Replace blank value with previous first column value using awk and sed

To get the first columns in each line using awk you can do the following:

< testfile awk '{print $1}'
aaa
aaa
aaa
www
hhh
hhh

These are your keys for the rest of the lines. So you may create a hash table, using the first column as a key and the second column of the line as the value:

< testfile awk '{table[$1]=table[$1] $2;} END {for (key in table) print key " => " table[key];}'
www => yyy
aaa => bbbNULLbbb
hhh => 111111

To get the whole rest of the line, starting with column 2, you need to collect all columns:

< testfile awk '{line="";for (i = 2; i <= NF; i++) line = line $i " "; table[$1]=table[$1] line;} END {for (key in table) print key " => " table[key];}'
www => yyy hhh NULL NULL NULL NULL 
aaa => bbb ccc ddd NULL NULL NULL NULL NULL NULL NULL NULL NULL bbb ccc    NULL NULL NULL NULL 
hhh => 111 333 yyy ooo hyy uuuioooy 111 333 yyy ooo hyy NULL 

In sed:

sed 's! \([^ ]\+\)\( \|$\)!\1 !g' your_file

This will make the substitutions and print the result to standard out. To modify the file in place, add the -i switch:

sed -i 's! \([^ ]\+\)\( \|$\)!\1 !g' your_file

Explanation

This sed command will look for a space, followed by at least one non-space character, followed by a space or the end of the line. It substitutes this sequence with whatever non-space characters it found followed by a single space. The substitution is applied as many times as possible across the line (this is called a global substitution) because the g modifier is supplied at the end. So, basically, with a sequence like A B C, sed will find the pattern " B " and substitute it with "B " leaving you with AB C as the final result.

Assumptions made by this code

This code assumes the spaces between your columns are really spaces and not TABs for example. This can be easily fixed at the expense of readability:

sed 's![[:blank:]]\+\([^[:blank:]]\+\)\([[:blank:]]\+\|$\)!\1 !g' your_file