Linux shell dynamically generate array series SEQ usage tips __linux



If you are not familiar with the Linux shell array, see an article: Linux Shell array Building and use techniques , this article is mainly about the dynamic generation of 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 + $st   ART)); start=$ (($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); Todo

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 [$start-le 1000];d o [[$ ($start%2)] = = 0]]&&total=$ (($tot   al+ $start)); start=$ (($start + 1));d One;echo $total;)
250500

Real 0m0.073s
User 0m0.069s
SYS 0m0.004s

Method is 6 times times more time-consuming than method two.

seq Use:

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

[chengmo@centos5 ~]$ seq 1000 ' starts by default is 1, and the interval defaults to 1

[Chengmo@centos5 ~] $seq 2 1000 ' interval default is 1

[Chengmo@centos5 ~] $seq 1 3 10 ' starting from 1, to 10 interval for 3 The result is: 1 4 7 10

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

[CHENGMO@CENTOS5 ~] $seq-S ' # ' 1 3 10

1#4#7#10

Application tip: generate continuous Array series:

[Chengmo@centos5 ~]$ a= ($ (SEQ 1 3 10))
[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.

Http://www.cnblogs.com/chengmo/archive/2010/09/30/1839668.html