How to append Line to previous Line?

A version in perl, using negative lookaheads:

$ perl -0pe 's/\n(?!([0-9]{8}|$))//g' test.txt
20141101 server contain dump
20141101 server contain nothing    {uekdmsam ikdas jwdjamc ksadkek} ssfjddkc * kdlsdlsddsfd jfkdfk
20141101 server contain dump

-0 allows the regex to be matched across the entire file, and \n(?!([0-9]{8}|$)) is a negative lookahead, meaning a newline not followed by 8 digits, or end of the line (which, with -0, will be the end of the file).

May be a little bit easy with sed

sed -e ':1 ; N ; $!b1' -e 's/\n\+\( *[^0-9]\)/\1/g'

• first part :1;N;$!b1 collect all lines in file divided by \n in 1 long line

• second part strip newline symbol if it followed non-digit symbol with possible spaces between its.

To avoid memory limitation (espesially for big files) you can use:

sed -e '1{h;d}' -e '1!{/^[0-9]/!{H;d};/^[0-9]/x;$G}' -e 's/\n\+\( *[^0-9]\)/\1/g'

Or forget a difficult sedscripts and to remember that year starts from 2

tr '\n2' ' \n' | sed -e '1!s/^/2/' -e 1{/^$/d} -e $a

One way would be:

 $ perl -lne 's/^/\n/ if $.>1 && /^\d+/; printf "%s",$_' file
 20141101 server contain dump
 20141101 server contain nothing    {uekdmsam ikdas jwdjamc ksadkek} ssfjddkc * kdlsdlsddsfd jfkdfk 
 20141101 server contain dump

However, .that also removes the final newline. To add it again, use:

$ { perl -lne 's/^/\n/ if $.>1 && /^\d+/; printf "%s",$_' file; echo; } > new

Explanation

The -l will remove trailing newlines (and also add one to each print call which is why I use printf instead. Then, if the current line starts with numbers (/^\d+/) and the current line number is greater than one ($.>1, this is needed to avoid adding an extra empty line at the beginning), add a \n to the beginning of the line. The printf prints each line.

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Alternatively, you can change all \n characters to \0, then change those \0 that are right before a string of numbers to \n again:

$ tr '\n' '\0' < file | perl -pe 's/\0\d+ |$/\n$&/g' | tr -d '\0'
20141101 server contain dump
20141101 server contain nothing    {uekdmsam ikdas jwdjamc ksadkek} ssfjddkc * kdlsdlsddsfd jfkdfk 
20141101 server contain dump

To make it match only strings of 8 numbers, use this instead:

$ tr '\n' '\0' < file | perl -pe 's/\0\d{8} |$/\n$&/g' | tr -d '\0'