Summary of shell's method for determining whether a variable is empty

How to determine whether a variable is empty in shell

In shell programming, the error checking items for parameters include whether the variable is assigned a value (that is, whether a variable is empty). The method to determine whether a variable is empty 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 result] "IS NULL"

2. Judge directly by variables

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

[Output result] "IS NULL"

3. Use test to judge

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

【Output result】"dmin is not set!"

4. Use "" to judge

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

【Output result】"dmin is not set!"