Sometimes, removing a few unnecessary characters from an input text is important. For example, our input text is in the tr.txt file:

$ cat tr.txt
This is a text file for demonstrating
tr command.
This input file contains digit 2 3 4 and 5
as well.
THIS IS CAPS LINE
this a lowercase line

Suppose we want to remove all the caps letters from this file. We can use the -d option with SET1 as 'A-Z':

$ tr -d 'A-Z' < tr.txt
This is a text file for demonstrating
tr command.
This input file contains digit 2 3 4 and 5
as well.
   
this a lowercase line

We see that the output doesn't have any caps letter. We can also removed a new line and space from a file as follows:

$ tr -d ' 
' < tr.txt > tr_out1.txt

Here, we have redirected the output to tr_out1.txt:

$ cat tr_out1.txt
Thisisatextfilefordemonstratingtrcommand.Thisinputfileconatainsdigit234and5aswell.THISISCAPSLINEthisalowercaseline

The -s option is useful when we don't want to delete a character throughout the input text, instead we want to squeeze down to a single occurrence if consecutive multiple occurrences of the given character is there.

One of the use-case where it will prove useful is when we have multiple spaces in between two words that we want to bring down to a single space between any two words/strings in the input text. Consider the tr1.txt file as an example:

$ cat tr1.txt
India            China              Canada
USA    Japan               Russia
Germany        France               Italy
Australia   Nepal

By looking into this file, it's quite clear that texts are not properly aligned. There are multiple spaces between two words. We can squeeze multiple spaces to one space using the tr option with -s:

$ tr -s ' ' < tr1.txt 
India China Canada
USA Japan Russia
Germany France Italy
Australia Nepal

Command tr also provides the -c or -C options to invert a character set to be translated. This is useful when we know what is not to be translated.

For example, we want to keep only alphanumeric, newline, and white-space in the text string. Everything should be deleted from the input text. Here, it's easy to specify what not to delete rather than what to delete.

For example, consider the tr2.txt file whose content is as follows:

$ cat tr2.txt
This is an input file.
It conatins special character like ?, ! etc
&^var is an invalid shll variable.
_var1_ is a valid shell variable

To delete characters other than alphanumeric, newline, and white-space, we can run the following command:

tr -cd '[:alnum:] 
' < tr2.txt
This is an input file
It conatins special character like   etc
var is an invalid shll variable
var1 is a valid shell variable