Advanced Bash-scripting Guide (19th): maximum common divisor of two integers, bashscripting

Advanced Bash-scripting Guide (19th): maximum common divisor of two integers, bashscripting

The example selected in this article is from the book "Advanced Bash-scripting Gudie", translated by Yang chunmin Huang Yi

1 #! /Bin/bash 2 # calculate the maximum common divisor of two integers 3 4 E_BADARGS = 65 5 6 # if the number of parameters is not 2, exit 7 if [$ #-ne 2] 8 then 9 echo "Usage: 'basename $ 0' first-number second-number "10 exit $ E_BADARGS11 fi12 13 # If the parameter is not an integer or the parameter value is 0, exit 14 for I in $ @ 15 do16 if [$ I = ~ [0-9] +] # "= ~ "Followed by the regular expression, + in the regular expression, it indicates that the previous content matches at least once 17 then18 if [$ I-eq 0] 19 then20 echo" Usage: 'basename $ 0' parameter can't be zero "21 exit $ E_BADARGS22 fi23 else24 echo" Usage: 'basename $ 0' parameter must be integer "25 exit $ E_BADARGS26 fi27 done28 29 # design a gcd () function and calculate the maximum approx. 30 gcd () using the moving phase division (Euclidean Algorithm () 31 {32 remainder = 133 dividend = $134 divisor = $235 36 until [$ remainder-eq 0] 37 do38 let "remainder = $ dividend % $ divisor" 39 dividend = $ divisor40 divisor = $ remainder41 done42} 43 44 gcd $1 $245 46 echo "gcd of $1 and $2 is: $ devidend "47 48 exit 0

When adapting this script, I mainly consider the following:

1. Do you want to exclude non-Integer Parameters?

For the first time, I use echo $ I | sed '/s/^ [0-9] * $/''/G' & echo $? If the first command is correctly executed, $? 0 should be returned, but we have a better method, that is, "= ~ "Followed by regular expressions

2. If the parameter value is 0, do you want to exclude it?

When $ I is judged as an integer, another nested judgment [$ I-eq 0]

3. How to control the number of parameters?

[$ #-Eq 2] or [$ #-ne 2] can exclude null parameters or the number of parameters is not 2.

4. What is the processing of $1 <$2 in Euclidean algorithms?

First look at the situation of $1> $2

$1 = 65 $2 = 15

First loop: 5 = 65% 15

Dividend = 15

Divisor = 5

The second cycle 0 = 15% 5

Dividend = 5

Divisor = 0

Exit the loop, gcd = $ dividend = 5

Let's look at the situation of $1 <$2.

$1 = 15 $2 = 65

First cycle: 15 = 15% 65

Dividend = 65

Divisor = 15

Second cycle: 5 = 65% 15

Dividend = 15

Divisor = 5

Third Cycle: 0 = 15% 5

Dividend = 5

Divisor = 0

Exit the loop, gcd = $ dividend = 5

We can see that $1 <$2 has a loop more than $1> $2, and the result is the same.