Shell Programming and Scripting

Hello Gurus,

Can anybody please help me to understand as what does this lines(Marked in bold) does:

_TABLESPACE_OFFLINE=`cat ${_tmp_res} | grep -iv "online" | grep -v "^[ ]*$" | grep -iv "SQL>"` ------->1

if [ "X${_TABLESPACE_OFFLINE}" != "X" ];then
_tmp_tablespace_status=1
if_need_raise_alarm


Output of _TABLESPACE_OFFLINE from line 1 is:

TABLESPACE_NAME STATUS
------------------------------ ---------
EMA_PROCLOG_20120918 READ ONLY
EMA_PROCLOG_20120919 READ ONLY
EMA_PROCLOG_20120920 READ ONLY
TABLESPACE_NAME STATUS
------------------------------ ---------
EMA_PROCLOG_20120921 READ ONLY
EMA_PROCLOG_20120922 READ ONLY
EMA_PROCLOG_20120923 READ ONLY
EMA_PROCLOG_20120924 READ ONLY
EMA_PROCLOG_20120925 READ ONLY
EMA_PROCLOG_20120926 READ ONLY
EMA_PROCLOG_20120927 READ ONLY
20 rows selected.

It is actually checking whether the variabe _TABLESPACE_OFFLINE is empty or not.