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Jawk 6.4.00

Custom Output

Output is specified per-call on the builder, to capture print() and printf(...) output in a stream, an Appendable, a custom AwkSink[1], or simply as a String.

Capture as a String

A no-argument execute() returns the printed output as a String:

Awk awk = new Awk();
String result = awk.script("BEGIN { print \"hello\" }").execute();
// result == "hello\n"

Redirect to a Stream

Pass an OutputStream or PrintStream to execute(...):

Awk awk = new Awk();
awk.script("BEGIN { print \"logged\" }").execute(new FileOutputStream("output.txt"));

To print to System.out explicitly:

awk.script("BEGIN { print \"hello\" }").execute(System.out);

Capture into an Appendable

Pass an Appendable to execute(...) to collect output in a StringBuilder, StringWriter, or any Appendable:

Awk awk = new Awk();
StringBuilder output = new StringBuilder();
awk.script("BEGIN { print \"captured\" }").execute(output);
// output.toString() == "captured\n"

Custom Output with AwkSink

Use AwkSink[1] when plain text is not the right abstraction. An AwkSink receives raw print(...) and printf(...) calls together with the current AWK formatting state.

Implementing an AwkSink

Extend AwkSink and override print(...), printf(...), and getPrintStream():

public final class CollectingSink extends AwkSink {
    private final List<List<Object>> prints = new ArrayList<List<Object>>();
    private final PrintStream processOutput = System.out;

    public CollectingSink() {
        super(Locale.US);
    }

    @Override
    public void print(String ofs, String ors, String ofmt, Object... values) {
        prints.add(Arrays.asList(Arrays.copyOf(values, values.length)));
    }

    @Override
    public void printf(String ofs, String ors, String ofmt, String format, Object... values) {
        // store format + values however your application wants
    }

    @Override
    public PrintStream getPrintStream() {
        return processOutput;
    }

    public List<List<Object>> getCollectedPrints() {
        return prints;
    }
}

Sink print and printf Parameters

print(...)

print(...)

Parameter AWK Variable Description
ofs OFS Output Field Separator, inserted between values
ors ORS Output Record Separator, appended after the record
ofmt OFMT Default numeric output format
values — The raw AWK values passed to print
printf(...)

printf(...)

Parameter AWK Variable Description
ofs OFS Output Field Separator, inserted between values
ors ORS Output Record Separator, appended after the record
ofmt OFMT Default numeric output format
format — The AWK format string
values — The AWK values to be formatted

getPrintStream

getPrintStream() provides the PrintStream used for pumping the stdout of spawned processes (system("...") and pipe output from print ... | "cmd") back into the host output. File redirection (print > "file") does not use this stream — it creates its own file-backed sink internally.

The default implementation returns a no-op stream that silently discards output. Override this method in sinks that need to capture subprocess output. Implementations typically return System.out or a custom stream.

Using a Custom Sink

Pass the sink to execute(...) on the builder:

Awk awk = new Awk();
CollectingSink sink = new CollectingSink();

awk.script("{ print $1, $2 }")
        .input("alpha beta\ngamma delta\n")
        .execute(sink);

// sink.getCollectedPrints() contains [[alpha, beta], [gamma, delta]]

Built-In Sink Implementations

Jawk provides two built-in AwkSink implementations:

  • AwkSink.from(PrintStream) / AwkSink.from(PrintStream, Locale) creates a sink that renders output to a PrintStream. This is the default behavior.
  • AwkSink.from(Appendable) / AwkSink.from(Appendable, Locale) renders output to any Appendable such as StringBuilder or StringWriter.

The overloads without a Locale parameter default to Locale.US.

Numeric Locale

The locale controls how AWK formats numbers in print and printf output (e.g. decimal separator). The default is Locale.US.

With the Builder

When you use the AwkRunBuilder methods (execute(), execute(OutputStream), execute(Appendable)), the locale is taken automatically from AwkSettings. Set it once on the Awk instance:

Awk awk = new Awk();
awk.getSettings().setLocale(Locale.FRANCE);

// All execute() variants use the French locale for number formatting
String result = awk.script("BEGIN { print 3.14 }").execute();
// result == "3,14\n"

With a Custom AwkSink

When you pass an AwkSink to execute(AwkSink), the sink carries its own locale — AwkSettings is not involved. Specify the locale when constructing the sink:

AwkSink frenchSink = AwkSink.from(System.out, Locale.FRANCE);
awk.script("BEGIN { print 3.14 }").execute(frenchSink);

When extending AwkSink directly, pass the locale to the super(...) constructor:

public class MySink extends AwkSink {
    public MySink(Locale locale) {
        super(locale);
    }
    // ...
}

Choosing the Right Output Strategy

Goal API Example
Capture as String execute() awk.script(s).execute()
Print to a PrintStream execute(PrintStream) awk.script(s).execute(System.out)
Print to an OutputStream execute(OutputStream) awk.script(s).execute(fileOut)
Capture to Appendable execute(Appendable) awk.script(s).execute(sb)
Structured collection execute(AwkSink) awk.script(s).execute(mySink)

Subprocess Output

When AWK runs an external command via system("...") or a pipe (print ... | "cmd"), the command's stdout is pumped into the sink's PrintStream returned by getPrintStream(). The built-in OutputStreamAwkSink and AppendableAwkSink both override this method, so subprocess stdout is captured alongside normal output. For a custom AwkSink that does not override getPrintStream(), subprocess stdout is silently discarded (the default no-op stream). To capture subprocess stdout, override getPrintStream() in your sink so it returns a real stream.

Subprocess stderr defaults to the sink's PrintStream as well. To redirect it to a separate stream, use errorStream(PrintStream) (this only affects stderr, not stdout):

awk.script("BEGIN { system(\"mycommand\") }")
        .errorStream(System.err)
        .execute(System.out);

The CLI uses .errorStream(System.err) so that command errors appear on the console rather than mixing with normal output.

See Also

output awksink print printf custom output appendable awk
Links:
  • [1] apidocs/io/jawk/jrt/AwkSink.html
  • [2] java.html
  • [3] java-variables.html
  • [4] java-input.html
  • [5] java-advanced.html
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