os, os/exec: using redirection symbol '<' '>' failed

[liugangnhm]

when an using exec.Cmd to fock an subprocess,with redirection like cat xxx > /tmp/xxxx ,it will always fail.

cmd:=exec.Command("cat","./aaa",">","/tmp/aaa")
cmd.Start()
err:=cmd.Wait()
if err!=nil {
    t.Errorf("exit error %s", err)
}

wheather /tmp/aaa is exist or not, error is always "exit status 1" .
file aaa is always exist.

when i set cmd's Stderr to bytes.Buffer ,and i see detail error:"cat: >d: no such file or directory". so i think go maybe treat '>' as a file name.

so how should i do when i want to use redirection symbol in command

[liugangnhm]

go version: 1.4.1 linux/amd64
os version:ubuntu 14.10 amd64

[crawshaw]

The > is a feature of your shell language. To use it, you need to run your command inside a shell. Perhaps something like

cmd := exec.Command("bash", "-c", "echo hello > /tmp/aaa")

Be careful with this though. If you construct a string and pass it to a shell, you need to think carefully about escaping and security. For the particular example you give, you may want to consider using os.Open, os.Create, and io.Copy.

[liugangnhm]

just talk about cat

package main

import (
    "bytes"
    "os/exec"
    "testing"
)

func TestExe1(t *testing.T) {

    out := &bytes.Buffer{}

    cmd := exec.Command("bash", "-c", "cat ./file_not_exist > /tmp/aaa")
    cmd.Stderr = out
    err := cmd.Start()
    if err != nil {
        t.Errorf("start error %s [%s]", err, out.String())
    }
    err = cmd.Wait()
    if err == nil {
        t.Errorf("exit error %s [%s]", err, out.String())
    }

    cmd = exec.Command("bash", "-c", "echo hello > /tmp/aaa")
    err = cmd.Start()
    if err != nil {
        t.Errorf("start error %s [%s]", err, out.String())
    }
    err = cmd.Wait()
    if err == nil {
        t.Errorf("exit error %s [%s]", err, out.String())
    }

    cmd = exec.Command("bash", "-c", "cat ./myself_test.go > /tmp/aaa")
    err = cmd.Start()
    if err != nil {
        t.Errorf("start error %s [%s]", err, out.String())
    }
    err = cmd.Wait()
    if err != nil {
        t.Errorf("exit error %s [%s]", err, out.String())
    }
}

output is
golang_test.go:41: exit error exit status 1 [bash: /tmp/aaa: Permission denied

why cat is particular?

and i wonder why cmd := exec.Command("bash", "-c", "cat ./file_not_exist > /tmp/aaa") did not throw an error.

[ianlancetaylor]

@liugangnhm We prefer to use the issue tracker for bug reports. Please ask questions on the golang-nuts mailing list, not on the issue tracker. Thanks.

[liugangnhm]

all right,thanks.