Add a negation flag for the success command.

Modifies the trigger to run when the command fails instead of
when it succeeds.

src/exec.rs

@@ -67,6 +67,7 @@ fn is_cmd_success(cmd: &str, env: Option<Vec<&str>>) -> bool {
 
 pub struct ExecProcess<'a> {
     test_cmd: &'a str,
+    negate: bool,
     cmd: &'a str,
     env: Option<Vec<&'a str>>,
     poll: Duration,
@@ -75,10 +76,12 @@ pub struct ExecProcess<'a> {
 impl<'a> ExecProcess<'a> {
     pub fn new(test_cmd: &'a str,
                cmd: &'a str,
+               negate: bool,
                env: Option<Vec<&'a str>>,
                poll: Duration) -> ExecProcess<'a> {
         ExecProcess {
             test_cmd: test_cmd,
+            negate: negate,
             cmd: cmd,
             env: env,
             poll: poll,
@@ -90,7 +93,8 @@ impl<'a> Process for ExecProcess<'a> {
     fn run(&self) -> Result<(), CommandError> {
         loop {
             // TODO(jwall): Should we set the environment the same as the other command?
-            if is_cmd_success(self.test_cmd, None) {
+            let test_result = is_cmd_success(self.test_cmd, None);
+            if (test_result && !self.negate) || (!test_result && self.negate) {
                 if let Err(err) = run_cmd(self.cmd, &self.env) {
                     println!("{:?}", err)
                 }

src/main.rs

@@ -62,6 +62,8 @@ fn do_flags<'a>() -> clap::ArgMatches<'a> {
              (about: "Trigger that fires if a command runs successful.")
              (@arg ifcmd: --if +required +takes_value
               "The command to test for successful exit from")
+             (@arg not: --not
+              "Negate the test command so we run on failure instead of success.")
              (@arg wait: --poll +takes_value
               "How frequently to test command (default 5s)")
             )
@@ -122,9 +124,10 @@ fn main() {
     } else if let Some(matches) = app.subcommand_matches("success") {
         // unwrap because this is required.
         let ifcmd = matches.value_of("ifcmd").expect("ifcmd flag is required");
+        let negate = matches.is_present("not");
         let dur = humantime::parse_duration(matches.value_of("poll").unwrap_or("5s"));
         process = match dur {
-            Ok(duration) => Some(Box::new(ExecProcess::new(ifcmd, cmd, maybe_env, duration))),
+            Ok(duration) => Some(Box::new(ExecProcess::new(ifcmd, cmd, negate, maybe_env, duration))),
             Err(msg) => {
                 println!("Malformed poll {:?}", msg);
                 process::exit(1)