I just do some practice on ballgown and stringtie, and I got some GTF file and ballgown`s file.

However, I just find I can't use R or something else to deliver a excel file which contain FPKM.

I think the excel file I want maybe looks like this:    

    gene_id      FPKM
        A        124
        B        541   
        C        122

Please help me

Thanks a lot :)
You can use standard unix commands for this as follows:

    $ cat stringtie.txt
    1  StringTie  transcript  337772  338047  1000  +  .  gene_id "Zm00001d027250"; transcript_id "Zm00001d027250_T001"; cov "66.076088"; FPKM "8.407302"; TPM "14.141658";
    1  StringTie  exon        337772  338047  1000  +  .  gene_id "Zm00001d027250"; transcript_id "Zm00001d027250_T001"; exon_number "1"; cov "66.076088";
    1  StringTie  transcript  426764  432130  1000  +  .  gene_id "Zm00001d027254"; transcript_id "Zm00001d027254_T001"; cov "2.043083"; FPKM "0.259955"; TPM "0.437262";
    1  StringTie  exon        426764  426798  1000  +  .  gene_id "Zm00001d027254"; transcript_id "Zm00001d027254_T001"; exon_number "1"; cov "0.000000";
    1  StringTie  exon        426869  426970  1000  +  .  gene_id "Zm00001d027254"; transcript_id "Zm00001d027254_T001"
    $ grep 'FPKM' temp.txt | cut -f9 | sed -r 's/gene_id "([^"]*).*FPKM "([^"]*).*/\1\t\2/g' | sed '1i#gene_id\tFPKM'
    #gene_id        FPKM
    Zm00001d027250  8.407302
    Zm00001d027254  0.259955