[Linux 文章收集] Shell Scripting: Convert Uppercase to Lowercase

Source From Here 
Question 
I've a small shell script and I would like to convert all incoming user input to lowercase using a shell script. How do I convert uppercase words or strings to a lowercase or vise versa on Unix-like / Linux bash shell? 

How-To 
Use the tr command to convert all incoming text / words / variable data from upper to lower case or vise versa (translate all uppercase characters to lowercase). Bash version 4.x+ user can use parameter expansion to modify the case of alphabetic characters in parameter. 

Convert all text in a file from UPPER to lowercase 
To translate or delete characters use tr command. The basic syntax is: 

# tr 'set1' 'set2' input
OR
# tr 'set1' 'set2' input > output

Type the following command at shell prompt: 

# cat input.txt
THIS IS For Testing
# tr '[:upper:]' '[:lower:]' < input.txt > output.txt
# cat output.txt
this is for testing

Task: Convert Data Stored in a Shell Variable From UPPER to lowercase: 
Type the following command: 

# export VAR_NAME='This is for Testing'
# echo $VAR_NAME
This is for Testing
# echo $VAR_NAME | tr '[:upper:]' '[:lower:]'
this is for testing
# echo $VAR_NAME | tr '[:lower:]' '[:upper:]'
THIS IS FOR TESTING

Bash version 4.x+: Uppercase to lowercase or vice versa 
The bash version 4.x+ got some interesting new features. Type the following commands to convert $y into uppercase: 

# echo $BASH_VERSION
4.1.2(1)-release
# bash --version
bash --version
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
...
# echo ${VAR_NAME}
This is for Testing
# echo ${VAR_NAME,,}
this is for testing
# echo ${VAR_NAME^^}
THIS IS FOR TESTING

Sample Shell Script 
- test.sh 

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1. #!/bin/bash  
2. # get filename  
3. echo -n "Enter File Name : "  
4. read fileName  
5.   
6. # make sure file exits for reading  
7. if [ ! -f $fileName ]; then  
8.     echo "Filename $fileName does not exists."  
9.     exit 1  
10. fi  
11.   
12. # convert uppercase to lowercase using tr command  
13. tr '[A-Z]' '[a-z]' < $fileName  
14.   
15. # Note Bash version 4 user should use builtins as discussed above  

Test the above script: 

# ./test.sh
Enter File Name : input.txt
this is for testing

Supplement 
* [Linux 命令] tr : 更換或變更檔案中的字元