exec.cmd fails to direct stdout to file when using systemd to start a service

Question

The code is like this:

cmd := exec.Command(os_.Args[0], os_.Args[1:]...)
        cmd.SysProcAttr = &builtin_syscall.SysProcAttr{
            Credential: &builtin_syscall.Credential{
                Uid:    uint32(runUid),
                Gid:    uint32(runGids[0]),
                Groups: gidsUint,
            },
            Setsid: true,
        }
        f,err :=os_.Open("/var/logs/std_0.log")
        if err != nil{
            fmt.Printf("failed to open file: %s\n",err.Error())
            cmd.Stdout = f
            cmd.Stderr = f
        } else {
            cmd.Stdout = os_.Stdout
            cmd.Stderr = os_.Stderr
        }

The /proc/${pid}/fd files list:

lr-x------ 1 steve coder 64 9月  26 15:29 0 -> /dev/null
lrwx------ 1 steve coder 64 9月  26 15:29 1 -> /var/logs/std_0.log
lrwx------ 1 steve coder 64 9月  26 15:29 10 -> socket:[1137628899]
lrwx------ 1 steve coder 64 9月  26 15:29 11 -> socket:[1137631303]
lrwx------ 1 steve coder 64 9月  26 15:29 12 -> socket:[1137630589]

However the log is still redirected to systemd's journal log instead of the dedicated file. Can somebody explain it?

Answer 1

There is probably easier ways to leverage systemd to redirect your output to another destination, rather than doing it in your service code.

Regardless, your if statement logic is reversed: on open success (err == nil) you want to use the the file-handle f:

f, err := os_.Open("/var/logs/std_0.log")

// if err != nil {   // <- here
if err == nil {
    
    cmd.Stdout = f
    cmd.Stderr = f
} else {

    fmt.Printf("failed to open file: %v\n",err)

    cmd.Stdout = os_.Stdout
    cmd.Stderr = os_.Stderr
}