   I have a .gtf file as below I would like to add "chr" to the first column of the file but not in first 5 rows?

    #!genome-build GRCh37.p13
    #!genome-version GRCh37
    #!genome-date 2009-02
    #!genome-build-accession NCBI:GCA_000001405.14
    #!genebuild-last-updated 2013-09
    1       ensembl chromosome      1       300239041       .       .       .       ID=1;Name=chromosome:AGPv1:1:1:300239041:1
    1       ensembl exon    3       104     .       +       .       Parent=GRMZM2G060082_T01;Name=GRMZM2G060082_E07


output:

    #!genome-build GRCh37.p13
    #!genome-version GRCh37
    #!genome-date 2009-02
    #!genome-build-accession NCBI:GCA_000001405.14
    #!genebuild-last-updated 2013-09
    chr1       ensembl chromosome      1       300239041       .       .       .       ID=1;Name=chromosome:AGPv1:1:1:300239041:1
    chr1       ensembl exon    3       104     .       +       .       Parent=GRMZM2G060082_T01;Name=GRMZM2G060082_E07


I used the foloww cods but it add "chr" to the first 5 lines, as well.

    cat Homo_sapiens.GRCh37.gtf | sed 's/^/chr/' > chr.gtf
    awk 'OFS="\t" {if (NR > 5) $1="chr"$1; print}' in.gtf

Could have been found on google easily...