Read stderr after process finished - shell

Question

I call imagemagick's convert command with some data I have in memory (from html form upload/web server). This works fine, but I'd like to get the error output of convert in case of an error. How can I do that?
This is my code:
package main
import (
"bytes"
"io"
"io/ioutil"
"log"
"os/exec"
"path/filepath"
)
func runImagemagick(data []byte, destfilename string) error {
data_buf := bytes.NewBuffer(data)
cmd := exec.Command("convert", "-", destfilename)
stdin, err := cmd.StdinPipe()
if err != nil {
return err
}
err = cmd.Start()
if err != nil {
return err
}
_, err = io.Copy(stdin, data_buf)
if err != nil {
return err
}
stdin.Close()
err = cmd.Wait()
if err != nil {
return err
}
return nil
}
func main() {
data, err := ioutil.ReadFile("source.gif")
if err != nil {
log.Fatal(err)
}
err = runImagemagick(data, filepath.Join("/tmp", "abc", "dest.png"))
if err != nil {
log.Fatal(err)
}
}
Now the artificial problem is that the directory /tmp/abc/ does not exist. Normally convert would give me this result:
$ convert - /tmp/abc/foo.png < source.gif
convert: unable to open image `/tmp/abc/foo.png': No such file or directory # error/blob.c/OpenBlob/2617.
convert: WriteBlob Failed `/tmp/abc/foo.png' # error/png.c/MagickPNGErrorHandler/1755.
but I don't "see" this error message within my small program. How can I get the error message and show it to my user?
(And another sub-question is: can you give me an advice if this code looks OK? Are there any obvious flaws in it?)

Answer 1

Pipe stdout and stderr too. For example,
package main
import (
"bytes"
"io"
"io/ioutil"
"log"
"os/exec"
"path/filepath"
)
func runImagemagick(data []byte, destfilename string) error {
cmd := exec.Command("convert", "-", destfilename)
stdin, err := cmd.StdinPipe()
if err != nil {
return err
}
stdout, err := cmd.StdoutPipe()
if err != nil {
return err
}
stderr, err := cmd.StderrPipe()
if err != nil {
return err
}
err = cmd.Start()
if err != nil {
return err
}
_, err = io.Copy(stdin, bytes.NewBuffer(data))
if err != nil {
return err
}
stdin.Close()
outData, err := ioutil.ReadAll(stdout)
if err != nil {
return err
}
if len(outData) > 0 {
log.Print(string(outData))
}
errData, err := ioutil.ReadAll(stderr)
if err != nil {
return err
}
if len(errData) > 0 {
log.Print(string(errData))
}
err = cmd.Wait()
if err != nil {
return err
}
return nil
}
func main() {
data, err := ioutil.ReadFile("source.gif")
if err != nil {
log.Fatal(err)
}
err = runImagemagick(data, filepath.Join("/tmp", "abc", "dest.png"))
if err != nil {
log.Fatal(err)
}
}
Output:
2013/03/03 15:02:20 convert.im6: unable to open image `/tmp/abc/dest-0.png': No such file or directory # error/blob.c/OpenBlob/2638.
convert.im6: WriteBlob Failed `/tmp/abc/dest-0.png' # error/png.c/MagickPNGErrorHandler/1728.
2013/03/03 15:02:20 exit status 1
exit status 1

Answer 2

There's no need to use pipes because bytes.Buffer implements the io.Writer interface and so it can be used just fine to collect the program's output:
func runImagemagick(data []byte, destfilename string) error {
cmd := exec.Command("convert", "-", destfilename)
var stdout, stderr bytes.Buffer
cmd.Stdout = &stdout
cmd.Stderr = &stderr
err := cmd.Run()
if err != nil {
if ee, ok := err.(*exec.ExitError); ok {
return &imagemagickError{ee, stdout.Bytes(), stderr.Bytes()}
} else {
return err
}
}
if stderr.Len() > 0 {
return errors.New(fmt.Sprintf("imagemagick wrote to stderr: %s", stderr.Bytes()))
}
if stdout.Len() > 0 {
log.Print(stdout.Bytes())
}
return nil
}