Content one: set default values for variables on command Line
1.1 Assign Values to Ordinary Variables
# a=1
# b=（Add: let a be 1 and B be null as an example）
1.2 Set default values for variables on command Line
# var1=${a:-no}
# var2=${b:-no}（
Add：
If a is null, var1 is no, otherwise var1 is equal to a
If B is null, var2 is no, otherwise var2 is equal to B
）
1.3 View variable results
# echo $var1
1
# echo $var2
no（
Add：
The value of a is 1, so var1 is 1
The B is null, so var2 is no
）
Content two：set variable defaults in script
2.1 Setting variable defaults in scripts
# vim var_default.sh
Create the following：
#!/bin/bash
var_default="${1:-no}"
echo $var_default（Add：If the variable of $1 is null, then var_default is no, otherwise var_default is equal to $1, and print it out as an example）
2.2 Test variable defaults in scripts
# bash var_default.sh
no
# bash var_default.sh 1
1
# bash var_default.sh 2
2
# bash var_default.sh 3
3