Shell Programming and Scripting

Aigles, you're my hero! It works perfectly.

Would it be too much to ask for you to explain what exactly the script tells awk to do? I'd like to understand how this works, to improve myself and be able to use this in other contexts.

Thanks a lot!

awk -v Id=$123456 -v FS='[][]' '
. . .
' th.txt

Run awk with file th.txt for input.
Two variables are define :
Id : sessionId to extract
FS : input Field Separator. The two characters [ and ] acts as field separator.
In that case the thread id is field $4 and the text part is field $7.

$7 ~ "BEGIN REQUEST sessionID=" Id {
thread = $4;
}

If the text part (field $7) contains 'BEGIN REQUEST' for the required sessionId,
the thread (field $4) is memorized into the variable thread.

$4 == thread

All the lines with the memorized thread are selected and printed.

$7 ~/END REQUEST/ { thread="" }

When END REQUEST is found, the value of the thread is reseted.