setup-ssh-agent

#!/bin/sh
# Hey EMACS, this should be in -*- sh -*- mode
#
# This starts ssh-agent in a predictable location if it's not already configured
# The location is ${HOME}/.ssh/agents/${HOSTNAME}
#
# To use this script you must source it so that the environment variable SSH_AUTH_SOCK can be set
# This is best done from your bashrc
#  if [ -e "${HOME}"/.ssh/setup-ssh-agent ]; then
#    source "${HOME}"/.ssh/setup-ssh-agent
#  fi

debug() { ! "${log_debug-false}" || log "DEBUG: $*" >&2; }
log() { printf '%s\n' "$*"; }
warn() { log "WARNING: $*" >&2; }
error() { log "ERROR: $*" >&2; }
try() { "$@" || error "'$@' failed"; }

# Canonicalize hostname to just the short name
if [ -z "${HOSTNAME}" ]; then 
    HOSTNAME=$(hostname)
fi
HOSTNAME=$(echo ${HOSTNAME} | sed 's/\..*//')

SOCKET_FILENAME="${HOME}"/.ssh/agents/${HOSTNAME}

# start agent if instructed to
start_agent() {
    if [ -e "${HOME}/.ssh/.start-ssh-agent" ]; then
        debug "Starting agent with ${SOCKET_FILENAME}"        
        # remove any existing socket that might be left over
        try rm -f "${SOCKET_FILENAME}"
        
        try ssh-agent -a "${SOCKET_FILENAME}" > /dev/null 2>&1
        SSH_AUTH_SOCK="${SOCKET_FILENAME}"
        export SSH_AUTH_SOCK
        debug "agent started"
    else
        debug "Agent not started due to user request"
    fi
}

if [ -z "${SSH_AUTH_SOCK}" ]; then
    debug "No socket variable defined"
    if [ -e "${SOCKET_FILENAME}" ]; then
        debug "Attempting to use existing socket at '${SOCKET_FILENAME}'"
        SSH_AUTH_SOCK="${SOCKET_FILENAME}"
        export SSH_AUTH_SOCK
    fi
fi

# check if SOCKET_FILENAME is valid, if so use it
# this assumes that the oldest valid socket is the one to keep
if [ -e "${SOCKET_FILENAME}" -a -n "${SSH_AUTH_SOCK}" ]; then
    debug "Saving ${SSH_AUTH_SOCK} while testing ${SOCKET_FILENAME}"
    ORIG_SSH_AUTH_SOCK=${SSH_AUTH_SOCK}
    export ORIG_SSH_AUTH_SOCK
    
    debug "Attempting to use existing socket at '${SOCKET_FILENAME}'"
    SSH_AUTH_SOCK="${SOCKET_FILENAME}"
    export SSH_AUTH_SOCK

    # check if the socket is valid
    output=$(timeout 2s ssh-add -l 2>&1)
    ssh_add_status=$?
    debug "ssh-add status ${ssh_add_status}"
    debug "ssh-add output '${output}'"
    if [ ${ssh_add_status} -eq 0 -o "${output#The agent has no identities}" != "${output}" ]; then
        debug "Valid SSH socket at '${SSH_AUTH_SOCK}'"
    else
        debug "Socket at '${SSH_AUTH_SOCK}' is not valid using '${ORIG_SSH_AUTH_SOCK}'"
        SSH_AUTH_SOCK=${ORIG_SSH_AUTH_SOCK}
        export SSH_AUTH_SOCK
    fi
fi

if [ -z "${SSH_AUTH_SOCK}" -a -n "${ORIG_SSH_AUTH_SOCK}" ]; then
    SSH_AUTH_SOCK=${ORIG_SSH_AUTH_SOCK}
    export SSH_AUTH_SOCK
fi

# ensure the directory exists
if [ ! -d "${HOME}"/.ssh/agents ]; then
    try mkdir -p "${HOME}"/.ssh/agents
    chmod go-rwx "${HOME}"/.ssh
fi

if [ -n "${SSH_AUTH_SOCK}" ]; then
    debug "Socket variable is defined as ${SSH_AUTH_SOCK}"

    # check if the socket is valid
    output=$(timeout 2s ssh-add -l 2>&1)
    ssh_add_status=$?
    debug "ssh-add status ${ssh_add_status}"
    debug "ssh-add output '${output}'"
    if [ ${ssh_add_status} -eq 0 -o "${output#The agent has no identities}" != "${output}" ]; then
        debug "Valid SSH socket at '${SSH_AUTH_SOCK}'"

        if [ "${SSH_AUTH_SOCK}" != "${SOCKET_FILENAME}" ]; then
            debug "Valid socket is not in the expected location, setting symlink and environment variable"
            ln -sf "${SSH_AUTH_SOCK}" "${SOCKET_FILENAME}"
            SSH_AUTH_SOCK="${SOCKET_FILENAME}"
            export SSH_AUTH_SOCK
        fi
    elif [ -n "${ORIG_AUTH_SOCK}" ]; then
        debug "${SSH_AUTH_SOCK} is invalid trying ${ORIG_AUTH_SOCK}"
        SSH_AUTH_SOCK=${ORIG_SSH_AUTH_SOCK}
        export SSH_AUTH_SOCK
        output=$(timeout 2s ssh-add -l 2>&1)
        ssh_add_status=$?
        debug "ssh-add status ${ssh_add_status}"
        debug "ssh-add output '${output}'"
        if [ ${ssh_add_status} -eq 0 -o "${output#The agent has no identities}" != "${output}" ]; then
            debug "Valid SSH socket at '${SSH_AUTH_SOCK}'"
        else
            debug "Invalid SSH socket at '${SSH_AUTH_SOCK}', clearing"
            unset SSH_AUTH_SOCK
            unset ORIG_SSH_AUTH_SOCK
            start_agent
        fi
    else
        debug "Invalid SSH socket at '${SSH_AUTH_SOCK}', clearing"
        unset SSH_AUTH_SOCK
        start_agent
    fi
else
    debug "No socket defined, starting agent"
    start_agent
fi

# cleanup
unset SOCKET_FILENAME