Join two files based on a column

The easy way is via awk - just read the 2nd file, save each line into an array (where the index is $1) and when reading 1st file check if the line is already an index in array - if so replace with the value of that element:

awk -F: 'NR==FNR{z[$1]=$0;next}
($0 in z) {$0=z[$1]};1' file2 file1

You can do this with join too but it requires more work (basically, numbering lines in 1st file so as to be able to sort the result of join and restore the order):

join -a 1 -t: -1 2 -2 1 <(nl -s: -ba -nrz file1 | sort -t: -k2,2) \
<(sort -t: -k1,1 file2) | sort -t: -k2,2 | cut -d: -f1,3-

Basically like this for your 3 files example

$ join file2 file3| join file1 -
1 test1 example1 foo1
2 test3 example2 foo2
3 test4 example3 foo3
4 test5 example4 foo4

But important all your input files must be sorted already (sort -k 1b,1, numerical sorted like your example may not work!). So the example above sorted on-the-fly could be written in bash like this:

join <(sort -k 1b,1 file2) <(sort -k 1b,1 file3) | join <(sort -k 1b,1 file1) -\
  | sort -k 1n,1

And finally the generic case for n files using a recursive function (tested in bash).:

xjoin() {
    local f
    local srt="sort -k 1b,1"

    if [ "$#" -lt 2 ]; then
            echo "xjoin: need at least 2 files" >&2
            return 1
    elif [ "$#" -lt 3 ]; then
            join <($srt "$1") <($srt "$2")
    else
            f=$1
            shift
            join <($srt "$f") <(xjoin "$@")
    fi
}

xjoin file1 file2 file3 | sort -k 1n,1

If you know what you are doing you may omit the sort pipes. But from my experience join without explicit sort is very often the cause of trouble.