How to filter lowercase or uppercase words/letters from a file in Linux ?

Today we will look how to filter lowercase or uppercase letters/words from a file or string in Linux.

The easiest way to achieve this task is to use tr command with few options.

Create a new test file with lowercase and uppercase characters

[root@ngelinux ~]# cat testfile2 
hello HOW are YOU.
WELCOME to NGELinux.com
[root@ngelinux ~]# 

1. Filter lowercase characters

[root@ngelinux ~]# cat testfile2 | tr -d [:upper:]
hello  are .
 to inux.com

2. Filter uppercase characters.

[root@ngelinux ~]# cat testfile2 | tr -d [:lower:]
 HOW  YOU.
WELCOME  NGEL.
[root@ngelinux ~]# 

Hence we can see the usage of [:upper:] and [:lower:] options with tr command.

The above tip is very useful for string manipulation on linux.

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