Shell loop, shellloop

Shell loop, shellloop
1. Loop
The shell loop continues to execute a program until it meets the conditions. There are four types of loops: while LOOP, until loop, for fixed processing, and for numerical processing.
2. while Loop
The while loop is executed until the condition does not match. Syntax: while [condition] do // do somethingdone example: loop until user input is correct

#!/bin/bash# desc : while loopwhile [ "$yn" != "yes" -a "$yn" != "YES" ] do     read -p "Please input yes/YES to stop : " yndoneecho "OK"

Execution result:

[work@www sh]$ sh while.sh Please input yes/YES to stop : noPlease input yes/YES to stop : yesOK[work@www sh]$ 

For example, from 1 to 100

#!/bin/bash# desc : while loopsum=0i=0while [ $i -lt 100 ]do    i=$(($i+1));    sum=$(($sum+$i))doneecho "From 1 to 100, sum is : " $sum 

3. until Loop
The until loop is opposite to the while loop. When the condition is set, the loop is terminated. Syntax: until [condition] do // do somethingdone example:

#!/bin/bash# desc : while loopuntil [ "$yn" == "yes" -o "$yn" == "YES" ]do    read -p "Please input yes/YES to stop : " yndoneecho "OK"

Run:

[work@www sh]$ sh while.sh Please input yes/YES to stop : hi         Please input yes/YES to stop : yesOK[work@www sh]$ 

4. for fixed Loop
For is a known number of cycles. Syntax: for var in con1 con2 .. do // do somethingdone example:

#!/bin/bash# desc : for loopfor animal in dog cat pig do    echo "HI, ${animal}"done

Run:

[work@www sh]$ sh for.sh HI, dogHI, catHI, pig[work@www sh]$

Example: Current directory file

#!/bin/bash# desc : for loopfilelist=$(ls)for filename in $filelistdo    echo $filenamedone

5. for numeric Loop
For numeric loop, applicable to cyclic numerical calculation. Syntax for (initial value; limit value; step size) do // do somethingdone example:

#!/bin/bash# desc : for loopsum=0for (( i=0; i<=100; i++ ))do    sum=$(($sum+$i))doneecho "sum is : " $sum

Address: http://blog.csdn.net/yonggang7/article/details/40679701

Why are there errors in the for loop in shell?

#! /Bin/sh
X = 1
Y = 2
For (I = 1; I <$1; I ++ ))
Do
M = $ y
Let y = $ x + $ y
X = $ m
Done
Echo $ x/$ y

Why are two parentheses for in shell?

For loop syntax has the following two types:

(1) This is the classic for usage of shell:
For varname [in word...]
Do
...
Done

Example:
For I in a B c
Do
Echo $ I
Done

(2) This syntax is similar to the C/C ++ usage, and (...) is used for arithmetic operations in shell:

For ([init_expression]; [loop_condition]; [loop_expression])
Do
...
Done

Example:
For (I = 0; I <5; I ++ ))
Do
Echo $ I
Done

Note: This for usage is only supported by ksh after 1988-11-16, so many Unix systems do not support this for syntax. For example, HPUX or ibm aix.