A Small m4 Recipe

15 Feb 2016

“They” say that Linux m4 is a useful tool but I’ve never had the time to get my head around it. This is a small recipe.

I find m4 difficult because the examples are more complex than what I need, therefore I never use it, therefore I never get better at it. Here’s a starter example, which could’ve been done with any number of other tools (sed, perl, etc). But why m4? Because it does the job and no installation is required on (locked down) Linux servers.

This snippet dynamically generates an ssh config file (with ProxyCommand):

divert(-1)dnl
define(`SLAVEX',slavex)
define(`USERX',userx)
define(`MASTERX',masterx)
divert(0)dnl
StrictHostKeyChecking no
Host SLAVEX
  ProxyCommand ssh -W %h:%p USERX@MASTERX

In ruby use it like this:

m4cmd = "m4 -Dslavex=#{row[:ip]} -Duserx=#{row[:jump_user]}
  -Dmasterx=#{row[:jump_ip]} ssh_config.m4 > ../ssh_config.toll"
`#{m4cmd}`

I use variables like userx so they don’t accidently get rescanned by m4:

m4 copies its input (from files or standard input) to standard output. It checks each token (a name, a quoted string, or any single character that’s not a part of either a name or a string) to see if it’s the name of a macro. If so, the token is replaced by the macro’s value, and then that text is pushed back onto the input to be rescanned. (If you’re new to m4, this repeated scanning may surprise you, but it’s one key to m4’s power.)