execute script in java, read both error and output streams

Question

I want to be able to execute an external command from java, eg "ls", and get the output, both output and error streams, as a string, and in realtime, and in the order they were generated.

So, if the output from the command is something like:

blah     <- to stdout
foo      <- to stderr
bar      <- to stdout

Then, ideally I want the output string, and the realtime output, to look like:

blah
foo
bar

Naive methods produce either:

blah
bar

(ie no stderr output)

or:

blah
bar
foo

(ie the order is rearranged so that the stdout and stderr messages are not interlaced with each other).

Use : Apache commons exec library

Nandkumar Tekale
– Nandkumar Tekale

2012-10-12 13:37:10 +00:00
Commented Oct 12, 2012 at 13:37 — Nandkumar Tekale
– Nandkumar Tekale, Commented Oct 12, 2012 at 13:37

artbristol · Accepted Answer · 2012-10-12 13:39:26Z

3

Use ProcessBuilder and set redirectErrorStream(true).

answered Oct 12, 2012 at 13:39

artbristol

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Comments

Aaron Digulla · Accepted Answer · 2012-10-12 13:51:40Z

This isn't always possible.

If you use the ProcessBuilder API, you can merge the stdout and stderr streams into one (using redirectErrorStream(true)), so you can read both outputs in a single InputStream. But that means you can't tell from which stream the data originally came from.

If you read stdout and stderr as two streams, you will need NIO or two Java threads. In both cases, processing one output will block the other (or rather, the other stream won't be processed timely). That will lead to swapped lines.

If the child process doesn't flush the output and you use pipes, then this gets worse because stdout will be sent to your process in 4KB blocks while stderr will usually arrive line-by-line.

There is no platform-independent workaround, I'm afraid. If you only need a solution which works on Unix, you can use Pseudo TTYs (man 4 pty) to simulate a shell but these are hard to set up from Java since you need to call OS functions.

One approach might be to use Perl or similar to run your command in a PTY (this causes stdout to become line buffered), read stdout and stderr from there and prefix each line with 1 for stdout and 2 for stderr.

Nandkumar Tekale · Accepted Answer · 2012-10-12 13:47:37Z

1

I would suggest you to use Apache commons exec API as they are more sophisticated API to use.

See DefaultExecutor: You can :

set a current working directory for the subprocess
provide a set of environment variables passed to the subprocess
capture the subprocess output of stdout and stderr using an ExecuteStreamHandler
kill long-running processes using an ExecuteWatchdog
define a set of expected exit values
terminate any started processes when the main process is terminating using a ProcessDestroyer

answered Oct 12, 2012 at 13:47

Nandkumar Tekale

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1 Comment

Hugh Perkins Over a year ago

It seems really complicated to use, and not obviously documented. Does it do anything that I can't do with a ProcessBuilder?

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