Since you use fedora you have GNU sed and this should work:
s=" shipname NVARCHAR(40) NOT NULL,"
echo "$s" | sed -E '/NOT/{s/^ ([[:lower:]]+)\s*NVARCHAR\(([[:digit:]]+)\) NOT NULL,$/\1 TEXT NOT NULL CHECK \(LENGTH\(\1\) <= \2\),/;q0} ; s/^ ([[:lower:]]+)/\1 TEXT NULL,/'
This emulates a fake if.
if:
a NOT (/NOT/) is found inside the db structure then the first sed command is executed then, quit (q0) without executing the second statement.
else:
no NOT keyword is found, and the second instance is executed.
For the second reqirements :
sed "s/N'/'/g"
Globally search for N' and replace it with only '. I found useful to swap ' with " for sed command line delimiter and make it more clean without a lot of escaping.
Put the first sed inside a file:
#!/bin/sed -Ef
# If a NOT is found execute this:
# capture the column name and the value of this
/NOT/ {
s/^ ([[:lower:]]+)\s*NVARCHAR\(([[:digit:]]+)\) NOT NULL,$/\1 TEXT NOT NULL CHECK \(LENGTH\(\1\) <= \2\),/
# Quit without execute the other statement
q0
}
# Else: If we are here then the database
# structure does not contains a length for the column;
# so it should be NULL
s/^ ([[:lower:]]+)/\1 TEXT NULL,/
The { command is used to group more sed command togheter.
The q is the quit command, it is used to make sed quit. Here i have use it to force sed exiting before encounter the last line if the first test succeded.