Using our knowledge of administrative commands, let us examine a system
script. One of the shortest and simplest to understand scripts is
killall, used to suspend running processes at system shutdown.
Example 13-11. killall, from /etc/rc.d/init.d
#!/bin/sh
# --> Comments added by the author of this document marked by "# -->".
# --> This is part of the 'rc' script package
# --> by Miquel van Smoorenburg, <[email protected]>.
# --> This particular script seems to be Red Hat / FC specific
# --> (may not be present in other distributions).
# Bring down all unneeded services that are still running
#+ (there shouldn't be any, so this is just a sanity check)
for i in /var/lock/subsys/*; do
# --> Standard for/in loop, but since "do" is on same line,
# --> it is necessary to add ";".
# Check if the script is there.
[ ! -f $i ] && continue
# --> This is a clever use of an "and list", equivalent to:
# --> if [ ! -f "$i" ]; then continue
# Get the subsystem name.
subsys=${i#/var/lock/subsys/}
# --> Match variable name, which, in this case, is the file name.
# --> This is the exact equivalent of subsys=`basename $i`.
# --> It gets it from the lock file name
# -->+ (if there is a lock file,
# -->+ that's proof the process has been running).
# --> See the "lockfile" entry, above.
# Bring the subsystem down.
if [ -f /etc/rc.d/init.d/$subsys.init ]; then
/etc/rc.d/init.d/$subsys.init stop
else
/etc/rc.d/init.d/$subsys stop
# --> Suspend running jobs and daemons.
# --> Note that "stop" is a positional parameter,
# -->+ not a shell builtin.
fi
done
That wasn't so bad. Aside from a little fancy footwork with variable
matching, there is no new material there.
Exercise 1. In /etc/rc.d/init.d,
analyze the halt script. It is a bit longer
than killall, but similar in concept. Make
a copy of this script somewhere in your home directory and
experiment with it (do not run it as
root). Do a simulated run with the -vn flags
(sh -vn scriptname). Add extensive
comments. Change the "action" commands to
"echos".
Exercise 2. Look at some of the more complex scripts in
/etc/rc.d/init.d. See if
you can understand parts of them. Follow the above procedure
to analyze them. For some additional insight, you might also
examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??,
which is part of the "initscripts"
documentation.
