tr

Translate characters (lowercase to uppercase)

$ echo "hello" | tr 'a-z' 'A-Z'

$ echo "hello 123" | tr -d '0-9'

$ echo "helllo" | tr -s 'l'

$ tr '\n' ' ' < [file]

$ tr -cd '[:print:]' < [file]

$ tr ' ' '\t' < [file]

$ echo "hello123" | tr -cd '0-9'

tr [options] set1 [set2]

tr translates, deletes, or squeezes characters from standard input, writing to standard output. It operates character-by-character, making it efficient for simple transformations.
Translation replaces each character in set1 with the corresponding character in set2. If set2 is shorter, its last character is repeated. If longer, excess characters are ignored.
With -d, characters in set1 are deleted. With -s, repeated characters in set1 are squeezed to single occurrences. Both can be combined.
-c complements set1, meaning "all characters NOT in set1." Useful for keeping only specific characters.

-d, --delete

Delete characters in set1

Replace repeated characters with single occurrence

Use complement of set1

Truncate set1 to length of set2

[:alnum:]: Letters and digits
[:alpha:]: Letters
[:digit:]: Digits
[:lower:]: Lowercase letters
[:upper:]: Uppercase letters
[:space:]: Whitespace
[:blank:]: Spaces and tabs
[:print:]: Printable characters
[:punct:]: Punctuation
[:cntrl:]: Control characters

Tr operates on characters, not strings or patterns. For pattern-based substitution, use sed or awk.
Character classes in brackets must use the [:class:] syntax. Don't confuse with regex [a-z] which also works but differs from [:lower:] in locales.
Tr reads only from stdin; it cannot process files directly. Use redirection or pipes.

sed(1), awk(1), cut(1), iconv(1)