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This article mainly introduces how to achieve variable replacement technology under Linux, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let Xiaobian take you to understand.

1.1.1 ${value:-word} returns word if the variable value is not defined, or the value of the value variable if it has been assigned

Result=$ {jimyy:-UNSET}

Echo $result

Because jimyy is not defined, UNSET is returned

This variable function determines that a variable always has a return value when it is not defined.

1.1.2 ${value:=word} when value is not defined, word is assigned to value, and then the vaule value is printed with the ${value:=word} value.

Result=$ {jimyy:=UNSET}

Echo $result

Echo $jimyy

As a result, the values of both variables are UNSET

The function of this variable is to assign a value to the variable when it is not defined and return.

1.1.3 ${value:? "not defined"} when value is not defined, return bash: vaule: not defined

Used to catch errors caused by undefined variables

1.1.4 ${value:+word} returns word when the variable exists and is not null, otherwise null is used to test whether the variable exists

1.1.5 ${value-word} if value does not exist, replace it with word.

The variable substitution function has significant functions in some scenarios.

When operating on a variable, it is best to determine whether the variable is not empty, for example, to perform rm-rf operation on a variable of directory type, which must be judged.

Because if the variable is empty or no value is assigned, it may be deleted from the root directory or / root directory, and the impact is particularly great, so you should be cautious, so you can use the variable substitution feature in this scenario to prevent accidental deletion.

For example:

Path2=/opt/server/backup/

Find ${path2:=/tmp/}-name "* .tar.gz"-type f | xargs rm-f

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