scala.sys.process
This package handles the execution of external processes. The contents of this package can be divided in three groups, according to their responsibilities:
Indicating what to run and how to run it.
Handling a process input and output.
Running the process.
For simple uses, the only group that matters is the first one. Running an
external command can be as simple as "ls".!
, or as complex as building a
pipeline of commands such as this:
import scala.sys.process._
"ls" #| "grep .scala" #&& Seq("sh", "-c", "scalac *.scala") #|| "echo nothing found" lazyLines
We describe below the general concepts and architecture of the package, and then take a closer look at each of the categories mentioned above.
Concepts and Architecture
The underlying basis for the whole package is Java's Process
and
ProcessBuilder
classes. While there's no need to use these Java classes,
they impose boundaries on what is possible. One cannot, for instance,
retrieve a process id for whatever is executing.
When executing an external process, one can provide a command's name,
arguments to it, the directory in which it will be executed and what
environment variables will be set. For each executing process, one can
feed its standard input through a java.io.OutputStream
, and read from
its standard output and standard error through a pair of
java.io.InputStream
. One can wait until a process finishes execution and
then retrieve its return value, or one can kill an executing process.
Everything else must be built on those features.
This package provides a DSL for running and chaining such processes, mimicking Unix shells ability to pipe output from one process to the input of another, or control the execution of further processes based on the return status of the previous one.
In addition to this DSL, this package also provides a few ways of controlling input and output of these processes, going from simple and easy to use to complex and flexible.
When processes are composed, a new ProcessBuilder
is created which, when
run, will execute the ProcessBuilder
instances it is composed of
according to the manner of the composition. If piping one process to
another, they'll be executed simultaneously, and each will be passed a
ProcessIO
that will copy the output of one to the input of the other.
What to Run and How
The central component of the process execution DSL is the
scala.sys.process.ProcessBuilder trait. It is ProcessBuilder
that
implements the process execution DSL, that creates the
scala.sys.process.Process that will handle the execution, and return
the results of such execution to the caller. We can see that DSL in the
introductory example: #|
, #&&
and #!!
are methods on
ProcessBuilder
used to create a new ProcessBuilder
through
composition.
One creates a ProcessBuilder
either through factories on the
scala.sys.process.Process's companion object, or through implicit
conversions available in this package object itself. Implicitly, each
process is created either out of a String
, with arguments separated by
spaces -- no escaping of spaces is possible -- or out of a
scala.collection.Seq, where the first element represents the command
name, and the remaining elements are arguments to it. In this latter case,
arguments may contain spaces.
To further control what how the process will be run, such as specifying the directory in which it will be run, see the factories on scala.sys.process.Process's companion object.
Once the desired ProcessBuilder
is available, it can be executed in
different ways, depending on how one desires to control its I/O, and what
kind of result one wishes for:
Return status of the process (
!
methods)Output of the process as a
String
(!!
methods)Continuous output of the process as a
LazyList[String]
(lazyLines
methods)The
Process
representing it (run
methods)
Some simple examples of these methods:
import scala.sys.process._
// This uses ! to get the exit code
def fileExists(name: String) = Seq("test", "-f", name).! == 0
// This uses !! to get the whole result as a string
val dirContents = "ls".!!
// This "fire-and-forgets" the method, which can be lazily read through
// a LazyList[String]
def sourceFilesAt(baseDir: String): LazyList[String] = {
val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
cmd.lazyLines
}
We'll see more details about controlling I/O of the process in the next section.
Handling Input and Output
In the underlying Java model, once a Process
has been started, one can
get java.io.InputStream
and java.io.OutputStream
representing its
output and input respectively. That is, what one writes to an
OutputStream
is turned into input to the process, and the output of a
process can be read from an InputStream
-- of which there are two, one
representing normal output, and the other representing error output.
This model creates a difficulty, which is that the code responsible for actually running the external processes is the one that has to take decisions about how to handle its I/O.
This package presents an alternative model: the I/O of a running process
is controlled by a scala.sys.process.ProcessIO object, which can be
passed _to_ the code that runs the external process. A ProcessIO
will
have direct access to the java streams associated with the process I/O. It
must, however, close these streams afterwards.
