themselves of its built-in
shell functions, described below.

BUILT-IN SHELL FUNCTIONS
message is used for communicating with the user. It is a wrapper for the echo(1) command
and relies upon echo for its argument processing. This function may be given an
arbitrarily long message string, which is formatted to the user's terminal width (breaking
lines at whitespace) and sent to standard error. If the DISPLAY environment variable is
set and the xmessage(1) program is available, xmessage is also used to display the
message.

message_nonl is used for communicating with the user when a trailing newline is
undesirable; it omits a trailing newline from the message text. It otherwise works as
message.

errormsg is used for indicating an error condition and aborting the script. It works as
message, above, except that after displaying the message, it will exit Xsession with
status 1.

ENVIRONMENT

       The following environment variables affect the execution of Xsession:

       HOME   specifies the user's home directory; various files are searched for here.

       TMPDIR names a default directory for temporary files; if the standard X session error file
              cannot be opened, this variable is used to locate a place for one.

       COLUMNS
              indicates  the width of terminal device in character cells.  This value is used for
              formatting diagnostic messages.

INPUT FILES

       /etc/X11/Xsession.d/
              is a directory containing Bourne shell scripts to be executed by  Xsession.   Files
              in  this  directory  are matched using run-parts and are sourced, not executed in a
              subshell.

       /etc/X11/Xresources/
              is a directory containing files corresponding to  Debian  package  names,  each  of
              which contains system-wide X resource settings for X clients from the corresponding
              package.  The settings are loaded with xrdb -merge.  Files in  this  directory  are
              matched using run-parts.

       /etc/X11/Xsession.options
              contains   configuration   options   for   the   /etc/X11/Xsession   script.    See
              Xsession.options(5) for more information.

       $HOME/.Xresources
              contains X resources specific to the invoking user's environment.  The settings are
              loaded  with  xrdb -merge.  Note that $HOME/.Xdefaults is a relic from X Version 10
              (and X11R1)  days,  before  app-defaults  files  were  implemented.   It  has  been
              deprecated  for  over ten years at the time of this writing.  .Xresources should be
              used instead.

       $HOME/.xsession
              is a sequence of commands invoking X clients (or a session manager such as xsm(1)).
              See the manual page for xinit for tips on writing an .xsession file.

OUTPUT FILES

       $HOME/.xsession-errors
              is  where  standard  output and standard error for Xsession script and all X client
              processes are directed by default.

       $TMPDIR/filename
              is where the X session error file is placed  if  $HOME/.xsession-errors  cannot  be
              opened.   For  security  reasons,  the  exact  filename  is  randomly  generated by
              tempfile(1).

AUTHORS

       Stephen Early, Mark Eichin, and Branden Robinson developed  Debian's  X  session  handling
       scripts.  Branden Robinson wrote this manual page.