Why do I have "join: extra operand '/dev/fd/62'" error?

I have a script equijoin2: #! /bin/bash # default args delim="," # CSV by default outer="" outerfile="" # Parse flagged arguments: while getopts "o:td:" flag do case $flag in d) delim=$OPTARG;; t) delim="\t";; o) outer="-a $OPTARG";; ?) exit;; esac done # Delete the flagged arguments: shift $(($OPTIND -1)) # two input files f1="$1" f2="$2" # cols from the input files col1="$3" col2="$4" join "$outer" -t "$delim" -1 "$col1" -2 "$col2" <(sort "$f1") <(sort "$f2") and two files $ cat file1 c c1 b b1 $ cat file2 a a2 c c2 b b2 Why does the last command fail? Thanks. $ equijoin2 -o 2 -d " " file1 file2 1 1 a a2 b b1 b2 c c1 c2 $ equijoin2 -o 1 -d " " file1 file2 1 1 b b1 b2 c c1 c2 $ equijoin2 -d " " file1 file2 1 1 join: extra operand '/dev/fd/62'