how do I verify that the the checksum is correct?

[Jacques B.]

The -c option is the easiest in my opinion as it uses the features of
the checksum application.

If you want to do it otherwise, you could do something like:

test [ "`sha1sum cd.iso`" = "`cat checksumfile.txt`" ] && echo
"matches" || "echo "no match"
where checksumfile.txt is the one provided by the web site.

I'm not in my Linux partition right now so I can't check the syntax to
be exactly certain that is right.  If it's off I'm sure someone can
correct it.  But ultimately why do that when you can simply use the -c
option?

Alternatively you could do
sha1sum cd.iso >> checksumfile.txt
where "checksumfile.txt" is the one provided by the web site.
Then you can cat the file and easily compare the two because they will
be one under the other so at a glance easy to see if they match or
not.

Jacques B.