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expr:tldr:0561c
The command "expr ${expression1} ${select} ${expression2}" is used to evaluate and calculate arithmetic expressions in the shell scripting language.
Here's how it works:
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${expression1} and ${expression2} are placeholders for the numeric expressions that you want to evaluate. These expressions can be variables, constants, or a combination of both.
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${select} is a placeholder for the operator that you want to use to perform the calculation. It can be +, -, *, /, or %. These operators represent addition, subtraction, multiplication, division, and modulo respectively.
When you run the command, the shell evaluates the values of ${expression1} and ${expression2} and performs the arithmetic operation specified by ${select}. It then returns the result of the calculation.
For example, if you run the command "expr 5 + 3", the shell will replace ${expression1} with 5, ${select} with +, and ${expression2} with 3. The expression becomes "expr 5 + 3", and the shell calculates the sum of 5 and 3, which is 8. The result is then returned by the command.