Simpler abstractions are available, however. The components of this package that handle I/O are:
scala.sys.process.ProcessIO: provides the low level abstraction.
scala.sys.process.ProcessLogger: provides a higher level abstraction for output, and can be created through its companion object.
scala.sys.process.BasicIO: a library of helper methods for the creation of
ProcessIO
.This package object itself, with a few implicit conversions.
Some examples of I/O handling:
import scala.sys.process._
// An overly complex way of computing size of a compressed file
def gzFileSize(name: String) = {
val cat = Seq("zcat", name)
var count = 0
def byteCounter(input: java.io.InputStream) = {
while(input.read() != -1) count += 1
input.close()
}
val p = cat run new ProcessIO(_.close(), byteCounter, _.close())
p.exitValue()
count
}
// This "fire-and-forgets" the method, which can be lazily read through
// a LazyList[String], and accumulates all errors on a StringBuffer
def sourceFilesAt(baseDir: String): (LazyList[String], StringBuffer) = {
val buffer = new StringBuffer()
val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
val lazyLines = cmd lazyLines_! ProcessLogger(buffer append _)
(lazyLines, buffer)
}
Instances of the java classes java.io.File
and java.net.URL
can both
be used directly as input to other processes, and java.io.File
can be
used as output as well. One can even pipe one to the other directly
without any intervening process, though that's not a design goal or
recommended usage. For example, the following code will copy a web page to
a file:
import java.io.File
import java.net.URL
import scala.sys.process._
new URL("https://www.scala-lang.org/") #> new File("scala-lang.html") !
More information about the other ways of controlling I/O can be found in the Scaladoc for the associated objects, traits and classes.
Running the Process
Paradoxically, this is the simplest component of all, and the one least likely to be interacted with. It consists solely of scala.sys.process.Process, and it provides only two methods:
exitValue()
: blocks until the process exit, and then returns the exit value. This is what happens when one uses the!
method ofProcessBuilder
.destroy()
: this will kill the external process and close the streams associated with it.
Type members
Classlikes
This object contains factories for scala.sys.process.ProcessIO,
which can be used to control the I/O of a scala.sys.process.Process
when a scala.sys.process.ProcessBuilder is started with the run
command.
This object contains factories for scala.sys.process.ProcessIO,
which can be used to control the I/O of a scala.sys.process.Process
when a scala.sys.process.ProcessBuilder is started with the run
command.
It also contains some helper methods that can be used to in the creation of
ProcessIO
.
It is used by other classes in the package in the implementation of various features, but can also be used by client code.
- Source:
- BasicIO.scala
A scala.sys.process.ProcessLogger that writes output to a file.
A scala.sys.process.ProcessLogger that writes output to a file.
- Source:
- ProcessLogger.scala
Represents a process that is running or has finished running.
Represents a process that is running or has finished running.
It may be a compound process with several underlying native processes (such as a #&& b
).
This trait is often not used directly, though its companion object contains factories for scala.sys.process.ProcessBuilder, the main component of this package.
It is used directly when calling the method run
on a ProcessBuilder
,
which makes the process run in the background. The methods provided on Process
make it possible for one to block until the process exits and get the exit value,
or destroy the process altogether.
- See also:
- Companion:
- object
- Source:
- Process.scala
Methods for constructing simple commands that can then be combined.
Methods for constructing simple commands that can then be combined.
- Companion:
- class
- Source:
- Process.scala
Represents a sequence of one or more external processes that can be executed.
Represents a sequence of one or more external processes that can be
executed. A ProcessBuilder
can be a single external process, or a
combination of other ProcessBuilder
. One can control where the
output of an external process will go to, and where its input will come
from, or leave that decision to whoever starts it.
One creates a ProcessBuilder
through factories provided in
scala.sys.process.Process's companion object, or implicit conversions
based on these factories made available in the package object
scala.sys.process. Here are some examples:
import scala.sys.process._
// Executes "ls" and sends output to stdout
"ls".!
// Execute "ls" and assign a `LazyList[String]` of its output to "contents".
val contents = Process("ls").lazyLines
// Here we use a `Seq` to make the parameter whitespace-safe
def contentsOf(dir: String): String = Seq("ls", dir).!!
