extract subset of a fasta file

#fasta file: pa101.fasta
>gi|129295|sp|P01013|OVAX_CHICK GENE X PROTEIN (OVALBUMIN-RELATED)
QIKDLLVSSSTDLDTTLVLVNAIYFKGMWKTAFNAEDTREMPFHVTKQESKPVQMMCMNNSFNVATLPAE
KMKILELPFASGDLSMLVLLPDEVSDLERIEKTINFEKLTEWTNPNTMEKRRVKVYLPQMKIEEKYNLTS
VLMALGMTDLFIPSANLTGISSAESLKISQAVHGAFMELSEDGIEMAGSTGVIEDIKHSPESEQFRADHP
FLFLIKHNPTNTIVYFGRYWSP

#script: sequence_extractor.sh
#!/bin/bash

# The 1 based sequence extractor - sequence_extractor.sh
# No guarantees offered.

# usage: 
# 1) download the script or copy the contents
# of the script and save it as sequence_extractor.sh
# 2) make it executable: chmod 755 sequence_extractor.sh
# reads from standard input or command line 
# 3) run the script: ./sequence_extractor.sh ps101.fasta 4 6

# create a backup copy of the input fasta file
# and delete the header 
sed -i.tmp -e '1d' $1 || exit $?

# merge the lines 
temp_var1=`awk '{printf $0;}' $1` || exit $?

# select the region
temp_var2=$(((($3-1)-($2-1))+1)) || exit $?

# display the extracted sequence
echo ${temp_var1:$(($2-1)):$temp_var2} && mv $1.tmp $1 || exit $?