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, and there is no way to pass Xsession an argument from startx or xinit. By default, three different arguments are supported:

failsafe

invokes a session consisting solely of /usr/bin/x-terminal-emulator (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.)

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 an internal implementation of 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.)

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

/etc/X11/Xsession.d/20xfree86-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/30xfree86-common_xresources

X resources are merged. The internal 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/50xfree86-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/90xfree86-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/99xfree86-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.

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; * 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(1x):

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.

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(1x) 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.