A common workflow is to
sed -i '<expression> somefile.conf to change things in the file, followed by
cat somefile.conf to see if things changed.
Using word designators in Bash is pretty useful for this. e.g. In the scenario above, after the
sed command running
cat !$ will show you the (hopefully, correctly modified) file contents.
From this answer, to show the sections of the
bash manpage on history expansion:
man -P 'less -p ^HISTORY\ EXPANSION' bash