Join multiple files

man join:

NAME
       join - join lines of two files on a common field

SYNOPSIS
       join [OPTION]... FILE1 FILE2

it only works with two files.

if you need to join three, maybe you can first join the first two, then join the third.

try:

join file1 file2 | join - file3 > output

that should join the three files without creating an intermediate temp file. - tells the join command to read the first input stream from stdin

One can join multiple files (N>=2) by constructing a pipeline of joins recursively:

#!/bin/sh

# multijoin - join multiple files

join_rec() {
    if [ $# -eq 1 ]; then
        join - "$1"
    else
        f=$1; shift
        join - "$f" | join_rec "$@"
    fi
}

if [ $# -le 2 ]; then
    join "$@"
else
    f1=$1; f2=$2; shift 2
    join "$f1" "$f2" | join_rec "$@"
fi