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

Question

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

This looks more obvious, doesn't it?

# Just a comment... Check if output of command is hahaha
MY_COMMAND_OUTPUT="$(echo hahaha)"
if [[ "$MY_COMMAND_OUTPUT" != "hahaha" ]]; then
  echo "The command output is not hahaha"
  exit 2
else
  echo "The command output is hahaha"
fi