Dear Pierre, I have tried your second suggestion, because I need the same snp positions in all samples(this is a population genetic study). Unfortunately, it didn't work.
My command was:
find ./samples/ -type f -name "*.bam" > in.list
./gatk-4.1.9.0/gatk --java-options "-Xmx4g" HaplotypeCaller ‐R ./reference.fasta ‐I in.list ‐o ./snps_output.vcf
My result:
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A USER ERROR has occurred: Illegal argument value: Positional arguments were provided ',‐R{./reference.fasta{‐I{in.list{‐o{./snps_output.vcf}' but no positional argument is defined for this tool.
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Set the system property GATK_STACKTRACE_ON_USER_EXCEPTION (--java-options '-DGATK_STACKTRACE_ON_USER_EXCEPTION=true') to print the stack trace.