Scala Library: scala.sys.process
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:
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 object companion.
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
Stream[String]
(lines
methods) - The
Process
representing it (run
methods)
Some simple examples of these methods:
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 object companion
- 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:
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:
More information about the other ways of controlling I/O can be looked at 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. - Source
Type Members
class FileProcessLogger extends ProcessLogger with Closeable with Flushable
A scala.sys.process.ProcessLogger that writes output to a file.
trait Process extends AnyRef
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.
- Source
- See also
- scala.sys.process.ProcessBuilder
trait ProcessBuilder extends Source with Sink
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 a 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:
```scala import scala.sys.process._
// Executes “ls” and sends output to stdout “ls”.!
// Execute “ls” and assign a Stream[String]
of its output to “contents”.
val contents = Process(“ls”).lineStream
// 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:
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.lineStream
: returns immediately likerun
, and the output being generated is provided through aStream[String]
. Getting the next element of thatStream
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 thelineStream_!
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 lines
, 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 lines
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 lines_!
like this:
Extended Example
Let’s examine in detail one example of usage:
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.
Note: the lines
method, though deprecated, may conflict with the StringLike
method of the same name. To avoid this, one may wish to call the builders in
Process
instead of importing scala.sys.process._
. The example above would
be
trait ProcessCreation extends AnyRef
Factories for creating scala.sys.process.ProcessBuilder. They can be found on and used through scala.sys.process.Process ‘s companion object.
final class ProcessIO extends AnyRef
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.
- Source
- Note
- Failure to close the passed streams may result in resource leakage.
trait ProcessImplicits extends AnyRef
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.
trait ProcessLogger extends AnyRef
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:
- Source
- See also
- scala.sys.process.ProcessBuilder
Value Members
object BasicIO
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.
object Process extends ProcessImpl with ProcessCreation
Methods for constructing simple commands that can then be combined.
object ProcessBuilder extends ProcessBuilderImpl
This object contains traits used to describe input and output sources.
object ProcessLogger
Provides factories to create scala.sys.process.ProcessLogger, which are used to capture output of scala.sys.process.ProcessBuilder commands when run.
Value Members From scala.sys.process
def javaVmArguments: List[String]
The arguments passed to java
when creating this process
(defined at scala.sys.process)
def stderr: PrintStream
The error stream of this process
(defined at scala.sys.process)
def stdin: InputStream
The input stream of this process
(defined at scala.sys.process)
def stdout: PrintStream
The output stream of this process
(defined at scala.sys.process)
Value Members From scala.sys.process.ProcessImplicits
implicit def builderToProcess(builder: JProcessBuilder): ProcessBuilder
Implicitly convert a java.lang.ProcessBuilder
into a Scala one.
- Definition Classes
- ProcessImplicits
(defined at scala.sys.process.ProcessImplicits)
implicit def buildersToProcess[T](builders: Seq[T])(implicit convert: (T) ⇒ Source): Seq[Source]
Return a sequence of scala.sys.process.ProcessBuilder.Source from a sequence of
values for which an implicit conversion to Source
is available.
- Definition Classes
- ProcessImplicits
(defined at scala.sys.process.ProcessImplicits)
implicit def fileToProcess(file: File): FileBuilder
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:
- Definition Classes
- ProcessImplicits
(defined at scala.sys.process.ProcessImplicits)
implicit def stringSeqToProcess(command: Seq[String]): 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.
- Definition Classes
- ProcessImplicits
(defined at scala.sys.process.ProcessImplicits)
implicit def stringToProcess(command: String): ProcessBuilder
Implicitly convert a String
into a scala.sys.process.ProcessBuilder.
- Definition Classes
- ProcessImplicits
(defined at scala.sys.process.ProcessImplicits)
implicit def urlToProcess(url: URL): URLBuilder
Implicitly convert a java.net.URL
into a
scala.sys.process.ProcessBuilder.URLBuilder, which can be used as input to a
process. For example:
- Definition Classes
- ProcessImplicits (defined at scala.sys.process.ProcessImplicits)
Full Source:
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