Seeing some odd behaviour from os/exec.Command

[Ajith Ramanathan]

Hi.

I'm trying to execute 

git -c gc.reflogexpire=0 -c gc.reflogexpireunreachable=0 -c gc.rerereresolved=0 -c gc.rerereunresolved=0 -c gc.prunreexpire=now gc

from inside git.  When I run this in a terminal I get output like:

Enumerating objects: 539, done.
Counting objects: 100% (539/539), done.
Delta compression using up to 4 threads
Compressing objects: 100% (235/235), done.
Writing objects: 100% (539/539), done.
Total 539 (delta 298), reused 539 (delta 298), pack-reused 0 (from 0)
Enumerating cruft objects: 491, done.
Traversing cruft objects: 597, done.
Counting objects: 100% (501/501), done.
Delta compression using up to 4 threads
Compressing objects: 100% (216/216), done.
Writing objects: 100% (501/501), done.
Total 501 (delta 284), reused 501 (delta 284), pack-reused 0 (from 0)

However, when I run it using os/exec.Command I get nothing on stderr or stdout.  Other git commands do output to stderr and stdout, so I'm a little mystified.  Here is a test program demonstrating this:

package main

import (
"fmt"
"io"
"os/exec"
)

func run(cmd string, args ...string) {
runner := exec.Command(cmd, args...)

outp, _ := runner.StdoutPipe()
errp, _ := runner.StderrPipe()

if err := runner.Start(); err != nil {
panic(err)
}

outs, _ := io.ReadAll(outp)
errs, _ := io.ReadAll(errp)

err := runner.Wait()

fmt.Printf("cmd: %q\n", runner.String())
fmt.Printf("stdout: %q\n", outs)
fmt.Printf("stderr: %q\n", errs)
fmt.Printf("err: %v\n", err)
}

func main() {
run("git", "--version")
fmt.Println("-----------------------------------------------------------------------------")
run(
"git",
"-c", "gc.reflogexpire=0",
"-c", "gc.reflogexpireunreachable=0",
"-c", "gc.rerereresolved=0",
"-c", "gc.rerereunresolved=0",
"-c", "gc.prunreexpire=now",
"gc",
)
fmt.Println("-----------------------------------------------------------------------------")
run("git", "skidoosh")
}

and here is its output:

cmd: "/usr/bin/git --version"
stdout: "git version 2.51.2\n"
stderr: ""
err: <nil>
-----------------------------------------------------------------------------
cmd: "/usr/bin/git -c gc.reflogexpire=0 -c gc.reflogexpireunreachable=0 -c gc.rerereresolved=0 -c gc.rerereunresolved=0 -c gc.prunreexpire=now gc"
stdout: ""
stderr: ""
err: <nil>
-----------------------------------------------------------------------------
cmd: "/usr/bin/git skidoosh"
stdout: ""
stderr: "git: 'skidoosh' is not a git command. See 'git --help'.\n"
err: exit status 1

I feel like I'm missing something obvious, but it isn't clear what that might me.

[Ian Lance Taylor]

"git gc" changes its output based on whether it is connected to a tty.

You'll see the same effect if you run "git gc >/tmp/stdout 2>/tmp/stderr".

I don't know why "git gc" behaves that way.

Ian

[Dan Kortschak]

On Fri, 2025-11-21 at 09:08 -0800, Ajith Ramanathan wrote:
> However, when I run it using os/exec.Command I get nothing on stderr
> or stdout.

Do you know that the command is not checking whether it's running on a
tty?

[Kurtis Rader]

That behavior typically means the program is predicating the output on stdout/stderr being attached to a tty (terminal). You can confirm that hypothesis by running

git ... </dev/null >stdout 2>stderr

Then look at the content of stdout and stderr to see if the expected messages are present. If they aren't then that confirms the program only outputs that information if connected to a tty.

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Kurtis Rader

Caretaker of the exceptional canines Junior and Hank

[Ajith Ramanathan]

Thanks for the quick replies.  

I should have realized that: I can only plead a fart-of-brain.   

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