Example code to capture STDOUT and STDERR in Julia.

capture-streams.jl

# A wrapper function to capture STDOUT and STDERR into strings.
#
# Heavily inspired by
#   https://github.com/JuliaStats/RCall.jl/blob/c1ff136864964cf2ac04b679f0c1b3b243df7e37/src/iface.jl#L35-L46
# referred to by the issue comment
#   https://github.com/JuliaLang/julia/issues/12711#issuecomment-133045787
#
# The Base.start_reading(stream, cb) requires fixes in Base.
#

function capture_streams(f)
    _stdout, _stderr = STDOUT, STDERR
    stdout_rd, stdout_wr = redirect_stdout()
    stderr_rd, stderr_wr = redirect_stderr()

    # buf combines the stdout and stderr
    buf, buf_stdout, buf_stderr = IOBuffer(), IOBuffer(), IOBuffer()

    # the signature of the callback is cb(stream, n)
    function cb_stdout(s,n)
        bytes = read(s,n)
        write(buf_stdout, bytes)
        write(buf, bytes)
        false
    end
    Base.start_reading(stdout_rd, cb_stdout)

    function cb_stderr(s,n)
        bytes = read(s,n)
        write(buf_stderr, bytes)
        write(buf, bytes)
        false
    end
    Base.start_reading(stderr_rd, cb_stderr)

    ret = try
        f()
    catch e
        println("ERROR in capture_streams(): $e")
    finally
        # read and restore
        redirect_stdout(_stdout)
        redirect_stderr(_stderr)

        close(stdout_wr)
        close(stderr_wr)

        #stdout_buf = readstring(stdout_rd)
        #stderr_buf = readstring(stderr_rd)

        close(stdout_rd)
        close(stderr_rd)
    end
    ret, takebuf_string(buf), takebuf_string(buf_stdout), takebuf_string(buf_stderr)
end

# Test / examples

function foo(x)
    for n = 1:x
        info("$n/$x \t[to STDERR]")
        println("$n/$x \t[to STDOUT]")
    end
    x
end

function test_capture(f)
    ret, all, out, err = capture_streams(f)
    println("Return value: $(ret)")
    println("------ STDOUT+STDERR -------")
    println(all)
    println("---------- STDOUT ----------")
    println(out)
    println("---------- STDERR ----------")
    println(err)
    println("----------------------------")
end
info("Capturing normally:")
test_capture() do
    info("Calling foo()")
    foo(5)
end

println(); println()

info("Capturing when error is thrown:")
try
    test_capture() do
        f(200) # error -- function does not exist
    end
catch e
    println("ERROR: $e")
end

info("Streams restored.")

output.txt

Return value: 5
------ STDOUT+STDERR -------
INFO: Calling foo()
INFO: 1/5 	[to STDERR]
1/5 	[to STDOUT]
INFO: 2/5 	[to STDERR]
2/5 	[to STDOUT]
INFO: 3/5 	[to STDERR]
3/5 	[to STDOUT]
INFO: 4/5 	[to STDERR]
4/5 	[to STDOUT]
INFO: 5/5 	[to STDERR]
5/5 	[to STDOUT]

---------- STDOUT ----------
1/5 	[to STDOUT]
2/5 	[to STDOUT]
3/5 	[to STDOUT]
4/5 	[to STDOUT]
5/5 	[to STDOUT]

---------- STDERR ----------
INFO: Calling foo()
INFO: 1/5 	[to STDERR]
INFO: 2/5 	[to STDERR]
INFO: 3/5 	[to STDERR]
INFO: 4/5 	[to STDERR]
INFO: 5/5 	[to STDERR]

----------------------------


Return value: nothing
------ STDOUT+STDERR -------
ERROR in capture_streams(): UndefVarError(:f)

---------- STDOUT ----------
ERROR in capture_streams(): UndefVarError(:f)

---------- STDERR ----------

----------------------------
INFO: Capturing normally:
INFO: Capturing when error is thrown:
INFO: Streams restored.

stream.jl.diff

diff --git a/base/stream.jl b/base/stream.jl
index d649b88..963b785 100644
--- a/base/stream.jl
+++ b/base/stream.jl
@@ -445,7 +445,7 @@ function notify_filled(stream::LibuvStream, nread::Int)
     more = true
     while more
         if isa(stream.readcb,Function)
-            nreadable = (stream.line_buffered ? Int(search(stream.buffer, '\n')) : nb_available(stream.buffer))
+            nreadable = (stream.line_buffered ? Int(search(stream.buffer, UInt8('\n'))) : nb_available(stream.buffer))
             if nreadable > 0
                 more = stream.readcb(stream, nreadable)
             else
@@ -674,7 +674,7 @@ function start_reading(stream::LibuvStream)
 end
 
 function start_reading(stream::LibuvStream, cb::Function)
-    failure = start_reading(stream)
+    failure_code = start_reading(stream)
     stream.readcb = cb
     nread = nb_available(stream.buffer)
     if nread > 0

Aha, now I get it. Thanks for the clarification 👍