Bug 59613

PROBLEM:
The environment variable PROMPT_COMMAND provides a way to run an
arbitrary command before the prompt is shown (post-execution hook),
but there is no way to run a command after the command-line has been
entered but before it has been executed (pre-execution hook).

MOTIVATION:
I want to set the xterm title to show the command currently running,
so that I can see what a minimized or obscured window is doing by
looking at the taskbar.
Other uses for this could be menu systems, computer-based exercises
or monitoring.

PROPOSITION:
Before each command is executed, the environment variable
SPAWN_COMMAND is inspected and executed if filled. The command-line
is accessible for this command through the use of the environment
variable CMD_LINE.

IMPLEMENTATION:
A new global variable 'global_command_line' is introduced, and the
history module sets this variable to be the contents of the last
command-line parsed.
A new function 'execute_spawn_command' is introduced, that reads the
contents of the SPAWN_COMMAND environment variable, sets the CMD_LINE
variable to the last command-line known, executes the commands and
restores the old value of CMD_LINE.
A call to execute_spawn_command() is added right before the call to
execute_command() in the main loop. This a place that was found to be
safe for such hooking. Adding it in execute_command() or further down
the chain gives odd effects due to recursive calls.
The option '--no-spawn-command' was added to enable the user to
disable the functionality.

VERSION:
The patch is done against bash version 2.05.

KNOWN ISSUES:
The use of a savestring() unmatched by an xfree() as an argument to
bind_variable() seems logical since bind_variable() would adopt the
responsibility of the memory for the variable.
However, the same thing done for parse_and_execute() does not seem
logical to me, but the string is appearently corrupted if this is not
done. Please confirm that this is safe to do in respect to memory
leaks.
A new global variable to hold the contents of the command-line seems
redundant, but I'm having problems accessing the history records from
outside the history module, and I didn't bother to use any time to
figure out why it seemed state dependent.
A core dump may be triggered upon exit from a subshell if variable
substitution is used within the SPAWN_COMMAND variable.

COMPATABILITY:
Old scripts that happens to use the environment variable
SPAWN_COMMAND, will undoubtly behave very strange. For this reason,
the option --no-spawn-command is introduced to enable bash to revert
to 2.05 behavior, where SPAWN_COMMAND is not interpreted.

DOCUMENTATION:
I suggest that the following paragraphs is added to the manual:

--no-spawn-command
	If this option is NOT set (which is the default), the
	contents of the environment variable SPAWN_COMMAND is
	executed before the execution of the command-line. The
	environment variable CMD_LINE will be filled with the
	unexpanded contents of the command-line, for the duration of
	that command only (i.e. it reverts to its previous contents before
	the command-line executes).
Example:
	To display the current running process in the title bar of an
	xterm window, and reset the title before the prompt is shown
	again, put the following commands in your ~/.bashrc

	export SPAWN_COMMAND='echo -ne "\033]2;$CMD_LINE\007"'
	export PROMPT_COMMAND='echo -ne "\033]2;idle\007"'

	or, you may want to use a command that append information
	about the current user and directory for the command:

	export PROMPT_COMMAND='/bin/echo -ne "\033]0;idle in 
`/usr/bin/whoami`@`/bin/hostname | /bin/cut -d. -f 1`:`/bin/pwd | /bin/sed "s%
^$HOME%~%g"`\007"'
	export SPAWN_COMMAND='/bin/echo -ne "\033]2;${CMD_LINE} in 
`/usr/bin/whoami`@`/bin/hostname | /bin/cut -d. -f 1`:`/bin/pwd | /bin/sed "s%
^$HOME%~%g"`\007"'

AUTHORS:
Roland Kaufmann	<roland at ii dot uib dot no>
Hevard Lygre 	<hklygre at online dot no>