Shell Programming and Scripting

(
cat file1|while read line1
do
        # Avoid feeding blank to grep
        if [ "${line1}""X" = "X" ]
        then
                echo "${line1}"
                continue
        fi
        # Does key exist in file2 ? 
        found=""
        found=$(grep \^"${line1}" file2)
        if [ ! "${found}""X" = "X" ]
        then
                echo "${found}"
        else
                echo "${line1}"
        fi
done
) > file3

./scriptname
cat file3
aaaa 1 2 3 4
ssss 3 5 6 7
dddd 1 2

ffff
qqqq 1 6 y 9 
wwww

eeee

Update file1 with the data from file2 using each file1 record as the matching key and output the results to file3.

if [ "${line1}""X" = "X" ]

awk 'NR==FNR { a[$1]=$0;next} { if (a[$1]) print a[$1]; else print $0; }' file2 file1