nsg's blog

The ssh command has included bash completion in most systems, for example you can complete common host names and on some systems even hosts previously connected to from the known_hosts file, but this file do not contain useful hostnames since many years due to security reasons.

At work we I connect to hundred of hosts and I can't use the known_hosts for for tab completion. I use ^r a lot to search for old entries and that works in a few cases. It that fails, there is a lot of typing.

First I thought, we have a full inventory of all our hosts so it should be really simple for me just to write a bash function and then call it with complete -F and I'm done. I then realized that our hosts follows a pattern of nycprod-mariadb-master01.example.com. A ssh nyc<tab> would still list hundreds of hosts and I had to write all the way down to ssh nycprod-mariadb<tab> to get a useful short list. To make it worse, we have a old and an new naming pattern so it not always easy to guess the name. Old legacy systems that we have not bothered to rename.

So I thought, what if I could find the host above by just typing ssh mariadb<tab>, would that be possible? It is!

The code

The function maintains a cache of all hosts at /tmp/__s__list_all_hosts__cache. It will call a function called __s__cache_all_hosts that will update the cache file from our inventory if needed. All hosts are then printed.

__s__list_all_hosts() {

    local cache_path=/tmp/__s__list_all_hosts__cache
    local cache_age=86400

    if [ -f $cache_path ]; then
        local cache_age=$(( $(date +%s) - $(date -r $cache_path +%s) ))
    fi

    if [ $cache_age -gt 300 ]; then
        __s__cache_all_hosts $cache_path
    fi

    cat $cache_path
}

This small function wraps grep so it supports * as a glob.

__s__grep() {
    grep -E "${1//\*/.*}"
}

The actual magic, this lists all hosts, uses grep to filter out the matches and finally updates the COMPREPLY array with a little help from mapfile.

__s__complete() {
    local word="${COMP_WORDS[COMP_CWORD]}";
    mapfile -t COMPREPLY < <(__s__list_all_hosts | __s__grep $word)
}

A simple function that creates an "alias" called just "s", and finally the completion. I mapped this to "s" instead of "ssh" because it was easier, and I like to have the logic separated.

s() {
    /usr/bin/ssh $@
}

complete -F __s__complete s

It's now possible for me to type things like s mariadb<tab> or s nyc*maria<tab>.