Mercurial: The Definitive Guide

Conditionally Applying Patches with Guards

Perhaps the best way to maintain sanity with so many targets is to be able to choose specific patches to apply for a given situation. MQ provides a feature called “guards” (which originated from quilt’s guards command) that does just this. To start off, let’s create a simple repository for experimenting in.

$ hg qinit
$ hg qnew hello.patch
$ echo hello > hello
$ hg add hello
$ hg qrefresh
$ hg qnew goodbye.patch
$ echo goodbye > goodbye
$ hg add goodbye
$ hg qrefresh

This gives us a tiny repository that contains two patches that don’t have any dependencies on each other, because they touch different files.

The idea behind conditional application is that you can “tag” a patch with a guard, which is simply a text string of your choosing, then tell MQ to select specific guards to use when applying patches. MQ will then either apply, or skip over, a guarded patch, depending on the guards that you have selected.

A patch can have an arbitrary number of guards; each one is positive (“apply this patch if this guard is selected”) or negative (“skip this patch if this guard is selected”). A patch with no guards is always applied.