Functionality of a start-stop-restart shell script - linux

Question

I am a shell scripting newbie trying to understand some code, but there are some lines that are too complexe for me. The piece of code I'm talking about can be found here: https://gist.github.com/447191
It's purpose is to start, stop and restart a server. That's pretty standard stuff, so it's worth taking some time to understand it. I commented those lines where I am unsure about the meaning or that I completely don't understand, hoping that somone could give me some explanation.
#!/bin/bash
#
BASE=/tmp
PID=$BASE/app.pid
LOG=$BASE/app.log
ERROR=$BASE/app-error.log
PORT=11211
LISTEN_IP='0.0.0.0'
MEM_SIZE=4
CMD='memcached'
# Does this mean, that the COMMAND variable can adopt different values, depending on
# what is entered as parameter? "memcached" is chosen by default, port, ip address and
# memory size are options, but what is -v?
COMMAND="$CMD -p $PORT -l $LISTEN_IP -m $MEM_SIZE -v"
USR=user
status() {
echo
echo "==== Status"
if [ -f $PID ]
then
echo
echo "Pid file: $( cat $PID ) [$PID]"
echo
# ps -ef: Display uid, pid, parent pid, recent CPU usage, process start time,
# controling tty, elapsed CPU usage, and the associated command of all other processes
# that are owned by other users.
# The rest of this line I don't understand, especially grep -v grep
ps -ef | grep -v grep | grep $( cat $PID )
else
echo
echo "No Pid file"
fi
}
start() {
if [ -f $PID ]
then
echo
echo "Already started. PID: [$( cat $PID )]"
else
echo "==== Start"
# Lock file that indicates that no 2nd instance should be started
touch $PID
# COMMAND is called as background process and ignores SIGHUP signal, writes it's
# output to the LOG file.
if nohup $COMMAND >>$LOG 2>&1 &
# The pid of the last background is saved in the PID file
then echo $! >$PID
echo "Done."
echo "$(date '+%Y-%m-%d %X'): START" >>$LOG
else echo "Error... "
/bin/rm $PID
fi
fi
}
# I don't understand this function :-(
kill_cmd() {
SIGNAL=""; MSG="Killing "
while true
do
LIST=`ps -ef | grep -v grep | grep $CMD | grep -w $USR | awk '{print $2}'`
if [ "$LIST" ]
then
echo; echo "$MSG $LIST" ; echo
echo $LIST | xargs kill $SIGNAL
# Why this sleep command?
sleep 2
SIGNAL="-9" ; MSG="Killing $SIGNAL"
if [ -f $PID ]
then
/bin/rm $PID
fi
else
echo; echo "All killed..." ; echo
break
fi
done
}
stop() {
echo "==== Stop"
if [ -f $PID ]
then
if kill $( cat $PID )
then echo "Done."
echo "$(date '+%Y-%m-%d %X'): STOP" >>$LOG
fi
/bin/rm $PID
kill_cmd
else
echo "No pid file. Already stopped?"
fi
}
case "$1" in
'start')
start
;;
'stop')
stop
;;
'restart')
stop ; echo "Sleeping..."; sleep 1 ;
start
;;
'status')
status
;;
*)
echo
echo "Usage: $0 { start | stop | restart | status }"
echo
exit 1
;;
esac
exit 0

Answer 1

1)
COMMAND="$CMD -p $PORT -l $LISTEN_IP -m $MEM_SIZE -v" — -v in Unix tradition very often is a shortcut for --verbose. All those dollar signs are variable expansion (their text values are inserted into the string assigned to new variable COMMAND).
2)
ps -ef | grep -v grep | grep $( cat $PID ) - it's a pipe: ps redirects its output to grep which outputs to another grep and the end result is printed to the standard output.
grep -v grep means "take all lines that do not contain 'grep'" (grep itself is a process, so you need to exclude it from output of ps). $( $command ) is a way to run command and insert its standard output into this place of script (in this case: cat $PID will show contents of file with name $PID).
3) kill_cmd.
This function is an endless loop trying to kill the LIST of 'memcached' processes' PIDs. First, it tries to send TERM signal (politely asking each process in $LIST to quit, saving its work and shutting down correctly), gives them 2 seconds (sleep 2) to do their shutdown job and then tries to make sure that all processes are killed using signal KILL (-9), which slays the process immediately using OS facilities: if a process has not done its shutdown work in 2 seconds, it's considered hung). If slaying with kill -9 was successful, it removes the PID file and quits the loop.
ps -ef | grep -v grep | grep $CMD | grep -w $USR | awk '{print $2}' prints all PIDs of processes with name $CMD ('memcached') and user $USR ('user'). -w option of grep means 'the Whole word only' (this excludes situations where the sought name is a part of another process name, like 'fakememcached'). awk is a little interpreter most often used to take a word number N from every line of input (you can consider it a selector for a column of a text table). In this case, it prints every second word in ps output lines, that means every PID.
If you have any other questions, I'll add answers below.

Answer 2

Here is an explanation of the pieces of code you do not understand:
1.
# Does this mean, that the COMMAND variable can adopt different values, depending on
# what is entered as parameter? "memcached" is chosen by default, port, ip address and
# memory size are options, but what is -v?
COMMAND="$CMD -p $PORT -l $LISTEN_IP -m $MEM_SIZE -v"
In the man, near -v:
$ man memcached
...
-v Be verbose during the event loop; print out errors and warnings.
...
2.
# ps -ef: Display uid, pid, parent pid, recent CPU usage, process start time,
# controling tty, elapsed CPU usage, and the associated command of all other processes
# that are owned by other users.
# The rest of this line I don't understand, especially grep -v grep
ps -ef | grep -v grep | grep $( cat $PID )
Print all processes details (ps -ef), exclude the line with grep (grep -v grep) (since you are running grep it will display itself in the process list) and filter by the text found in the file named $PID (/tmp/app.pid) (grep $( cat $PID )).
3.
# I don't understand this function :-(
kill_cmd() {
SIGNAL=""; MSG="Killing "
while true
do
## create a list with all the pid numbers filtered by command (memcached) and user ($USR)
LIST=`ps -ef | grep -v grep | grep $CMD | grep -w $USR | awk '{print $2}'`
## if $LIST is not empty... proceed
if [ "$LIST" ]
then
echo; echo "$MSG $LIST" ; echo
## kill all the processes in the $LIST (xargs will get the list from the pipe and put it at the end of the kill command; something like this < kill $SIGNAL $LIST > )
echo $LIST | xargs kill $SIGNAL
# Why this sleep command?
## some processes might take one or two seconds to perish
sleep 2
SIGNAL="-9" ; MSG="Killing $SIGNAL"
## if the file $PID still exists, delete it
if [ -f $PID ]
then
/bin/rm $PID
fi
## if list is empty
else
echo; echo "All killed..." ; echo
## get out of the while loop
break
fi
done
}
This function will kill all the processes related to memcached slowly and painfully (actually quite the opposite).
Above are the explanations.