awk command to merge the content of the same file

Question

I have an input file with the following content

1 1
2 1
3 289
4 1
5 2 
0   Clear
1   Warning
2   Indeterminate
3   Minor
4   Major
5   Critical

I want to merge the first type of lines with the messages by the first column and obtain

  1 1 Warning
  2 1 Indeterminate
  3 289 Minor 
  4 1 Major
  5 2 Critical

Answer 1

Just use awk:

awk '$1 in a { print $1, a[$1], $2; next } { a[$1] = $2 }' file

Output:

1 1 Warning
2 1 Indeterminate
3 289 Minor
4 1 Major
5 2 Critical

Answer 2

Using join/sed, sed creates different views of the file for each part and join joins on the common field:

join <(sed '/^[0-9]* [0-9]* *$/!d' input) <(sed '/^[0-9]* [0-9]* *$/d' input)

Gives:

1 1 Warning
2 1 Indeterminate
3 289 Minor
4 1 Major
5 2  Critical