Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 73. Don't Let Elvis Leave the Building

Keep a program running in the background by
automatically restarting it whenever it exits.

Some programs behave
contrary to our wishes and exit
prematurely, either because they are designed to do so, or because
they are flaky and prone to crashing. This hack provides a neat trick
to restart such programs automatically every time they exit.

The xcompmgr program
that
provides drop shadows and other special effects for Xorg is still a
work in progress, and it often exits unexpectedly. With a simple
script, you can automatically restart it every time it exits. First,
log in as root, fire up your favorite editor, and create the
following script, naming it
/usr/local/bin/keep-xcompmgr-running:

#!/bin/bash
# start up xcompmanager with drop shadows and fade effects
instances=`ps ax | grep "xcompmgr -cCfF" | grep -v grep | wc -l`
if [ $instances == 0 ]; then
while true; do xcompmgr -cCfF -l 0 -t 0 -r 5 -o .6 ; done
else
exit 1
fi

The first thing the script does is check to see if the command is
already running. If so, the script exits. Perhaps you forgot you
already started this script, or perhaps you started the command
manually. In either case, you don't want to start
two instances, and this portion of the script prevents that.

How to Ignore grep

Many people use the expression ps ax to see which
processes are running, and then pipe the command through
a grep command (a search utility) to see if a
certain program is in the list. A common problem often occurs when
you use a command such as the following in a script to see if
xcompmgr is already running:

ps ax | grep xcompmgr

The problem is that even if xcompmgr is not
running, the command might return a resultthe
grep command itself. The grep
command finds the word "xcompmgr"
in the process list, because it put the word there:

22833 pts/1    R+     0:00 grep xcompmgr

One way to prevent this is to pipe the results through a second
grep command. This time, you pipe it through the
command grep -v grep. The -v
switch tells grep to ignore lines that include the
word "grep."

This is how the following line uses the technique to make sure
xcompmgr is not running:

instances=`ps ax | grep "xcompmgr -cCfF" | grep -v grep | wc -l`

First, it generates a process list and pipes it through
grep to find lines that contain the string
xcompmgr -cCfF. If xcompmgr -cCfF is running, this command might return two lines. One
line will show the xcompmgr process running, and
the second line will show the grep command that is
looking for this program. Then those lines are piped through
grep again, this time eliminating lines that
contain the word "grep." Finally,
it pipes the results through wc -l, which is a
command that counts lines of text. If one line is left over after
grep removes itself from the list,
xcompmgr -cCfF must be running. If no lines are
left over after the grep command is removed from
the list, you know xcompmgr -cCfF is not running.

You might also be wondering why the script looks for
xcompmgr -cCfF rather than something simpler such
as xcompmgr. I confess that it's
force of habit. Suppose you were looking to see if the
xset program is running. If we look for the word
"xset" it might find the
xsetpointer program, because the
xset string appears in that program, too. The
script would get the mistaken idea that xset is
running, because it found a program with a name that contains the
xset string. To avoid this, I have developed the
habit of finding ways to make the search string as unique as
possible.

Notice that the xcompmgr command is not followed
with an ampersand. That would launch xcompmgr in
the background and return control to the script. Then the script
would proceed to try to launch more instances of
xcompmgr over and over
again. Trust me. That's a bad thing.

Now save your work and make this script executable with this command:

# chmod +x /usr/local/bin/keep-xcompmgr-running

The script is a simple infinite loop. But it doesn't
just start new instances of xcompmgr over and
over again. The xcompmgr program does not return
control to the script unless it fails, so this script will get to the
point where it launches the xcompmgr command and
its arguments, and then stop running. If
xcompmgr encounters a bug that causes it to exit
unexpectedly, control returns automatically to the script, and the
loop continues by starting xcompmgr all over
again. Then the script stops running until
xcompmgr fails again. (See the sidebar, How to Ignore grep.)

9.5.1. Putting the Respawn Trick to Work

One great place to use this
trick is in a
.xinitrc startup script (or any other method of
starting up an application automatically when you run a window
manager or graphical desktop). The following
.xinitrc script will work, but if
xcompmgr crashes, it will stay crashed:

xcompmgr -cCfF -l 0 -t 0 -r 5 -o .6 &
exec /usr/bin/fluxbox

If you instead start up the script that keeps
xcompmgr running, it will restart every time it
crashes:

/usr/local/bin/keep-xcompmgr-running &
exec /usr/bin/fluxbox

Linux.Desktop.Hacks [Electronic resources] نسخه متنی

فارسی

کردی

العربیه

اردو

Türkçe

Русский

English

Français

کانال فیلم من

تبیان من

فایلهای من

کتابخانه من

پنل پیامکی

وبلاگ من

اینجــــا یک کتابخانه دیجیتالی است

با بیش از 100000 منبع الکترونیکی رایگان به زبان فارسی ، عربی و انگلیسی