$ curl cheat.sh/
# tr
# Translate characters: run replacements based on single characters and character sets.

# Replace all occurrences of a character in a file, and print the result:
tr find_character replace_character < filename

# Replace all occurrences of a character from another command's output:
echo text | tr find_character replace_character

# Map each character of the first set to the corresponding character of the second set:
tr 'abcd' 'jkmn' < filename

# Delete all occurrences of the specified set of characters from the input:
tr -d 'input_characters' < filename

# Compress a series of identical characters to a single character:
tr -s 'input_characters' < filename

# Translate the contents of a file to upper-case:
tr "[:lower:]" "[:upper:]" < filename

# Strip out non-printable characters from a file:
tr -cd "[:print:]" < filename

Follow @igor_chubin cheat.sh tldr