Multiple methods of shell dynamically generating arrays _linux shell

If you're not familiar with the Linux shell array, see an article: The Linux shell array build and use techniques, this article is mainly about dynamically generating array series. There should be a lot of methods here, I mainly take a sum calculation of the problem as an example to analyze.

Topic: Please write a script with Linux shell, implement from 1. 1000 and values of all even numbers.

Method One:

The desired result is obtained through a while loop:

Start=1;

total=0;

While [$start-le 1000];d o

  [[$ ($start%2)] = = 0]]&&total=$ (($total + $start));

  start=$ (($start + 1));

Done;

echo $total;

 

[Chengmo@centos5 ~]$ start=1;total=0;while [$start-le 1000];d o  [[$ ($start%2)] = = 0]]&&total=$ ($total +$ Start));  start=$ (($start + 1));d One;echo $total;
250500

The above results are: 249500, in the Linux shell, ";" As a command line separator. If you are not very understanding of the $ (()) operation symbol, you can view: Linux shell Implementation arithmetic (integer and floating-point) simple method, if for: [[]] [] symbol, you can refer to another article Linux shell logical operators, logical expression of the detailed explanation.

Method Two:

Get results with a For loop:

start=0;

total=0;

For I in $ (Seq $start 2 1000); Do

  total=$ (($total + $i));

Done;

echo $total;

[Chengmo@centos5 ~]$ start=0;total=0;for i in $ (Seq $start 2 1000); Do  total=$ (($total + $i));d One;echo $total;    
250500

The above statement is obviously better than method one in terms of code, and performance is also good. Here's a comparison to find out:

Compare performance:

[Chengmo@centos5 ~]$ Time (start=0;total=0;for i $ (seq $start 2 1000), do  total=$ ($total + $i));d One;echo $total;)       250500

Real  0m0.016s
user  0m0.012s
sys   0m0.003s
[chengmo@centos5 ~]$ Time (Start=1;total=0;while [$start-le 1000];d o  [[$ ($start%2)] = = 0]]&&total=$ (($total + $start));  start=$ (($start + 1));d one;echo $total;) 
250500

real  0m0.073s
user  0m0.069s
sys   0m0.004s

Method one time is 6 times times of method two!

seq Use:

seq [OPTION] ... Last
seq [OPTION] ... The last
seq [OPTION] ... First INCREMENT

[chengmo@centos5 ~]$ seq 1000  ' starts by default is 1, interval default is also 1

[chengmo@centos5 ~] $seq 2 1000 ' interval default is 1< C6/>[CHENGMO@CENTOS5 ~] $seq 1 3  ' starting from 1, to 10 intervals of 3 The result is: 1 4 7

Note: The default interval is "space" if you want to change to another can with parameters:-S

[ CHENGMO@CENTOS5 ~] $seq-S ' # ' 1 3

1#4#7#10

Application tips:

To generate a continuous array series:

[Chengmo@centos5 ~]$ a= ($ (SEQ 1 3))  
[Chengmo@centos5 ~]$ echo ${a[1]}
4
[chengmo@centos5 ~]$ Echo ${a[@ ]}
1 4 7 10

Generate consecutive same characters:

[Chengmo@centos5 ~]$ seq-s ' # ' 30 | Sed-e ' s/[0-9]*//g '
#############################

The above example: by adding the interval character ' # ', replace the number, generate a continuous same character ' # ', this in the future writing will still have a lot of help.