tr(1)

tr [opt] str1 [str2]
    -d      delete characters in str1
    -s      squeeze repeating sequence of characters in str1

Examples

To lower

echo MoOsE | tr '[:upper:]' '[:lower:]'
# output: moose

Replace characters

echo moose | tr 'o' '-'
# output: m--se

echo moose | tr 'os' '-'
# output: m---e

Remove specific characters

echo moose | tr -d 'o'
# output: mse

echo moose | tr -d 'os'
# output: me

Squeeze character sequences

echo moooooossse | tr -s 'os'
# output: mose