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expr-表达式计算.md

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The expr command

The expr command calculates the given expression and displays its corresponding output. It is used for basic operations such as addition, subtraction, multiplication, division, and modulus on integers, evaluating regular expressions, and string operations such as substring and string length.

Syntax

expr [EXPRESSION | OPTION]

Parameters

  • --help: Output help information.
  • --version: Output version information.

Expressions

  • ARG1 | ARG2: ARG1 if it is neither null nor 0, otherwise ARG2.
  • ARG1 & ARG2: ARG1 if both arguments are neither null nor 0, otherwise 0.
  • ARG1 < ARG2: ARG1 is less than ARG2.
  • ARG1 <= ARG2: ARG1 is less than or equal to ARG2.
  • ARG1 = ARG2: ARG1 is equal to ARG2.
  • ARG1 != ARG2: ARG1 is not equal to ARG2.
  • ARG1 >= ARG2: ARG1 is greater than or equal to ARG2.
  • ARG1 > ARG2: ARG1 is greater than ARG2.
  • ARG1 + ARG2: Arithmetic sum of ARG1 and ARG2.
  • ARG1 - ARG2: Arithmetic difference of ARG1 and ARG2.
  • ARG1 * ARG2: Arithmetic product of ARG1 and ARG2.
  • ARG1 / ARG2: Arithmetic quotient of ARG1 divided by ARG2.
  • ARG1 % ARG2: Arithmetic remainder of ARG1 divided by ARG2.
  • STRING : REGEXP: Pattern match of the regular expression REGEXP in STRING.
  • match STRING REGEXP: Same as STRING matching REGEXP.
  • substr STRING POS LENGTH: Substring of STR starting at POS, which starts counting from 1.
  • index STRING CHARS: Index of any of CHARS in STRING, or 0 if not found.
  • length STRING: Length of the string.
  • + TOKEN: Treat TOKEN as a string, even if it is a keyword like match or an operator like /.
  • (EXPRESSION): Value of EXPRESSION.

Examples

Calculate 12 + 9.

expr 12 + 9
# 21

Calculate 12 * 2.

expr 12 \* 2
# 24

Perform operations on variables in a shell script. Remember to save it as a .sh file, grant it 755 permission, and then execute.

echo "Enter two numbers"
read x 
read y
sum=`expr $x + $y`
echo "Sum = $sum"

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://www.computerhope.com/unix/uexpr.htm
https://www.runoob.com/linux/linux-comm-expr.html
https://www.geeksforgeeks.org/expr-command-in-linux-with-examples/