I tried running bowtie2-build as below

bowtie2-build \
--threads 8 \
"$path_genome" \
"$path_index"

and it gives the following error that threads is not defined.

Bowtie2 indexing genome 
bowtie2-build: unrecognized option '--threads' 
Bowtie 2 version 2.2.6 by Ben Langmead (langmea@cs.jhu.edu, www.cs.jhu.edu/~langmea) 
Usage: bowtie2-build [options]* <reference_in> <bt2_index_base>
    reference_in            comma-separated list of files with ref sequences
    bt2_index_base          write bt2 data to files with this dir/basename
*** Bowtie 2 indexes work only with v2 (not v1).  Likewise for v1 indexes. *** Options:
    -f                      reference files are Fasta (default)
    -c                      reference sequences given on cmd line (as
                            <reference_in>)
    --large-index           force generated index to be 'large', even if ref
                            has fewer than 4 billion nucleotides
    -a/--noauto             disable automatic -p/--bmax/--dcv memory-fitting
    -p/--packed             use packed strings internally; slower, less memory
    --bmax <int>            max bucket sz for blockwise suffix-array builder
    --bmaxdivn <int>        max bucket sz as divisor of ref len (default: 4)
    --dcv <int>             diff-cover period for blockwise (default: 1024)
    --nodc                  disable diff-cover (algorithm becomes quadratic)
    -r/--noref              don't build .3/.4 index files
    -3/--justref            just build .3/.4 index files
    -o/--offrate <int>      SA is sampled every 2^<int> BWT chars (default: 5)
    -t/--ftabchars <int>    # of chars consumed in initial lookup (default: 10)
    --seed <int>            seed for random number generator
    -q/--quiet              verbose output (for debugging)
    -h/--help               print detailed description of tool and its options
    --usage                 print this usage message
    --version               print version information and quit 
Error: Encountered internal Bowtie 2 exception (#1)

But, if I check the help for bowtie2-build, it's there.

[user@server dir]$ bowtie2-build --help
Bowtie 2 version 2.2.9 by Ben Langmead (langmea@cs.jhu.edu, www.cs.jhu.edu/~langmea)
Usage: bowtie2-build [options]* <reference_in> <bt2_index_base>
    reference_in            comma-separated list of files with ref sequences
    bt2_index_base          write bt2 data to files with this dir/basename
*** Bowtie 2 indexes work only with v2 (not v1).  Likewise for v1 indexes. ***
Options:
    -f                      reference files are Fasta (default)
    -c                      reference sequences given on cmd line (as
                            <reference_in>)
    --large-index           force generated index to be 'large', even if ref
                            has fewer than 4 billion nucleotides
    -a/--noauto             disable automatic -p/--bmax/--dcv memory-fitting
    -p/--packed             use packed strings internally; slower, less memory
    --bmax <int>            max bucket sz for blockwise suffix-array builder
    --bmaxdivn <int>        max bucket sz as divisor of ref len (default: 4)
    --dcv <int>             diff-cover period for blockwise (default: 1024)
    --nodc                  disable diff-cover (algorithm becomes quadratic)
    -r/--noref              don't build .3/.4 index files
    -3/--justref            just build .3/.4 index files
    -o/--offrate <int>      SA is sampled every 2^<int> BWT chars (default: 5)
    -t/--ftabchars <int>    # of chars consumed in initial lookup (default: 10)
    --threads <int>         # of threads
    --seed <int>            seed for random number generator
    -q/--quiet              verbose output (for debugging)
    -h/--help               print detailed description of tool and its options
    --usage                 print this usage message
    --version               print version information and quit

I am not sure what I am doing wrong.