About variable settings in the shell

Defining variables

A=

Assigning values to variables

A=5

' Number equals $ ()

[Email protected] ~]# hostname

Localhost.localdomain

[Email protected] ~]# a= ' hostname '

[Email protected] ~]# echo $a

Localhost.localdomain

${-} to determine whether the variable is set, if not set, use-the next assignment value is null value

[Email protected] ~]# c=

[[email protected] ~]# Ab=${path-/usr/sbin}

[Email protected] ~]# echo $ab

/usr/sbin

[Email protected] ~]# echo $path

[Email protected] ~]#

If path has no value, it is assigned to AB using the/usr/sbin value.

If path has a value, assign the value of path to AB

${:-} Determines whether the variable is not set, or is set to a null value, and if it is not set or is empty, the following content is used to assign the value

So generally in the judgment we can completely abandon ${-}, directly using ${:-string} to achieve

About variable settings in the shell