yadm/_docs/060_alternates.md

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Alternate Files	/docs/alternates

When possible, it is best to use the same files across all systems. However, there are occasions when you need different files in some places. Below are features and strategies for dealing with those occasions.

Symlink alternates

It can be useful to have an automated way of choosing an alternate version of a file for a different operating system, host, user, etc.

yadm will automatically create a symbolic link to the appropriate version of a file, when a valid suffix is appended to the filename. The suffix contains the conditions that must be met for that file to be used.

The suffix begins with ##, followed by any number of conditions separated by commas.

##<condition>[,<condition>,…]

Each condition is an attribute/value pair, separated by a period. Some conditions do not require a "value", and in that case, the period and value can be omitted. Most attributes can be abbreviated as a single letter. Values are compared case-insensitive.

Attribute	Meaning
arch, a	Valid if the value matches the architecture. Architecture is calculated by running uname ‑m.
class, c	Valid if the value matches the local.class configuration. Class must be manually set using yadm config local.class <class>.
default	Valid when no other alternate is valid.
distro, d	Valid if the value matches the distro. Distro is calculated by running lsb_release ‑si or inspecting ID from /etc/os-release
distro_family, f	Valid if the value matches the distro family. Distro family is calculated by inspecting ID_LIKE from /etc/os-release (or ID if ID_LIKE isn't found)
extension, e	A special "condition" that doesn't affect the selection process. Its purpose is instead to allow the alternate file to end with a certain extension to e.g. make editors highlight the content properly.
hostname, h	Valid if the value matches the short hostname. Hostname is calculated by running uname ‑n, and trimming off any domain.
os, o	Valid if the value matches the OS. OS is calculated by running uname ‑s. *
template, t	Valid when the value matches a supported template processor. See the Templates section for more details.
user, u	Valid if the value matches the current user. Current user is calculated by running id ‑u ‑n.

<sub><sup>* The OS for "Windows Subsystem for Linux" is reported as "WSL", even though uname identifies as "Linux".
* The OS for Linux-like runtimes for Windows (e.g. MinGW, Cygwin) is obtained by running uname -o.</sup></sub>

You may use any number of conditions, in any order. An alternate will only be used if ALL conditions are valid. For all files managed by yadm's repository or listed in $HOME/.config/yadm/encrypt, if they match this naming convention, symbolic links will be created for the most appropriate version.

The "most appropriate" version is determined by calculating a score for each version of a file. A template is always scored higher than any symlink condition. The number of conditions is the next largest factor in scoring. Files with more conditions will always be favored. Any invalid condition will disqualify that file completely.

If you don't care to have all versions of alternates stored in the same directory as the generated symlink, you can place them in the $HOME/.config/yadm/alt directory. The generated symlink or processed template will be created using the same relative path.

Alternate linking may best be demonstrated by example. Assume the following files are managed by yadm's repository:

$HOME/path/example.txt##default
$HOME/path/example.txt##class.Work
$HOME/path/example.txt##os.Darwin
$HOME/path/example.txt##os.Darwin,hostname.host1
$HOME/path/example.txt##os.Darwin,hostname.host2
$HOME/path/example.txt##os.Linux
$HOME/path/example.txt##os.Linux,hostname.host1
$HOME/path/example.txt##os.Linux,hostname.host2

If running on a MacBook named host2, yadm will create a symbolic link which looks like this:

$HOME/path/example.txt → $HOME/path/example.txt##os.Darwin,hostname.host2

However, on another MacBook named host3, yadm will create a symbolic link which looks like this:

$HOME/path/example.txt → $HOME/path/example.txt##os.Darwin

Since the hostname doesn't match any of the managed files, the more generic version is chosen. If running on a Linux server named host4, the link will be:

$HOME/path/example.txt → $HOME/path/example.txt##os.Linux

If running on a Solaris server, the link will use the default version:

$HOME/path/example.txt → $HOME/path/example.txt##default

If running on a system, with class set to Work, the link will be:

$HOME/path/example.txt → $HOME/path/example.txt##class.Work

If no ##default version exists and no files have valid conditions, then no link will be created.

Links are also created for directories named this way, as long as they have at least one yadm managed file within them (at the top level).

yadm will automatically create these links by default. This can be disabled using the yadm.auto-alt configuration. Even if disabled, links can be manually created by running yadm alt.

Class and Overrides

Class is a special value which is stored locally on each host (inside the local repository). To use alternate symlinks using ##class.<CLASS>, you must set the value of class using the configuration local.class. This is set like any other yadm configuration—with the yadm config command. The following sets the local.class to be "Work".

yadm config local.class Work

Similarly, the values of arch, os, hostname, user, distro, and distro_family can be manually overridden using the configuration options local.arch, local.os, local.hostname, local.user, local.distro, and local.distro-family.

Additional local classes can be defined using the --add switch.

yadm config --add local.class <additional-class>

You can display all of the classes using --get-all.

yadm config --get-all local.class

Templates

Templates are another powerful tool for creating alternate content on each host. See the Templates documentation for full details.

Strategies for alternate files on different systems

Where possible, you should try to use the same file on every system. Here are a few examples:

.vimrc

let OS=substitute(system('uname -s'),"\n","","")
if (OS == "Darwin")
    " do something that only makes sense on a Mac
endif

.tmux.conf

# use reattach-to-user-namespace as the default command on OSX
if-shell "test -f /usr/local/bin/reattach-to-user-namespace" 'set -g default-command "reattach-to-user-namespace -l bash"'

.bash_profile

system_type=$(uname -s)
if [ "$system_type" = "Darwin" ]; then
    eval $(gdircolors $HOME/.dir_colors)
else
    eval $(dircolors -b $HOME/.dir_colors)
fi

.gitconfig

However, sometimes the type of file you are using doesn't allow for this type of logic. If a configuration can do an "include", you can include a specific alternate version using yadm. Consider these three files:

.gitconfig

[log]
    decorate = short
    abbrevCommit = true
[include]
    path = .gitconfig.local

.gitconfig.local##os.Darwin

[user]
    name = Tim Byrne
    email = tim@personal.email.org

.gitconfig.local##os.Linux

[user]
    name = Dr. Tim Byrne
    email = dr.byrne@work.email.com

Configuring Git this way includes .gitconfig.local in the standard .gitconfig. yadm will automatically link the correct version based on the operating system. The bulk of your configurations can go in a single file, and you just put the exceptions in OS-specific files.