From 3497688ab98a793f570d72441619f1aeb065e292 Mon Sep 17 00:00:00 2001
From: Andreas Ntaflos <daff@pseudoterminal.org>
Date: Wed, 6 Jul 2016 19:14:45 +0200
Subject: [PATCH] Add tip to "Automatically install or update dotfiles when
 ssh'ing into a remote machine"

---
 Tips-and-Tricks.md | 48 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/Tips-and-Tricks.md b/Tips-and-Tricks.md
index 570aa4b..d433119 100644
--- a/Tips-and-Tricks.md
+++ b/Tips-and-Tricks.md
@@ -1,2 +1,46 @@
-**How can I have different groups of tasks for different hosts with different configurations?**
-See [here](https://github.com/anishathalye/dotbot/pull/11#issuecomment-73082152) for information on using machine-specific configs.
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+## How can I have different groups of tasks for different hosts with different configurations?
+
+See [here](https://github.com/anishathalye/dotbot/pull/11#issuecomment-73082152) for information on using machine-specific configs.
+
+## Automatically install or update dotfiles when ssh'ing into a remote machine (or: let my dotfiles follow me)
+
+In your local `~/.ssh/config`:
+
+```
+Host some.remote.host.example.com
+    PermitLocalCommand yes
+    # Unfortunately ssh does not support line breaks in config files
+    LocalCommand ssh -o PermitLocalCommand=no %n "which git >/dev/null && ([[ -d ~/dotfiles ]] && (echo "Updating dotfiles on %h ..." && cd ~/dotfiles && git pull -q && ./install >/dev/null) || (echo "Installing dotfiles on %h ..." && git clone -q https://github.com/MYNAMESPACE/dotfiles && ./dotfiles/install >/dev/null))"
+```
+
+Relevant part broken down for readability:
+
+```bash
+LocalCommand ssh -o PermitLocalCommand=no %n "which git >/dev/null && ([[ -d ~/dotfiles ]] && \
+  (echo "Updating dotfiles on %h ..." && cd ~/dotfiles && git pull -q && ./install >/dev/null) || \
+  (echo "Installing dotfiles on %h ..." && git clone -q https://github.com/MYNAMESPACE/dotfiles && ./dotfiles/install >/dev/null))"
+```
+
+The main attraction here is the clever use of `LocalCommand`:
+
+> `LocalCommand` Specifies a command to execute on the *local* machine after successfully connecting to the server.
+
+We use `LocalCommand` to run a *second* SSH session to connect to the same remote machine and execute the defined command line in that SSH session. So what happens is this:
+
+1. Initiate SSH connection to remote machine (`ssh user@some.remote.host.example.com`)
+2. If the connection is successful (including authentication) then `LocalCommand` is executed
+3. The `LocalCommand` initiates a second SSH connection to the same remote machine and executes the command line specified. This commandline updates or installes the dotfiles.
+
+   The second SSH connection sets `-o PermitLocalCommand=no` so that *no local command* is executed for that SSH connection. Without this setting each SSH connection would initiate another SSH connection, which would initiate another SSH connection, ad infinitum. Wo don't want that.
+4. When the command line of the second SSH connection is finished then the `LocalCommand` is finished
+5. The initial SSH session is finally established and the remote shell becomes available
+
+The command line is in Bash syntax so adapt it if you use a different shell.
+
+First we check if `git` is even available (if it is not then nothing more will happen). Then, if there is a `dotfiles` directory in the user's home we `cd` into it, run `git pull` and `install`, thus updating the local dotfiles repository and installing any new or changed files or symlinks.
+
+If there isn't a `dotfiles` repository in the user's home we do a fresh installation by cloning the dotfiles repo from Github and running the `install` command.
+
+Obviously this `LocalCommand` is executed every time you connect to the remote machine so it will take a few seconds before your remote shell becomes available.
+
+This works best if you use public key authentication (or GSSAPI/Kerberos authentication) so SSH doesn't ask for a password when logging in. If you do use password authentication then you will need to enter your password once for each of the two SSH connections.
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