remove new line if next line does not begin with a number

Question

I have a file that is like

    1 test
    test

How can I remove the new line from the so the final output becomes:

1 test test

Answer 1

You can be a bit smarter and print a new line before the line if it starts with a digit (except for the first line);

awk 'BEGIN{ORS="";} NR==1 { print; next; } /^[[:digit:]]/ { print "\n"; print; next; } { print; }'

The awk script:

BEGIN { ORS=""; }                            # default: no newline between output records
NR==1 { print; next; }                       # first line: print
/^[[:digit:]]/ { print "\n"; print; next; }  # if starts with a digit: print newline before
{print;}                                     # other lines (next; has not been called yet)