[SOLVED] How to join lines not starting with specific pattern to the previous line in UNIX?

Issue

Please take a look at the sample file and the desired output below to understand what I am looking for.

It can be done with loops in a shell script but I am struggling to get an awk/sed one liner.

SampleFile.txt

These are leaves.
These are branches.
These are greenery which gives
oxygen, provides control over temperature
and maintains cleans the air.
These are tigers
These are bears
and deer and squirrels and other animals.
These are something you want to kill
Which will see you killed in the end.
These are things you must to think to save your tomorrow.

Desired output

These are leaves.
These are branches.
These are greenery which gives oxygen, provides control over temperature and maintains cleans the air.
These are tigers
These are bears and deer and squirrels and other animals.
These are something you want to kill Which will see you killed in the end.
These are things you must to think to save your tomorrow.

Solution

Please try the following:

awk 'BEGIN {accum_line = "";} /^These/{if(length(accum_line)){print accum_line; accum_line = "";}} {accum_line = accum_line " " $0;} END {if(length(accum_line)){print accum_line; }}' < data.txt

The code consists of three parts:

1. The block marked by BEGIN is executed before anything else. It's useful for global initialization
2. The block marked by END is executed when the regular processing finished. It is good for wrapping the things. Like printing the last collected data if this line has no These at the beginning (this case)
3. The rest is the code performed for each line. First, the pattern is searched for and the relevant things are done. Second, data collection is done regardless of the string contents.

Answered By - GMichael