The methods of ProcessBuilder
are divided in three categories: the ones that
combine two ProcessBuilder
to create a third, the ones that redirect input
or output of a ProcessBuilder
, and the ones that execute
the external processes associated with it.
Combining ProcessBuilder
Two existing ProcessBuilder
can be combined in the following ways:
They can be executed in parallel, with the output of the first being fed as input to the second, like Unix pipes. This is achieved with the
#|
method.They can be executed in sequence, with the second starting as soon as the first ends. This is done by the
###
method.The execution of the second one can be conditioned by the return code (exit status) of the first, either only when it's zero, or only when it's not zero. The methods
#&&
and#||
accomplish these tasks.
Redirecting Input/Output
Though control of input and output can be done when executing the process,
there's a few methods that create a new ProcessBuilder
with a
pre-configured input or output. They are #<
, #>
and #>>
, and may take
as input either another ProcessBuilder
(like the pipe described above), or
something else such as a java.io.File
or a java.io.InputStream
.
For example:
new URL("https://databinder.net/dispatch/About") #> "grep JSON" #>> new File("About_JSON") !
Starting Processes
To execute all external commands associated with a ProcessBuilder
, one
may use one of four groups of methods. Each of these methods have various
overloads and variations to enable further control over the I/O. These
methods are:
run
: the most general method, it returns a scala.sys.process.Process immediately, and the external command executes concurrently.!
: blocks until all external commands exit, and returns the exit code of the last one in the chain of execution.!!
: blocks until all external commands exit, and returns aString
with the output generated.lazyLines
: returns immediately likerun
, and the output being generated is provided through aLazyList[String]
. Getting the next element of thatLazyList
may block until it becomes available. This method will throw an exception if the return code is different than zero -- if this is not desired, use thelazyLines_!
method.
Handling Input and Output
If not specified, the input of the external commands executed with run
or
!
will not be tied to anything, and the output will be redirected to the
stdout and stderr of the Scala process. For the methods !!
and lazyLines
, no
input will be provided, and the output will be directed according to the
semantics of these methods.
Some methods will cause stdin to be used as input. Output can be controlled
with a scala.sys.process.ProcessLogger -- !!
and lazyLines
will only
redirect error output when passed a ProcessLogger
. If one desires full
control over input and output, then a scala.sys.process.ProcessIO can be
used with run
.
For example, we could silence the error output from lazyLines_!
like this:
val etcFiles = "find /etc" lazyLines_! ProcessLogger(line => ())
Extended Example
Let's examine in detail one example of usage:
import scala.sys.process._
"find src -name *.scala -exec grep null {} ;" #| "xargs test -z" #&& "echo null-free" #|| "echo null detected" !
Note that every String
is implicitly converted into a ProcessBuilder
through the implicits imported from scala.sys.process. These ProcessBuilder
are then
combined in three different ways.
#|
pipes the output of the first command into the input of the second command. It mirrors a shell pipe (|
).#&&
conditionally executes the second command if the previous one finished with exit value 0. It mirrors shell's&&
.#||
conditionally executes the third command if the exit value of the previous command is different than zero. It mirrors shell's||
.
Finally, !
at the end executes the commands, and returns the exit value.
Whatever is printed will be sent to the Scala process standard output. If
we wanted to capture it, we could run that with !!
instead.
Note: though it is not shown above, the equivalent of a shell's ;
would be
###
. The reason for this name is that ;
is a reserved token in Scala.
- Companion:
- object
- Source:
- ProcessBuilder.scala
This object contains traits used to describe input and output sources.
This object contains traits used to describe input and output sources.
- Companion:
- class
- Source:
- ProcessBuilder.scala
Factories for creating scala.sys.process.ProcessBuilder.
Factories for creating scala.sys.process.ProcessBuilder. They can be found on and used through scala.sys.process.Process's companion object.
- Source:
- Process.scala
This class is used to control the I/O of every scala.sys.process.Process.
This class is used to control the I/O of every
scala.sys.process.Process. The functions used to create it will be
called with the process streams once it has been started. It might not be
necessary to use ProcessIO
directly --
scala.sys.process.ProcessBuilder can return the process output to the
caller, or use a scala.sys.process.ProcessLogger which avoids direct
interaction with a stream. One can even use the factories at BasicIO
to
create a ProcessIO
, or use its helper methods when creating one's own
ProcessIO
.
