If the shell determines that a variable is empty

1. The most straightforward and straightforward judgment

Without those if statements, directly shorten the amount of code.

2. The variable is caused by "" quotation marks

As shown below:, you can get the result is NULL.

#!/bin/sh  a=  if [!-n "$a"]; then  echo ' is null '  else  echo ' NOT NULL '  

　

3. Judging directly by variables

The results are as follows: Is NULL, the same as the first method, but the code is a little longer, it is recommended to use the second way of judging, simple and clear.

#!/bin/sh  a=  if [! $a]; then echo ' is null '  else  echo ' NOT NULL '  

4. Use test to determine

The results are: A is not set!

#!/bin/sh  a=  if test-z "$a" then  echo "A was not set!"  else  echo "A is set!"  

5. Use "" to determine

#!/bin/sh  a=  If ["$a" = ""]; Then  echo "A was not set!"   else  echo "A is set!"  

　

This situation is easy to make the foot of the newspaper wrong

Transferred from: http://luxiaok.blog.51cto.com/2177896/925548

　

　　

　　

　

If the shell determines that a variable is empty