shell 变量的双重扩展

Question

我有一个 table，其中包含一个环境变量，我需要获取该变量的值并从 shell 脚本中导出另一个 java 实用程序：

    command="SELECT param_value FROM tableX WHERE param_name='ABCD';"
    #This param_value is ${PATHX} and PATHX is /home/users/pathx

    PARAM_VALUE=`sqlplus -s $CONN_STRING <<- END
    SET head off;
    set feedback off;
    ${command}
    exit;
    END`

    echo ${PARAM_VALUE} | grep -q "ERROR"
    if [ $? -eq 0 ]
    then
            echo "Failed in fetching param_value "
            exit 1
    else
            #Trimming the value fetched from DB
            PARAM_VALUE=`echo "${PARAM_VALUE}" | tr -d " " | tr -d "\n"`
            echo "Value fetched from DB=${PARAM_VALUE}" 
            #This prints ${PATHX}
            export PATH_VALUE="${PARAM_VALUE}" 
            #This is exporting PATH_VALUE as ${PATHX} instead of /home/users/pathx - WHICH IS WHERE I NEED HELP
            #If I put directly export PATH_VALUE="${PATHX}", it exports the value correctly as /home/users/pathx
    fi

搜索选项后，我尝试了如下各种选项但都失败了：

export PATH_VALUE="${PARAM_VALUE}"
export PATH_VALUE=`eval echo "$${PARAM_VALUE}"`
export PATH_VALUE=$(eval $$PARAM_VALUE)
export PATH_VALUE=${$PARAM_VALUE}
export PATH_VALUE=${!PARAM_VALUE}
export PATH_VALUE=`echo ${PARAM_VALUE}`
export PATH_VALUE=`expr ${PARAM_VALUE}`

请建议在这种情况下如何导出实际扩展值 - /home/users/pathx。

Answer 1

为了按您期望的方式工作，sqlplus 查询响应应该是 PATHX 而不是 ${PATHX}。修复它的一种方法是替换

PARAM_VALUE=`echo "${PARAM_VALUE}" | tr -d " " | tr -d "\n"`

和

PARAM_VALUE=`echo "${PARAM_VALUE}" | tr -dc '[:alnum:]\n\r'`

Answer 2

yes the query response is ${PATHX} and this is an environment variable which I need to expand.

你可以使用eval，但是eval是邪恶的，如果值被导出，做一个安全的envsubst:

PARAM_VALUE=$(envsubst <<<"$PARAM_VALUE")

shell 变量的双重扩展

Double expansion of shell variable

shell

variables

unix

expansion