added multifile document for linux things

docs/linux/commands.md

@@ -0,0 +1,13 @@
+# Linux commands
+
+- [Linux commands](#linux-commands)
+  - [combine commands with *xargs*](#combine-commands-with-xargs)
+
+## combine commands with *xargs*
+
+Some commands (e.g. `chmod`) can't handle input via pipe and require the input as arguments. `xargs` takes the standard input as list of arguments seperated by spaces or newlines and executes the given command with the input parsed to a list of arguments.
+
+Usage:
+```bash
+find . -type f | xargs chmod 644
+```