add man page about hooks

docs/tinyramfs.hooks.7.scd

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+tinyramfs.hooks(7)
+
+# NAME
+
+Tinyramfs - hooks and related stuff
+
+# DESCRIPTION
+
+Hooks can be located in */usr/share/tinyramfs/hooks* (system path) and
+*/etc/tinyramfs/hooks* (user path). Tinyramfs also allows you to specify custom
+location via *-H* option. See *tinyramfs*(8) for more information.
+
+Hooks provides a way to extend build and init process. Hooks must be written in
+POSIX shell. Bashisms and other non-portable extensions are forbidden. In order
+to write hooks you must know about hook structure.
+
+- <hook> - directory of hook scripts
+	- <hook>           - invoked in build process.
+	- <hook>.init      - invoked in init process.
+	- <hook>.init.late - invoked after root filesystem was mounted.
+
+## MAN PAGE SYNTAX
+
+```
+- B: extension can be used in build process
+- I: extension can be used in init process
+- BI: extension can be used in both processes
+```
+
+# EXTENSIONS
+
+Tinyramfs also provides some extensions and environment variables to easily
+interact with build and init system.
+
+## BI: print <message>
+
+	Print message to stdout.
+
+## BI: panic [message]
+
+	If message was not specified, then tinyramfs will print default error
+	message. otherwise message will be printed.
+
+## B: copy_module <full path>
+
+	Copy kernel module by path to tinyramfs working directory.
+
+## B: copy_binary <name or full path>
+
+	If full path was specified and it's has executable bit, then it's will
+	be copied to /bin location of tinyramfs working directory.
+	
+	If name was specified, then tinyramfs will try to find command by name
+	in PATH. If it's success, command will be copied to /bin location of
+	tinyramfs working directory. Otherwise error message will appear.
+
+## B: copy_file <file> <destination> <mode> <strip>
+
+	<file> must be full path to file.
+
+	<destination> must be directory where <file> should be stored. Tinyramfs
+	will automatically create that directory if it doesn't exist already. Also
+	no need to prepend path of tinyramfs working directory.
+
+	<mode> permissions in octal format.
+
+	<strip> if was set to 1, then tinyramfs will attempt to run strip
+	on file. Tinyramfs will silently ignore errors if strip doesn't
+	exists or failed to strip binary.
+
+# VARIABLES
+
+```
+- BI: debug - 1 if debug mode enabled
+- I: break  - breakpoint for debugging
+- B: tmpdir - full path of tinyramfs working directory (initramfs rootfs in future)
+- B: kernel - kernel version
+- B: moddir - modules directory
+- B: config - config location
+- B: srcdir - directory of tinyramfs system files
+- B: output - output path
+- BI: see tinyramfs.config(5)
+```
+
+# EXAMPLES
+
+This example will show how to handle soft dependencies of ext4 module.
+Create */etc/tinyramfs/hooks/ext4* directory and copy below scripts with
+appropriate names to that directory. After that, prepend *ext4* to *hooks*
+option in tinyrams config.
+
+ext4
+
+```
+print "Copying ext4 dependencies"
+
+for _mod in crc32c libcrc32c; do
+    copy_module "$_mod"
+done
+```
+
+ext4.init
+
+```
+modprobe -a crc32c libcrc32c
+```
+
+# SEE ALSO
+
+*tinyramfs*(8) *tinyramfs.config*(5) *tinyramfs.cmdline*(7) *tinyramfs.hooks*(7)