Set default values for variables in the shell

• b variable is nullthe time,var=a

var=${b-a}

• When the b variable is NULLan empty string,var=a

var=${b:-a}

Note that when the variable is ${1-9}time, there is no second case

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Special meaning

• $$ current shell process id
• $0 Current script file name
• $# The number of parameters passed to the function or script
• $\ast or$ @ All parameters passed to the function or script, "KaTeX parse error: Undefined control sequence: \* at position 1: \̲*̲" returns a string, "@" returns a list
• $? The exit status of the last command or the return value of the function

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Reference link: http://c.biancheng.net/cpp/view/2739.html