4 增加和检查整数值

脚本包含了一个计数集，用户将其赋予一个新值就可改变它，脚本然后将当前值100加入一个新值。 用户输入一个新值改变其值，如果输入回车键，则不改变它，打印当前值，脚本退出。如果用户用y或Y响应新值，将提示用户输入变量。如果键入回车键，原址人未变。加入一个增量，首先测试是否为数字，如果是，加入计数COUNTOR中，然后显示新值。

#!/bin/bash
# ifcounter.sh
COUNTER=100
echo "Do you wish to change the counter value crretnly set at $COUNTER ?[y..n]"
read ANS
if [ "$ANS" = "y" ] || [ "$ANS" = "Y" ]
#yes user wants to change the value
then
    echo "enter a sensible value"
    read VALUE
    # simple test to see if it's numeric,add any number to VALU
    # then check out return
    # code
    expr $VALUE + 10 > /dev/null 2>&1
    STATUS=$?
    # check return code of the expr
    if [ "$STATUS" = "" ] || [ "$STATUS" != "0" ]
    # send errors to standard error
    then
        echo "you either entered nothing or a non-numeric" >&2
        echo "sorry now exiting..counter stays at $COUNTER" >&2
        exit 1
    fi
    # if we are here,then it's a numer,so add it to COUNTER
    COUNTER=$(expr $COUNTER + $VALUE)
    echo "Counter stays at $COUNTER"
else
    # if we are here then user just hit return instedad of entering a number
    # or answered n to the change a value prompt
    echo "Counter stays at $COUNTER"
fi

ywx@ywx:~/Desktop/linux_shell$ ./ifcounter.sh
Do you wish to change the counter value crretnly set at 100 ?[y..n]
y
enter a sensible value
569
Counter stays at 669