When creating a ProcessIO
, it is important to close all streams when
finished, since the JVM might use system resources to capture the process
input and output, and will not release them unless the streams are
explicitly closed.
ProcessBuilder
will call writeInput
, processOutput
and processError
in separate threads, and if daemonizeThreads is true, they will all be
marked as daemon threads.
- Value parameters:
- daemonizeThreads
Indicates whether the newly spawned threads that will run
processOutput
,processError
andwriteInput
should be marked as daemon threads.- processError
Function that will be called with the
InputStream
from which all error output of the process must be read from. This will be called in a newly spawned thread.- processOutput
Function that will be called with the
InputStream
from which all normal output of the process must be read from. This will be called in a newly spawned thread.- writeInput
Function that will be called with the
OutputStream
to which all input to the process must be written. This will be called in a newly spawned thread.
- Note:
Failure to close the passed streams may result in resource leakage.
- Source:
- ProcessIO.scala
Provide implicit conversions for the factories offered by scala.sys.process.Process's companion object.
Provide implicit conversions for the factories offered by scala.sys.process.Process's companion object. These implicits can then be used to decrease the noise in a pipeline of commands, making it look more shell-like. They are available through the package object scala.sys.process.
- Source:
- Process.scala
Encapsulates the output and error streams of a running process.
Encapsulates the output and error streams of a running process. This is used
by scala.sys.process.ProcessBuilder when starting a process, as an
alternative to scala.sys.process.ProcessIO, which can be more difficult
to use. Note that a ProcessLogger
will be used to create a ProcessIO
anyway. The object BasicIO
has some functions to do that.
Here is an example that counts the number of lines in the normal and error output of a process:
import scala.sys.process._
var normalLines = 0
var errorLines = 0
val countLogger = ProcessLogger(line => normalLines += 1,
line => errorLines += 1)
"find /etc" ! countLogger
- See also:
- Companion:
- object
- Source:
- ProcessLogger.scala
Provides factories to create scala.sys.process.ProcessLogger, which are used to capture output of scala.sys.process.ProcessBuilder commands when run.
Provides factories to create scala.sys.process.ProcessLogger, which are used to capture output of scala.sys.process.ProcessBuilder commands when run.
- Companion:
- class
- Source:
- ProcessLogger.scala
Value members
Concrete methods
Implicits
Inherited implicits
Implicitly convert a java.lang.ProcessBuilder
into a Scala one.
Implicitly convert a java.lang.ProcessBuilder
into a Scala one.
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala
Return a sequence of scala.sys.process.ProcessBuilder.Source from a sequence
of values for which an implicit conversion to Source
is available.
Return a sequence of scala.sys.process.ProcessBuilder.Source from a sequence
of values for which an implicit conversion to Source
is available.
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala
Implicitly convert a java.io.File
into a
scala.sys.process.ProcessBuilder.FileBuilder, which can be used as
either input or output of a process.
Implicitly convert a java.io.File
into a
scala.sys.process.ProcessBuilder.FileBuilder, which can be used as
either input or output of a process. For example:
import scala.sys.process._
"ls" #> new java.io.File("dirContents.txt") !
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala
Implicitly convert a sequence of String
into a
scala.sys.process.ProcessBuilder.
Implicitly convert a sequence of String
into a
scala.sys.process.ProcessBuilder. The first argument will be taken to
be the command to be executed, and the remaining will be its arguments.
When using this, arguments may contain spaces.
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala
Implicitly convert a String
into a scala.sys.process.ProcessBuilder.
Implicitly convert a String
into a scala.sys.process.ProcessBuilder.
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala
Implicitly convert a java.net.URL
into a
scala.sys.process.ProcessBuilder.URLBuilder , which can be used as
input to a process.
Implicitly convert a java.net.URL
into a
scala.sys.process.ProcessBuilder.URLBuilder , which can be used as
input to a process. For example:
import scala.sys.process._
Seq("xmllint", "--html", "-") #< new java.net.URL("https://www.scala-lang.org") #> new java.io.File("fixed.html") !
- Inherited from:
- ProcessImplicits
- Source:
- Process.scala