The bash variable for Linux

Advanced parts of Bash variables

1. Variable expansion

${variable name} variable extension for example: ${filename}

$ (command) command replacement for example: $ (LS/)

$ (arithmetic) calculation extension for example: $ ((9+9))

Variable extension: A simple conditional judgment, the substitution value that assigns different variables to different conditions, is an implicit "if-then-else" program syntax

Basic syntax for judging whether a variable exists or not: ${the variable to be tested-default value}

Formula: There is a transmission exists, no, the default value is passed

Presence or absence or null: ${variable to be tested-: Default value}

If there is a pass, no or empty, the default value is passed in

Presence or absence or null: ${variables to be tested: = default}

Formulas: Set default values for variables that do not exist or are empty

Presence or absence or NULL, output hint: ${variables to be tested:? Prompt message}

Purpose: In the script, in order to detect the variable is empty or not present, output prompt information to prevent serious problems in the script

Testing the existence of variables: ${variables to be measured: + truth}

Purpose: To test the existence, existence and value of a variable, which is true

To organize your pictures, please see attachments

When a variable is a string, the use of the slice substring:

There are roughly two kinds: 1.${variable: Position start} 2.${variable: Position start: Length}

Take the length of the passed-in string: ${#变量名称} The extension of this method applies: ${#数组 [@]} or ${#数组 [*]} are the number of array elements taken

Style comparison application: ${variable # style} Starting from the far left, contrasting variable values, deleting "shortest match string"

${variable # #样式} ... "Best-fit string"

${variable% style} Starting from the far right, contrasting variable values, deleting "shortest match string"

${variable-percent-style} .... "Best-fit string"

Replace or delete a partial string

${variable/style/substitution string}: Replaces only the first matched string

${variable//style/replacement string}: Replace all matching strings

${variable/style}: Delete the first matching string

${variable//style}: Delete all matching strings

The bash variable for Linux