Shell Programming and Scripting

I guess you should use solution in post #6 OR in post #2.
If you are new to awk/sed, you may not understand it. U need to go through some basic tutorial 1st.

post #6:

Code:

sed -n 'H;/IM/h;/B2/{g;p;}' infile

Few meanings (Look into sed manual OR some sed tutorial for more details):
-n switch to supress default output. don't print unless explicitly requested
H = > Append the content of pattern space to the hold buffer
h = > Replace contents of hold space with the contents of the pattern space
g = > Replace contents of pattern space with the contents of the hold space
p = > Copy the pattern space to the standard output

Above command does following:
1. Append every line to hold space
2. If IM pattern found, write current line to hold space (overwrite, remove old data)
3. If B2 pattern found, move hold space content to pattern space and print it to standard output.

So here if input file doesn't have IM (starting) pattern, but only B2 (Ending)Pattern, everything upto ending pattern will be printed (which is not expected I believe). So this will work good if we are sure that if there is an Ending pattern in file, there is atleast one starting pattern before that.

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Post #2:

Code:

awk '$0 ~ /^IM/ {f=$0;next} $0 ~ /^B2/ {if(f){f=f"\n"$0;print f;f="";next}} {if(f) f=f"\n"$0;}' inputFile

== >> If a line has IM, store line in f
== >> If line doesn't has IM or B2, just append it to f (If f is already have some non-empty data, means IM has been found already in previous lines)
== >> If B2 is found, append it to f and print f. Set f to empty.

Code:

 awk '/IM/{h=$0};/B2/{print h"\n"$0;exit}' file