Test and content replacement of shell variables, replacement of shell Variables

Test and content replacement of shell variables, replacement of shell Variables

Reference: laruence's private house dish-Linux BASICS (Third edition).

In Chapter 11th of the Linux BASICS (Third edition, the final example illustrates '-', '=', and ','? 'Usage of three special symbols.

After reading these routines, I feel like var =$ {str-newvar}, var =$ {str = newvar}, or var =$ {str? Is the replacement of the newvar} variable the only three-object operator in the C language? The usage is similar:

Max = (x> y )? (X: y ). The analysis is as follows:

Var =$ {str-newvar}: If str does not exist, var = newvar; // select the latter

If str exists, var =$ {str}; // select the former

Similarly, "=" can be explained as follows:

Var =$ {str = newvar}: If str does not exist, var = newvar; // select the latter, but str = newvar because it is an equal sign;

If str exists, var =$ {str}; // select the former str to keep the original content.

For "?" Is interpreted as"

Var =$ {str? None of the variables}: If str does not exist, an error is returned: none of the variables; // select the latter

If str exists, var =$ {str}; // select the former str to keep the original content.