Bedtools Genomecoveragebed Usage : How To Create A Genome File?

I am using BEDTOOLS and the following command to get the coverage file:

$ ./genomeCoverageBed -ibam ~/GG_project/trim/ecoli.bam -g > ~/GG_project/trim/coverage

where ecoli.bam is my sorted bam file, and coverage is my output file

From where do I get the genome file? How do I create a genome file?? Specifically I would need a ecoli.genome file.