Removing all special characters from a string in Bash

I am not exactly certain where the text is coming from in your question but lets just say that the "lot of text in lowercase" is in the file called special.txt you could do something like the following but focused more on the characters you want to keep:

cat special.txt | sed 's/[^a-z  A-Z]//g'

It is a bit like doing surgery with an axe though.

Another possible solution in the post Remove non-ascii characters from ...

If the above don't solve your question, please try to provide a bit more details and I might be able to provide a more actionable answer.

Just wanted to add my bit to it. The code below will do a better job of getting rid of all characters as explained above and will replace them with space and preserve your newline character at the same time

    tr -s "[:punct:]" " "

From Manual Entry -s

Squeeze multiple occurrences of the characters listed in the last operand (either string1 or string2) in the input into a single instance of the character. This occurs after all deletion and translation is completed.

You can use tr to print only the printable characters from a string like below. Just use the below command on your input file.

tr -cd "[:print:]\n" < file1   

The flag -d is meant to the delete the character sets defined in the arguments on the input stream, and -c is for complementing those (invert what's provided). So without -c the command would delete all printable characters from the input stream and using it complements it by removing the non-printable characters. We also keep the newline character \n to preserve the line endings in the input file. Removing it would just produce the final output in one big line.

The [:print:] is just a POSIX bracket expression which is a combination of expressions [:alnum:], [:punct:] and space. The [:alnum:] is same as [0-9A-Za-z] and [:punct:] includes characters ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~