tr

tr (abbreviated from translate or transliterate) is a command in Unix-like operating systems.

echo "ABcd" | tr '[A-Z]' '[a-z]'
abcd

Change all Upper character to lower abcde...

echo "abcd" | tr 'abcd' 'jkmn'
jkml

maps 'a' to 'j', 'b' to 'k', 'c' to 'm', and 'd' to 'n'.

echo "123abcABC中文" | tr -cd '[:alnum:]'
123abcABC

The -c flag complements the first set of characters.
Therefore, removes all non-alphanumeric characters.

tr -s '\n' '\n'

The -s flag causes tr to compress sequences of identical adjacent characters in its output to a single token
Therefore, replaces sequences of one or more newline characters with a single newline.

cat /etc/passwd | tr -d '\t\n\r\f'

To delete the all whitespace , tab , breakline character in the output content
aaa bbb
cc, ddd, ee

----->
aaabbbcc,ddd,ee

cat /etc/passwd | tr -d ':'

To delete the : character in the output content.

cat /etc/passwd | tr '\t' ','

replace tag field to comma to the content
eg: aaa bbb ---> aaa,bbb

tr -d '\r'

The -d flag causes tr to delete all tokens of the specified set of characters from its input. In this case, only a single character set argument is used. The following command removes carriage return characters, thereby converting a file in DOS/Windows format to one in Unix format