You would have to use sed for this. Look at the following:
sed -e 's/.*\[\([^]]*\)\].*/\1/g'
This would be the command line equivelant to perform the operation you are looking for. As an example (I'm not within linux, so this might be off...):
echo "blah [Need this phrase] bblah" | sed -e 's/.*\[\([^]]*\)\].*/\1/g'
Which should return for you "Need this phrase".