Concise and portable “join” on the Unix command-line

Question

How can I join multiple lines into one line, with a separator where the new-line characters were, and avoiding a trailing separator and, optionally, ignoring empty lines?

Answer 1

I had a log file where some data was broken into multiple lines. When this occurred, the last character of the first line was the semi-colon (;). I joined these lines by using the following commands:

for LINE in 'cat $FILE | tr -s " " "|"'
do
    if [ $(echo $LINE | egrep ";$") ]
    then
        echo "$LINE\c" | tr -s "|" " " >> $MYFILE
    else
        echo "$LINE" | tr -s "|" " " >> $MYFILE
    fi
done

The result is a file where lines that were split in the log file were one line in my new file.