linux – start-stop-restart shell脚本的功能

我是一个 shell脚本新手试图理解一些代码,但有一些线对我来说太复杂了.我正在谈论的那段代码可以在这里找到： https://gist.github.com/447191
它的目的是启动,停止和重新启动服务器.这是非常标准的东西,所以值得花一些时间来理解它.我评论了那些我不确定其含义或我完全不理解的那些内容,希望somone可以给我一些解释.

#!/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