Basic preprocessor for your ~/.ssh/config. Useful if you use various versions of ssh regularly, or you want to create a bunch of host entries dynamically.

config.sh

#!/bin/bash
set -e

export SSH=${SSH:-/usr/bin/ssh}
SSH_FULL_NAME="$(${SSH} -V 2>&1)"
SSH_SHORT_NAME="${SSH_FULL_NAME%% *}"

export SSH_PROVIDER=${SSH_SHORT_NAME%%_*}
export SSH_VERSION=${SSH_SHORT_NAME##*_}

function ssh_config {
  cat <<EOF
##########################################################################
# My hosts

host some some.example.com
HostName some.example.com
User theasp

Host github.com
KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1

##########################################################################
# Default
Host *
$(ver-ge 5.9)ControlMaster auto
$(ver-ge 5.9)ControlPersist 15m
$(ver-between 5.9 7.1)ControlPath ~/.ssh/cm-%r@%h:%p
$(ver-ge 7.2)ControlPath ~/.ssh/cm-%C
$(ver-ge 7.2)AddKeysToAgent yes
EOF
}

function compare_ver {
  local -a version
  local op=$1
  local test_ver=${2//[\.p]/ }
  read -ra version <<< ${SSH_VERSION//[\.p]/ }

  local pos=0
  for test_num in $test_ver; do
    cur_num=${version[$pos]}
    cur_num=${cur_num:-0}
    pos=$(( pos + 1 ))

    # BUG: gt and lt act like ge and le
    case $op in
      eq)        
        [[ $cur_num -lt $test_num ]] && return 1
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -gt $test_num ]] && return 1
        ;;

      ne)
        [[ $cur_num -lt $test_num ]] && return 0
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -ge $test_num ]] && return 0
        ;;

      lt)
        [[ $cur_num -lt $test_num ]] && return 0
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -ge $test_num ]] && return 1
        ;;

      le)
        [[ $cur_num -lt $test_num ]] && return 0
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -gt $test_num ]] && return 1
        ;;

      gt)
        [[ $cur_num -gt $test_num ]] && return 0
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -le $test_num ]] && return 1
        ;;

      ge)
        [[ $cur_num -gt $test_num ]] && return 0
        [[ $cur_num -eq $test_num ]] && continue
        [[ $cur_num -lt $test_num ]] && return 1
        ;;

      *)
        log error "Unknown compare opration: $op"
    esac

    return 1
  done
}

function ver-gt {
  compare_ver gt $1 || echo "# Requires $SSH_VERSION > $1 # "
}

function ver-ge {
  compare_ver ge $1 || echo "# Requires $SSH_VERSION >= $1 # "
}

function ver-between {
  (compare_ver ge $1 && compare_ver le $2) || echo "# Requires $1 >= $SSH_VERSION >= $2 # "
}

ssh_config

ssh.sh

#!/bin/bash

set -e

export SSH=${SSH:-/usr/bin/ssh}
export SSH_CONFIG="$HOME/.cache/ssh_config.$(hostname -f)"
export SSH_GEN="${SSH_GEN:-$HOME/.ssh/config.sh}"

if [[ -x $SSH_GEN ]]; then
  if [[ ! -e $SSH_CONFIG ]] || [[ $SSH_GEN -nt $SSH_CONFIG ]]; then
    $SSH_GEN > $SSH_CONFIG
  fi
  
  exec ${SSH} -F $SSH_CONFIG "$@"
else
  exec $SSH "$@"
fi