examples/intro/tr.txt
$ cat examples/intro/words.txt
foo
foo
bar
foo
bar


$ cat examples/intro/words.txt | tr 'f' 'x' 
xoo
xoo
bar
xoo
bar


$ cat examples/intro/words.txt | tr '[a-z]' '[A-Z]' 
FOO
FOO
BAR
FOO
BAR