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diff --git a/_posts/2020-05-20-makefile-escaping.md b/_posts/2020-05-20-makefile-escaping.md new file mode 100644 index 0000000..f393c51 --- /dev/null +++ b/_posts/2020-05-20-makefile-escaping.md @@ -0,0 +1,319 @@ +--- +title: Escaping characters in Makefile +excerpt: Making less error-prone. +--- +I'm a big sucker for irrelevant neatpicks like properly quoting arguments in +shell scripts. +I've also recently started using GNU make as a substitute for one-line shell +scripts (so instead of a bunch of scripts like build.sh, deploy.sh, test.sh I +get to have a single Makefile and can just run `make build`, `make deploy`, +`make test`). + +As a side note, there's an excellent [Makefile style guide] available on the +web. +I'm going to be using the prologue suggested in the guide in all Makefiles in +this post: + +[Makefile style guide]: https://clarkgrubb.com/makefile-style-guide + +``` +MAKEFLAGS += --warn-undefined-variables +.DEFAULT_GOAL := all +.DELETE_ON_ERROR: +.SUFFIXES: +SHELL := bash +.SHELLFLAGS := -e -o pipefail -c +``` + +`make` invokes a shell program to execute recipes. +As issues of properly escaping "special" characters are going to be discussed, +the choice of shell is very relevant. +The Makefiles in this post specify `bash` explicitly using the `SHELL` +variable, but the same rules should apply for all similar `sh`-like shells. + +TL;DR +----- + +* Quote command arguments in Makefiles using single quotes `'`. +* Don't use `'` and `$` in stuff like file paths/environment variable values, +and you're good to go. +* To escape `$(shell)` function output, define a helper function: + + escape = $(subst ','\'',$(1)) + + You can then replace `'$(shell your-command arg1 arg2)'` with +`'$(call escape,$(shell your-command arg1 arg2))'`. +* To escape environment variable values, define another helper function: + + escape_var = $(call escape,$(value $(1))) + + You can then replace `'$(VAR_NAME)'` with `'$(call escape_var,VAR_NAME)'`. +* Don't override variables using `make var1=value1 var2=value2`, use your shell +instead: `var1=value1 var2=value2 make`. + +Quoting arguments +----------------- + +You should quote command arguments in `make` rule recipes, just like in shell +scripts. +This is to prevent a single argument from being expanded into multiple +arguments by the shell. + +``` +$ cat > Makefile +test_var := Same line? +export test_var + +test: + printf '%s\n' $(test_var) + printf '%s\n' '$(test_var)' + printf '%s\n' $$test_var + printf '%s\n' "$$test_var" + +$ make test +printf '%s\n' Same line? +Same +line? +printf '%s\n' 'Same line?' +Same line? +printf '%s\n' $test_var +Same +line? +printf '%s\n' "$test_var" +Same line? +``` + +This is quite often sufficient to write valid recipes. + +One thing to note is that you shouldn't use double quotes `"` for quoting +arguments, as they might contain literal dollar signs `$`, interpreted by the +shell as variable references, which is not something you always want. + +Escaping quotes +--------------- + +What if `test_var` included a single quote `'`? +In that case, even the quoted `printf` invocation would break because of the +mismatch. + +``` +$ cat > Makefile +test_var := Includes ' quote + +test: + printf '%s\n' '$(test_var)' + +$ make test +printf '%s\n' 'Includes ' quote' +bash: -c: line 0: unexpected EOF while looking for matching `'' +make: *** [Makefile:11: test] Error 2 +``` + +One solution is to take advantage of how `bash` parses command arguments, and +replace every quote `'` by `'\''`. +This works because `bash` merges a string like `'Includes '\'' quote'` into +`Includes ' quote`. + +``` +$ cat > Makefile +escape = $(subst ','\'',$(1)) + +test_var := Includes ' quote + +test: + printf '%s\n' '$(call escape,$(test_var))' + +$ make test +printf '%s\n' 'Includes '\'' quote' +Includes ' quote +``` + +Surprisingly, this works even in much more complicated cases. +You can have a recipe that executes a command that takes a whole other command +(with its own separate arguments) as an argument. +I guess the most common use case is doing something like `ssh 'rm -rf +$(junk_dir)'`, but I'll use nested `bash` calls instead for simplicity. + +``` +$ cat > Makefile +escape = $(subst ','\'',$(1)) + +test_var := Includes ' quote + +echo_test_var := printf '%s\n' '$(call escape,$(test_var))' +bash_test_var := bash -c '$(call escape,$(echo_test_var))' + +test: + printf '%s\n' '$(call escape,$(test_var))' + bash -c '$(call escape,$(echo_test_var))' + bash -c '$(call escape,$(bash_test_var))' + +$ make test +printf '%s\n' 'Includes '\'' quote' +Includes ' quote +bash -c 'printf '\''%s\n'\'' '\''Includes '\''\'\'''\'' quote'\''' +Includes ' quote +bash -c 'bash -c '\''printf '\''\'\'''\''%s\n'\''\'\'''\'' '\''\'\'''\''Includes '\''\'\'''\''\'\''\'\'''\'''\''\'\'''\'' quote'\''\'\'''\'''\''' +Includes ' quote +``` + +That's somewhat insane, but it works. + +Shell output +------------ + +The `shell` function is one of the two most common ways to communicate with the +outside world in a Makefile (the other being environment variables). +This little `escape` function we've defined is actually sufficient to deal with +the output of the `shell` function safely. + +``` +$ cat > Makefile +escape = $(subst ','\'',$(1)) + +cwd := $(shell basename -- "$$( pwd )") +export cwd + +echo_cwd := printf '%s\n' '$(call escape,$(cwd))' +bash_cwd := bash -c '$(call escape,$(echo_cwd))' + +.PHONY: test +test: + @printf '%s\n' '$(call escape,$(cwd))' + @printf '%s\n' "$$cwd" + @bash -c '$(call escape,$(echo_cwd))' + @bash -c '$(call escape,$(bash_cwd))' + +$ ( mkdir -p -- "Includes ' quote" && cd -- "Includes ' quote" && make -f ../Makefile test ; ) +Includes ' quote +Includes ' quote +Includes ' quote +Includes ' quote + +$ ( mkdir -p -- 'Maybe a comment #' && cd -- 'Maybe a comment #' && make -f ../Makefile test ; ) +Maybe a comment # +Maybe a comment # +Maybe a comment # +Maybe a comment # + +$ ( mkdir -p -- 'Variable ${reference}' && cd -- 'Variable ${reference}' && make -f ../Makefile test ; ) +Variable ${reference} +Variable ${reference} +Variable ${reference} +Variable ${reference} +``` + +Environment variables +--------------------- + +Makefiles often have parameters that modify their behaviour. +The most common example is doing something like `make install +PREFIX=/somewhere/else`, where the `PREFIX` argument overrides the default +value "/usr/local". +These parameters are often defined in a Makefile like this: + +``` +param_name ?= Default value +``` + +They should be quoted when passed to external commands, of course. +To prevent mismatched quotes, the `escape` function might seem useful, but in +case of environment variables, a complication arises when they contain dollar +signs `$`. +`make` variables may contain references to other variables, and they're +expanded recursively either when defined (for `:=` assignments) or when used +(in all other cases, including `?=`). + + +``` +$ cat > Makefile +escape = $(subst ','\'',$(1)) + +test_var ?= This is safe. +export test_var + +echo_test_var := printf '%s\n' '$(call escape,$(test_var))' +bash_test_var := bash -c '$(call escape,$(echo_test_var))' + +.PHONY: test +test: + @printf '%s\n' '$(call escape,$(test_var))' + @printf '%s\n' "$$test_var" + @bash -c '$(call escape,$(echo_test_var))' + @bash -c '$(call escape,$(bash_test_var))' + +$ test_var='Variable ${reference}' make test +Makefile:18: warning: undefined variable 'reference' +Variable +Variable ${reference} +Variable +Variable +``` + +Here, `$(test_var)` is expanded recursively, substituting an empty string for +the `${reference}` part. +One attempt to solve this is to escape the dollar sign in the variable value, +but that breaks the `"$$test_var"` case: + +``` +$ test_var='Variable $${reference}' make test +Variable ${reference} +Variable $${reference} +Variable ${reference} +Variable ${reference} +``` + +A working solution would be to use the `escape` function on the unexpanded +variable value. +Turns out, you can do just that using the `value` function in `make`. + +``` +$ cat > Makefile +escape = $(subst ','\'',$(1)) +escape_var = $(call escape,$(value $(1))) + +test_var ?= This is safe. +export test_var + +echo_test_var := printf '%s\n' '$(call escape_var,test_var)' +bash_test_var := bash -c '$(call escape_var,echo_test_var)' + +.PHONY: test +test: + @printf '%s\n' '$(call escape_var,test_var)' + @printf '%s\n' "$$test_var" + @bash -c '$(call escape_var,echo_test_var)' + @bash -c '$(call escape_var,bash_test_var)' + +$ test_var='Variable ${reference}' make test +Variable ${reference} +Variable ${reference} +Variable ${reference} +Variable ${reference} +``` + +You can still use the original `escape` function to escape `shell` output. + +One thing to note is that I couldn't find a way to prevent variable values from +being expanded when [overriding variables] on the command line. +For example, this doesn't work: + +[overriding variables]: https://www.gnu.org/software/make/manual/html_node/Overriding.html#Overriding + +``` +$ make test test_var='Variable ${reference}' +make: warning: undefined variable 'reference' +Variable ${reference} +Variable +Variable ${reference} +Variable ${reference} + +$ make test test_var='Variable $${reference}' +Variable $${reference} +Variable ${reference} +Variable $${reference} +Variable $${reference} +``` + +As a workaround, set parameter values using your shell: `var_name=value make +...`. |