Common script programming knowledge points

1, the length of the characters in the variable: ${#VARNAME}

#A = ' Hello World '

#echo ${#A}--Displays the length of the defined variable A, with an output of 11

#11

2, variable assignment value, etc.:

${parameter:-word}: If parameter is empty or undefined, the variable expands to "word"; otherwise, expands to the parameter value;

${parameter:+word}: If parameter is empty or undefined, do nothing, otherwise expand to Word value;

${parameter:=word}: If parameter is empty or undefined, the variable expands to "word" and assigns the expanded value to parameter;

${parameter:offset}: Remove the defined offset number

${parameter:offset:length}: Takes a substring, starts at the last character at the offset, and takes the lenth long substring

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The most commonly used is the first, such as:

${parameter:-word}

#A =3

#echo ${a:-30}---indicates that the variable A is not empty, the expression uses the value of the variable itself

#3

#unset A

#echo ${a:-30}---indicates that the variable A is empty, the expression uses its own value

#30

#A =${a:-30}---indicates that variable A is empty, it is assigned to the variable a with its own value

#echo $A---to indicate that the variable A is not empty, it is assigned to the variable a again using the value of variable a

#30

${parameter:+word}

#A =3

#echo ${a:+30}---indicates that the value of Word is used when variable A is not empty

#3

#unset A

#echo ${a:+30}---indicates that the variable A is empty and the value is null

#为空

${parameter:=word}

#unset A

#echo ${a:=30}--Indicates that the variable a is empty, not only expands the expression to this string, but assigns the string value to a

#30

${parameter:offset}

#A = ' Hello World '---define a variable

#echo ${a:2}---Number 2 indicates that the string in variable A is offset 2 from left to right, showing everything from the offset

#llo World

${parameter:offset:length}

#echo ${a:2:3}---Number 2 indicates that the string in variable A is offset from left to right 2, and the number 3 indicates that 3 characters are taken from the offset position

#llo

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