shell单引号与变量、双引号与变量、如何在多重引号里面取到shell变量的值？

如何在多重引号里面取到shell变量的值？

　　 双引号是不会屏蔽对变量和某些特殊符号的转义的，而单引号里的所有内容都会原封不动的输出，而单引号里再用单引号将变量引起来，变量就又可以正常的显示，有点像数学里的负负为正。

演示如何取到变量BUILD_NUMBER的值：

num=
while true
do
    result_code=`curl -d '{"appName":"offline-index-web","branch":"security-test1-1-20190128","id":"ab432040-b5ca-48ff-946f-ec984748dcae","build_number":'$BUILD_NUMBER'}' http://10.5.25.4:8088/api/v1/job_finish -H "Content-type: application/json" -H "Accept: application/json" -m 10 -w %{http_code} || echo "failed"`
    if [[ $result_code == \"ok\"200 ]] ; then
        echo "callback successed!"
        break
    else
        echo "callback failed, retry after 10s!"
        sleep
    fi
    num=`expr $num + `
    if [[ $num ==  ]]; then
        echo "failed after 5 times"
        exit -
        break
    fi
done