zsh: functions: add watch_file function

this can be used to watch a file for changes and execute the
given action on changes.

functions/misc.zsh

@@ -284,3 +284,19 @@ function mangrep() {
 
     MANPAGER="less -p \"$pattern\"" man "$cmd"
 }
+
+
+# watches the given file and executes the given action whenever the file is changed. Usage: watch_file <file> <command>
+function watch_file() {
+  if ! which inotifywait &>/dev/null ; then echo "$0 requires inotifywait"; return 1; fi
+  [[ -z "$1" ]] && "Usage: $0 <file> <action>" && return 1
+  local file
+  file="$1"
+  shift
+
+  [[ -z "$@" ]] && "Usage: $0 <file> <action>" && return 1
+
+  local action
+  action="$@"
+  while inotifywait -e close_write "$file"; do zsh -c "$action"; done
+}