shell脚本里怎样判断某个字符串是否为空

1、例1
#!/bin/sh
cid=$1
if [[ $1 == '' ]];then
echo "Usage: $0 cid "
exit 0;
fi
echo $id
2、例2
#!/bin/sh
cid=$1
if [[ -z $1 ]];then
echo "Usage: $0 cid "
exit 0;
fi
echo $id本回答被网友采纳