A shortcut to the last command is to press <Ctrl><Alt><Esc>, to which the cursor changes into something looking like a death sentence--you point at the window of the offending program, click your mouse, and the window closes and the program is gone.
If your X-windows system crashes so that it cannot recover, or you just get stuck, it may be the easiest to kill the X-server by pressing <Ctrl><Alt><BkSpace>. After that, it might be a good idea to run ps axu, find any possible X-programs that might still be running, and kill them. If you don't do this, and there really is a misbehaving program that caused your X-windows to crash, it might cause trouble again.
If you have programs in the background, the operating systems will object your logging out, and issue a message like "There are stopped jobs". To override and logout anyway, just repeat the logout (or exit) command --the background program(s) will be automatically terminated and you will be logged out.
4.1.7.2 Core files
When a program crashes, it often dumps a "core" into your home directory. This is accompanied by an appropriate message. A core is a memory image (plus debugging info) and is meant to be a debugging tool. If you are a user who does not intend to debug the program, you may simply delete the core:
rm core
or do nothing (the core will be overwritten when another core is ever dumped). You can also disable dumping the core using the command:
ulimit -c 0
Checked if it worked using:
ulimit -a
(This shows "user limits", the option "-a" stands for "all".) To make the option of disabling core dumps permanent for all users, edit the file /etc/profile (as root), where ulimit is set, and adjust the setting. Re-login for the changes to /etc/profile to take effect.
If you would like to see how a core file can be used, try (in the directory where you have a core file):
gdb -c core
This launches GNU debugger (gdb) on the core file "core" and displays the name of the program that created the core, signal on which the program was terminated, etc. Type "quit" to exit the debugger. To learn the meaning of different signals, try:
cat /usr/include/bits/signum.h |more
4.1.8 Command options
Most commands accept numerous "options". An option can be introduced with an "-" (dash). For example:
dir -l
shows me the listing of the current directory but in a long format (the default format is "short"). Multiple options can be introduced in two, equivalent ways:
dir -l -a
or
dir -la
Either of the above commands will show me the listing of the current directory in the long file format (option -l), and include all files in the listing, i.e., also the hidden files (option -a).
Most popular options are marked with one letter. This follows the traditional UNIX way of invoking options. There is also a new style, which looks like this:
dir --help
Here, a single option is more than one character long, and it must be introduced with two dashes. The above command displays a brief help for the dir command, including the listing of all options. Because there are so many of those (more than a screen-full), I would probably do:
dir --help | more
