Shell Custom Variables: Definition, Format, Types, and Usage

Variable definition: a variable is a storage space that can hold a mutable value, which can be changed in different environments. By default, each shell in Linux can be seen as a separate execution environment, so the same variable name may have different values in different shells.

1.1 Variable format

VariableName = VariableValue

Variable names cannot start with a digit or special character.

"=" is used for assignment.

Variable values can be numbers, strings, file paths, commands, or command results.

1.2 Classification of shell variables

Custom variables, environment variables, positional variables, predefined variables.

1.3 Variable output

a=1

echo $a

echo outputs the variable; note the $ prefix before the variable name.

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2. Custom variables

Example 1:

[root@node1 ~]# Linux=7.2

[root@node1 ~]# echo $Linux

7.2

[root@node1 ~]# linux=6.5

[root@node1 ~]# echo $linux

6.5

[root@node1 ~]# echo $Linux

7.2

Conclusion: there must be a space between echo and the variable, and variable values are case‑sensitive.

Example 2: invoking multiple variables

[root@node1 ~]# system=centos

[root@node1 ~]# version=7.2

[root@node1 ~]# echo $system $version

centos 7.2

[root@node1 ~]# echo sys{$Linux}

sys{7.2}

[root@node1 ~]# echo sys$Linux

sys7.2

Conclusion: you can directly echo the variable name.