Extract sequence from fasta file

Hi,

I want to match the sequence id (sub-string of line starting with '>' and extract the information upto next '>' line ). Please help .

input 

> fefrwefrwef X900
AGAGGGAATTGG
AGGGGCCTGGAG
GGTTCTCTTC
> fefrwefrwef X932
AGAGGGAATTGG
AGGAGGTGGAG
GGTTCTCTTC
> fefrwefrwef X937
AGAGGGAATTGG
AGGAGACTTTAG
GGTTCTCTTC

So if my input parameter is X937 I get an output of

> fefrwefrwef X937
AGAGGGAATTGG
AGGAGACTTTAG
GGTTCTCTTC

If my input parameter is X932 I should get

> fefrwefrwef X932
AGAGGGAATTGG
AGGAGGTGGAG
GGTTCTCTTC

If input is X9, I should get the entire file.

Using awk :

awk -v F="X9" '$1 ~ F {printf "%s", RS $0} ' RS='> ' FS="\n" infile

Very simple one:

awk '/X937/' RS='> ' input

...using variable:

awk -v P="$s" '$0~P' RS='> ' input

... adding leading RS:

awk -v P="$s" '!c++{printf RS}$0~P' RS='> ' input

$ s=X937
$ awk -v P="$s" '$0~P' RS='> ' input
fefrwefrwef X937
AGAGGGAATTGG
AGGAGACTTTAG
GGTTCTCTTC