[Linux 文章收集] Bash For Loop Examples

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Preface : 
A ‘for loop’ is a bash programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement i.e. it is the repetition of a process within a bash script. For example, you can run UNIX command or task 5 times or read and process list of files using a for loop. A for loop can be used at a shell prompt or within a shell script itself. 

for loop syntax : 
Numeric ranges for syntax is as follows : 

for VARIABLE in 1 2 3 4 5 .. N
do
command1
command2
commandN
done

This type of for loop is characterized by counting. The range is specified by a beginning (#1) and ending number (#5). The for loop executes a sequence of commands for each member in a list of items. A representative example in BASH is as follows to display welcome message 5 times with for loop : 

- test1.sh :

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1. #!/bin/sh  
2. for i in 1 2 3 4 5  
3. do  
4.         echo "Welcome $i times"  
5. done  

Sometimes you may need to set a step value (allowing one to count by two’s or to count backwards for instance). It can be done easily with seq command. A representative example in bash as follows : 

- test2.sh :

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1. #!/bin/sh  
2. for i in $(seq 1 2 20)  
3. do  
4.         echo "Welcome $i times"  
5. done  

Latest bash version 3.0+ has inbuilt support for setting up ranges : 

- test3.sh :

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1. #!/bin/sh  
2. for i in {1..5}  
3. do  
4.         echo "Welcome $i times"  
5. done  

Bash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax : 

- test4.sh :

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1. #!/bin/sh  
2. echo "Bash version ${BASH_VERSION}..."  
3. for i in {0..10..2}  
4. do  
5.         echo "Welcome $i times"  
6. done  

上面執行結果 : 

$ ./test4.sh
Bash version 4.2.8(1)-release...
Welcome 0 times
Welcome 2 times
Welcome 4 times
Welcome 6 times
Welcome 8 times
Welcome 10 times

Three-expression bash for loops syntax : 
This type of for loop share a common heritage with the C programming language. It is characterized by a three-parameter loop control expression; consisting of an initializer (EXP1), a loop-test or condition (EXP2), and a counting expression (EXP3). 

for (( EXP1; EXP2; EXP3 ))
do
command1
command2
command3
done

A representative three-expression example in bash as follows : 

- test5.sh :

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1. #!/bin/sh  
2. for (( c=1; c<=5; c++ ))  
3. do  
4.         echo "Welcome $c times..."  
5. done  

How do I use for as infinite loops ? 
Infinite for loop can be created with empty expressions, such as : 

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1. #!/bin/bash  
2. for (( ; ; ))  
3. do  
4. echo “infinite loops [ hit CTRL+C to stop]“  
5. done  

Conditional exit with break : 
You can do early exit with break statement inside the for loop. You can exit from within a FOR, WHILE or UNTIL loop using break : 

- test6.sh :

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1. #!/bin/sh  
2. for i in 1 2 3 4 5  
3. do  
4.         echo "Test $i..."  
5.         if [ "$i" -eq "3" ]; then  
6.                 echo "Break!"  
7.                 break  
8.         fi  
9. done  

執行結果 : 

$ ./test6.sh
Test 1...
Test 2...
Test 3...
Break!

Early continuation with continue statement : 
To resume the next iteration of the enclosing FOR, WHILE or UNTIL loop use continue statement. 

- test7.sh :

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1. #!/bin/sh  
2. for i in {1..10}  
3. do  
4.         if [ "$i" -eq "5" ]; then  
5.                 echo "Continue...(Skip 5)"  
6.                 continue  
7.         fi  
8.         echo "Test $i..."  
9. done  

執行結果 : 

$ ./test7.sh
Test 1...
Test 2...
Test 3...
Test 4...
Continue...(Skip 5)
Test 6...
Test 7...
Test 8...
Test 9...
Test 10...

Supplement 
* nixCraft - Bash For Loop Examples