Procs

Procs is a library to make working with command line applications a little nicer.

The primary use case is when you have to use a command line client in place of an API. Often times you want to do things like output stdout within your own logs or ensure that every time the command is called, there are a standard set of flags that are used.

Basic Usage

The majority of this functionality is intended to be included the procs.Process.

Defining a Command

A command can be defined by a string rather than a []string. Normally, this also implies that the library will run the command in a shell, exposing a potential man in the middle attack. Rather than using a shell, procs lexically parses the command for the different arguments. It also allows for pipes in order to string commands together.

p := procs.NewProcess("kubectl get events | grep dev")

You can also define a new Process by passing in predefined commands.

cmds := []*exec.Cmd{
	exec.Command("kubectl", "get", "events"),
	exec.Command("grep", "dev"),
}

p := procs.Process{Cmds: cmds}

Output Handling

One use case that is cumbersome is using the piped output from a command. For example, lets say we wanted to start a couple commands and have each command have its own prefix in stdout, while still capturing the output of the command as-is.

p := procs.NewProcess("cmd1")
p.OutputHandler = func(line string) string {
	fmt.Printf("cmd1 | %s\n")
	return line
}
out, _ := p.Run()
fmt.Println(out)

Whatever is returned from the OutputHandler will be in the buffered output. In this way you can choose to filter or skip output buffering completely.

You can also define a ErrHandler using the same signature to get the same filtering for stderr.

Environment Variables

Rather than use the exec.Cmd []string environment variables, a procs.Process uses a map[string]string for environment variables.

p := procs.NewProcess("echo $FOO")
p.Env = map[string]string{"FOO": "foo"}

Also, environment variables defined by the Process.Env can be expanded automatically using the os.Expand semantics and the provided environment.

There is a ParseEnv function that can help to merge the parent processes' environment with any new values.

env := ParseEnv(os.Environ())
env["USER"] = "foo"

Finally, if you are building commands manually, the Env function can take a map[string]string and convert it to a []string for use with an exec.Cmd. The Env function also accepts a useEnv bool to help include the parent process environment.

cmd := exec.Command("knife", "cookbook", "show", cb)
cmd.Env = Env(map[string]string{"USER": "knife-user"}, true)

Example Applications

Take a look in the cmd dir for some simple applications that use the library. You can also make all to build them. The examples below assume you've built them locally.

Prelog

The prelog command allows running a command and prefixing the output with a value.

$ ./prelog -prefix foo -- echo 'hello world!'
Running the command
foo | hello world!
Accessing the output without a prefix.
hello world!
Running the command with Start / Wait
foo | hello world!

Cmdtmpl

The cmdtmpl command uses the procs.Builder to create a command based on some paramters. It will take a data.yml file and template.yml file to create a command.

$ cat example/data.json
{
  "source": "https://my.example.org",
  "user": "foo",
  "model": "widget",
  "action": "create",
  "args": "-f new -i improved"
}
$ cat example/template.json
[
  "mysvc ${model} ${action} ${args}",
  "--endpoint ${source}",
  "--username ${user}"
]
$ ./cmdtmpl -data example/data.json -template example/template.json
Command: mysvc foo widget create -f new -i imporoved --endpoint https://my.example.org --username foo
$ ./cmdtmpl -data example/data.json -template example/template.json -field user=bar
Command: mysvc foo widget create -f new -i imporoved --endpoint https://my.example.org --username bar

Procmon

The procmon command acts like foreman with the difference being it uses a JSON file with key value pairs instead of a Procfile. This example uses the procs.Manager to manage a set of procs.Processes.

$ cat example/procfile.json
{
  "web": "python -m SimpleHTTPServer"
}
$ ./procmon -procfile example/procfile.json
web | Starting web with python -m SimpleHTTPServer

You can then access http://localhost:8000 to see the logs. You can also kill the child process and see procmon recognizing it has exited and exit itself.