17.1. Analyzing a System Script
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 17-12. killall, from /etc/rc.d/init.d
# --> Comments added by the author of this document marked by "# -->".
# --> This is part of the 'rc' script package
# --> by Miquel van Smoorenburg, <email@example.com>.
# --> 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.
# --> 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
# --> Suspend running jobs and daemons.
# --> Note that "stop" is a positional parameter,
# -->+ not a shell builtin.
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 commands to echos.
Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. Try to understand at least portions 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.