Shell Programming and Scripting

echo "string" | tr -s '[:lower:]' '[:upper:]'
echo "string" | tr -s '[a-z]'  '[A-Z]'

Try:
type tr
(Make sure that it is the right tr).

alias
(Look for aliases of tr).

And an obscure one ... but quite likely to be your problem:
tr is affected by single character filenames in the directory if you use the (wrong) syntax you quote.

This sequence of commands in ksh prompt illustrates the problem:

1) Just show that I have no single character filenames.

ls ?
? not found

2) Create the first file called "a".  tr misbehaves !

>a
echo "abcdef"|tr [a-z] [A-Z]
Abcdef

3) Create a second file called "c". tr fails !

>c   
echo "abcdef"|tr [a-z] [A-Z]
tr: The combination of options and String parameters is not legal.
Usage: tr [ -c | -cds | -cs | -ds | -s ] [-A] String1 String2
       tr [ -cd | -cs | -d | -s ] [-A] String1

4) But it works if we use the correct syntax.

echo "abcdef"|tr '[a-z]' '[A-Z]'
ABCDEF


QED

touch a
echo echo "abcdef"|tr [a-z] [A-Z]