Following script might help.
#!/bin/bash
decrypt=credentials.txt
encrypt=${decrypt}.encrypted
if [[ $# -eq 0 ]] ; then
echo "Gimme username"
read username
echo "Gimme password"
read -s password
echo ${username}:${password} | openssl des3 -salt -out $encrypt
echo "File encrypted into $encrypt"
elif [[ $1 = '-d' ]] ; then
openssl des3 -d -salt -in $encrypt -out $decrypt
echo "File decrypted into $decrypt"
else
echo "Argument $1 not recognized. Either run with no argument for encryption or with '-d' for encryption" >&2
exit 1
fi