The For loop summary in the shell _linux shell

About the shell in the for loop usage a lot, always want to sum up, today on the Internet to see a summary of the use of the loop, feeling very comprehensive, so turn around research, HEI ...

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For ((i=1;i<=10;i++));d o echo $ (expr $i \* 4);d One

Used in the shell is for I in $ (seq 10)

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For i in ' ls '
For i in ${arr[@]}
For i in $*; Todo
For File in/proc/sys/net/ipv4/confaccept_redirects: '
For File in/proc/sys/net/ipv4/conf/*/accept_redirects; Todo
Echo $File
Done
echo "Specify loop content directly"
For I in F1 F2 F3;d o
Echo $i
Done
Echo
echo "C syntax for loop:"
for ((i=0; i<10; i++)); Todo
Echo $i
Done

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For -loop usage in the shell

Shell syntax Good trouble, a loop has been done for a while, and found several different ways to achieve output 1-100 can be divisible by 3 number
1. Use (())

Copy Code code as follows:

#!/bin/bash
Clear
For ((i=1;i<100;i++))
For
Todo
if ((i%3==0))
Then
Echo $i
Continue
Fi
Done

2. Use ' SEQ 100 '

Copy Code code as follows:

#!/bin/bash
Clear
For i in ' SEQ 100 '
Todo
if ((i%3==0))
Then
Echo $i
Continue
Fi
Done

3. Use while

Copy Code code as follows:

#!/bin/bash
Clear
I=1
while (($i <100))
Todo
if (($i%3==0))
Then
Echo $i
Fi
i=$ (($i + 1))
Done

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When the shell uses the for loop to increase the number of digits to discover the problem, it lists several methods of the shell for the loop:
1.

Copy Code code as follows:

For i in ' seq 1 1000000 ';d o
Echo $i
Done

With the SEQ 1 10000000 increment, before using this method did not encounter problems, because the previous I was useless to millions (1000000), because the project needs me this number is far greater than million, found in SEQ value to 1000000 when converted to 1e+06, Can not be used as a number of other operations, or the $i effective, correct access, and then find other ways to solve, as follows
2.

Copy Code code as follows:

For ((i=1;i<10000000;i++));d o
Echo $i
Done
3.
I=1
while (($i <10000000));d o
Echo $i
i= ' expr $i + 1 '
Done

Because this method calls expr, it will run faster than the 1th, 2nd, but can be slightly improved, will be i= ' expr $i + 1 ' to i=$ (($i + 1)) can be a little speed, but specifically to see if the corresponding Shell environment support
4.

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For I in {1..10000000;do
Echo $i
Done

In fact, choose which method is specific or by the corresponding Shell environment support, to achieve the desired effect, and then consider the problem of speed.
Example:

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#!/bin/sh
I=1
function Test_while () {
I=1
While [$i]
Todo
Echo $i
i= ' expr $i + 1 '
If [$i-ge 10]; Then
Break
Fi
Done
}
function Test_for () {
I=1
for ((I=1; i<=100; i++)); Todo
Echo $i
If [$i-ge 10]; Then
Break
Fi
Done
}
function Test_continue () {
I=1
For I in $ (seq 100); Todo
if ((i==0)); Then
Echo $i
Continue
Fi
Done
}
echo "Test_while ..."
Test_while
echo "Test_for ..."
Test_for
echo "Test_continue ..."
Test_continue

Run Result:

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Test_while ...
1
2
3
4
5
6
7
8
9
Test_for ...
1
2
3
4
5
6
7
8
9
10
Test_continue ...
10
20
30
40
50
60
70
80
90
100