This has already been addressed as Giovanni pointed out:

• Perl solution
• Python solution
• Fasta parsing in Perl
• Extracting multiple entries from a fasta file

Please try at least a cursory search of the site before posting a new question!

Default grep can't find multiline patterns.

You can use pcregrep.

Check this topic from stackoverflow.

If you want to extract multiple contigs at different times, it is highly suggested that you build an index. Here some commands:

samtools faidx in.fasta #(create an index for the fasta file with all the contigs)
samtools faidx in.fasta contigname > contigname.fasta #(extract the single contig you need)

#to extract seqnames from MS blast result(seqtool) files:
grep contigname filename > out.txt

If you need more lines from contig add -A number after grep like: (for 3 lines)

grep -A 3 contigname filename > out.txt