Longest common prefix of two strings in bash

In sed, assuming the strings don't contain any newline characters:

string1="test toast"
string2="test test"
printf "%s\n%s\n" "$string1" "$string2" | sed -e 'N;s/^\(.*\).*\n\1.*$/\1/'

An improved version of the sed example, this finds the common prefix of N strings (N>=0):

string1="test toast"
string2="test test"
string3="teaser"
{ echo "$string1"; echo "$string2"; echo "$string3"; } | sed -e 'N;s/^\(.*\).*\n\1.*$/\1\n\1/;D'

If the strings are stored in an array, they can be piped to sed with printf:

strings=("test toast" "test test" "teaser")
printf "%s\n" "${strings[@]}" | sed -e '$!{N;s/^\(.*\).*\n\1.*$/\1\n\1/;D;}'

You can also use a here-string:

strings=("test toast" "test test" "teaser")
oIFS=$IFS
IFS=$'\n'
<<<"${strings[*]}" sed -e '$!{N;s/^\(.*\).*\n\1.*$/\1\n\1/;D;}'
IFS=$oIFS
# for a local IFS:
(IFS=$'\n'; sed -e '$!{N;s/^\(.*\).*\n\1.*$/\1\n\1/;D;}' <<<"${strings[*]}")

The here-string (as with all redirections) can go anywhere within a simple command.