1. External Processes

External Processes 

Usage 

Scala includes a process library to simplify working with external processes. Use import scala.sys.process._ to bring the implicit conversions into scope.

To run an external command, follow it with an exclamation mark !:

"find project -name *.jar" !

An implicit converts the String to scala.sys.process.ProcessBuilder, which defines the ! method. This method runs the constructed command, waits until the command completes, and returns the exit code. Alternatively, the run method defined on ProcessBuilder runs the command and returns an instance of scala.sys.process.Process, which can be used to destroy the process before it completes. With no arguments, the ! method sends output to standard output and standard error. You can pass a Logger to the ! method to send output to the Logger:

"find project -name *.jar" ! log

If you need to set the working directory or modify the environment, call scala.sys.process.Process explicitly, passing the command sequence (command and argument list) or command string first and the working directory second. Any environment variables can be passed as a vararg list of key/value String pairs.

Process("ls" :: "-l" :: Nil, Path.userHome, "key1" -> value1, "key2" -> value2) ! log

Operators are defined to combine commands. These operators start with # in order to keep the precedence the same and to separate them from the operators defined elsewhere in sbt for filters. In the following operator definitions, a and b are subcommands.

  • a #&& b Execute a. If the exit code is nonzero, return that exit code and do not execute b. If the exit code is zero, execute b and return its exit code.
  • a #|| b Execute a. If the exit code is zero, return zero for the exit code and do not execute b. If the exit code is nonzero, execute b and return its exit code.
  • a #| b Execute a and b, piping the output of a to the input of b.

There are also operators defined for redirecting output to Files and input from Files and URLs. In the following definitions, url is an instance of URL and file is an instance of File.

  • a #< url or url #> a Use url as the input to a. a may be a File or a command.
  • a #< file or file #> a Use file as the input to a. a may be a File or a command.
  • a #> file or file #< a Write the output of a to file. a may be a File, URL, or a command.
  • a #>> file or file #<< a Append the output of a to file. a may be a File, URL, or a command.

There are some additional methods to get the output from a forked process into a String or the output lines as a Stream[String]. Here are some examples, but see the ProcessBuilder API for details.

val listed: String = "ls" !!
val lines2: Stream[String] = "ls" lines_!

Finally, there is a cat method to send the contents of Files and URLs to standard output.

Examples 

Download a URL to a File:

url("http://databinder.net/dispatch/About") #> file("About.html") !
// or
file("About.html") #< url("http://databinder.net/dispatch/About") !

Copy a File:

file("About.html") #> file("About_copy.html") !
// or
file("About_copy.html") #< file("About.html") !

Append the contents of a URL to a File after filtering through grep:

url("http://databinder.net/dispatch/About") #> "grep JSON" #>> file("About_JSON") !
// or
file("About_JSON") #<< ( "grep JSON" #< url("http://databinder.net/dispatch/About") )  !

Search for uses of null in the source directory:

"find src -name *.scala -exec grep null {} ;"  #|  "xargs test -z"  #&&  "echo null-free"  #||  "echo null detected"  !

Use cat:

val spde = url("http://technically.us/spde/About")
val dispatch = url("http://databinder.net/dispatch/About")
val build = file("project/build.properties")
cat(spde, dispatch, build) #| "grep -i scala" !