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I'm attempting to back up a db from Go using the sqlite3 .backup command in the manner give by this SO answer https://stackoverflow.com/a/25684912/426853.

The following always works from the command line:

sqlite3 /home/pi/pgclogs/smartlog.db ".backup '/home/pi/pgcdata/smartlog.db.bak'"

I've coded it in Go as follows:

func DbBackup() (err error) {
    dbpath := "/home/pi/pgclogs/smartlog.db"
    bakpath := "/home/pi/pgcdata/smartlog.db.bak"
    cmd := exec.Command("sqlite3", dbpath, fmt.Sprintf("\".backup '%s'\"", bakpath))
    out, err := cmd.CombinedOutput()
    if err != nil {
        return fmt.Errorf("dbBackup failed : %s : %v", string(out), err)
    }
    return
}

and I have a test file that invokes it like this:

func TestDbBackup(t *testing.T) {
    var err error
    err = DbBackup()
    if err != nil {
        t.Errorf("backup failed : %v", err)
    }
}

The test reports a syntax error from sqlite3.

--- FAIL: TestDbBackup (0.01s)
    db_backup_test.go:22: backup failed : dbBackup failed : Error: near "".backup '/home/pi/pgcdata/smartlog.db.bak'"": syntax error
         : exit status 1

I suspect the problem is the way cmd.CombinedOutput is passing the arguments to the shell and that the problem is in the quoting in the fmt.Sprintf call. I've tried changing it to

fmt.Sprintf(`".backup '%s'"`, bakpath)

but the result is the same. I also tried putting echo in front of the command, i.e exec.Command("echo", "sqlite3", dbpath, ... and printing the output. The output looks absolutely correct

sqlite3 /home/pi/pgclogs/smartlog.db ".backup '/home/pi/pgcdata/smartlog.db.bak'"

and it runs correctly when pasted to the command line. I'm sure I'm overlooking something simple, but I've spent over an hour and am still not seeing it.

Mike Ellis
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1 Answers1

1

You don't need the additional quotes at all, so this should do:

exec.Command("sqlite3", dbpath, fmt.Sprintf(".backup '%s'", bakpath))

The quotes are parsed by the shell so that .backup 'somefile' gets passed to sqlite3 as a single argument. When using exec.Command, there is no shell involved, that's also the reason why you are passing the command splitted into multiple parameters.

Julian
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  • That almost works but not quite. Just before spotting your answer I (finally!) got it working with no inner quotes at all, `fmt.Sprintf(".backup %s", bakpath)`. I'm going to accept your answer since you took the trouble to post it and your reasoning is correct.. I'm not sure if the inner single quotes would be necessary if the filepath contained spaces but they definitely cause a problem for my case. If you agree, please change your example accordingly. Thanks! – Mike Ellis Mar 26 '19 at 16:43
  • It should also work with the single quotes around the filename, see https://gist.github.com/julianbrost/57251afb708cd851fdbca58740353db9 – Julian Mar 26 '19 at 20:17