shell determines whether a variable is null

How to determine whether a variable is empty in shell

In shell programming, the error check item for parameters contains whether to assign a variable (that is, whether a variable is null). The method to determine whether a variable is null is as follows:

1. Variables are enclosed in "" quotes

#!/bin/sh
para1=
if [ ! -n "$para1" ]; then
  echo "IS NULL"
else
  echo "NOT NULL"
fi

[Output] "IS NULL"

2. Judge directly by variables

#!/bin/sh
para1=
if [ ! $para1 ]; then
  echo "IS NULL"
else
  echo "NOT NULL"
fi

[Output] "IS NULL"

3. Use test

#!/bin/sh
dmin=
if test -z "$dmin"
then
  echo "dmin is not set!"
else  
  echo "dmin is set !"
fi

[Output] "dmin is not set!

4. Use "" judgment

#!/bin/sh 
dmin=
if [ "$dmin" = "" ]
then
  echo "dmin is not set!"
else  
  echo "dmin is set !"
fi

[Output] "dmin is not set!"