Bash: test for newline in a string.

test-newline.sh

#!/bin/bash

# Use pattern matching: http://goo.gl/q6qNco.
# The newline in the pattern string needs to be quoted.
# See ANSI-C quoting: http://goo.gl/I3HZTR

string=$(echo -e 'hello\nworld')
if [[ ${string} == *$'\n'* ]]; then
    echo 'newline detected' >&2
else
    echo 'newline not detected' >&2
fi