Shell Programming and Scripting

 
awk 'NF=0{print NR}' file1 >> blank_line_numbers
for i in `cat blank_line_numbers`
do
insert_line=`head -$i file2 | tail -1`
sed 's/^$/$insert_line/$i' file1 >> temp
mv temp file1
done

#!/bin/bash
PATH=/usr/bin:/bin
export PATH

exec 3<file1
exec 4<file2

while read a 0<&3; do
        read b 0<&4
        echo ${a:-$b}
done

# this line will output any remaining lines in file2
# if you don't need this, then you can take it out
cat <&4

awk 'NR==FNR{a[NR]=$0;next}{b[FNR]=$0}END { for(i=1;i<=FNR;i++) if (b[i]=" ") b[i]=a[i]}{print i,b[i]}' file1 file2

nawk 'FNR==NR {f1[FNR]=$0;next} {print NF?$0:f1[FNR]}' file2 file1

nawk 'FNR==NR {f1[FNR]=$0;next} {print NF?$0:f1[FNR]}'

nawk 'FNR==NR {f1[FNR]=$0;next} 
                        {
                           if (NF!=0) 
                              print $0
                           else
                              print f1[FNR]
                        }' file2 file1