Checking if output of a command contains a certain string in a shell script

I'm writing a shell script, and I'm trying to check if the output of a command contains a certain string. I'm thinking I probably have to use grep, but I'm not sure how. Does anyone know?

Testing $? is an anti-pattern.

if ./somecommand | grep -q 'string'; then
  echo "matched"
fi

Test the return value of grep:

./somecommand | grep 'string' &> /dev/null
if [ $? == 0 ]; then
   echo "matched"
fi

which is done idiomatically like so:

if ./somecommand | grep -q 'string'; then
   echo "matched"
fi

and also:

./somecommand | grep -q 'string' && echo 'matched'

Another option is to check for regular expression match on the command output.

For example:

[[ "$(./somecommand)" =~ "sub string" ]] && echo "Output includes 'sub string'"

A clean if/else conditional shell script:

if ./somecommand | grep -q 'some_string'; then
  echo "exists"
else
  echo "doesn't exist"
fi