A statement like this is used when matching strings
if[ $ timeofday = ” yes”]; then
echo ” Good morning”
exit 0
The reason for the error is: if the value of the variable timeofday is empty, the if statement becomes if [ =” yes” ], This is not a legal condition. To avoid this, we must quote the variable if [ “$ timeofdat”=”yes” ]， In this way, even null variables provide legal test conditions, if [ ” “=” yes” ]
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