[SOLVED] busybox system without sync command: what's best to do?

Ratamahatta · 09-06-2014, 06:26 PM

Hi!

I'm configuring a busybox system as a wire-bound downloader. I'm using a USB thumb drive to save the downloaded files to using a script. That script is meant to continue to run and check a file for more download links. Obviously I'd like to be able to unplug that device immediately when all downloads are finished. (At other times I'd let it connected and add more download links to the file.) But there's no sync command on this particular busybox system.
What's the best thing to do here? (Updating to an newer firmware is not an option as the internet provider this old box is from removed that telnet root access I'm using in later versions.)
Should I just umount/remount after all downloads are finished?
How long would it take until the system flushes the disk cache to the disk if I just waited?

Tried to add a sync link and this was the output when executing it:

Code:

BusyBox v1.00 (2007.04.23-03:09+0000) multi-call binary

Usage: busybox [function] [arguments]...
   or: [function] [arguments]...

        BusyBox is a multi-call binary that combines many common Unix
        utilities into a single executable.  Most people will create a
        link to busybox for each function they wish to use, and BusyBox
        will act like whatever it was invoked as.

Currently defined functions:
        [, ash, basename, busybox, cat, chmod, chown, chroot, cp, crond,
        crontab, cut, date, dd, df, dmesg, du, dumpleases, echo, egrep,
        env, expr, false, fdisk, fgrep, find, free, getopt, getty, grep,
        gunzip, gzip, hostname, httpd, ifconfig, inetd, init, insmod,
        kill, killall, klogd, ln, logger, login, logread, ls, lsmod, md5sum,
        mkdir, mknod, modprobe, more, mount, msh, mv, netstat, passwd,
        ping, ps, pwd, rdate, reboot, rm, rmdir, rmmod, route, sed, seq,
        sh, sha1sum, sleep, sysctl, syslogd, tail, tar, telnetd, test,
        tftp, top, touch, tr, true, tty, udhcpc, udhcpd, udhcpr, umount,
        uname, uptime, usleep, vi, wc, wget, xargs, zcat

unSpawn · 09-07-2014, 11:38 AM

Calling it as 'busybox mount --help' should show mount options. No idea if there are any sysctls that could influence VFS layer writes but I'm pretty sure the underlying umount syscall takes care of flushing, depending on the file system.

jefro · 09-07-2014, 03:26 PM

Create an alias or just add in a static build. Otherwise you have to rebuild busybox.

Ratamahatta · 09-07-2014, 04:08 PM

Hi again!

Thanks for the responses.
Would have gone with jefro's suggestion (the alias). Then I realised what unSpawn was trying to tell me.
mount --help (the system has symbolic links to the busybox binary) only resulted in

Code:

BusyBox v1.00 (2007.04.23-03:09+0000) multi-call binary
Usage: mount [flags] DEVICE NODE [-o options,more-options]

but cat /etc/mtab yielded

Code:

/dev/sda1 /ramdisk/mnt/usb-storage/unknown-USB-0 vfat rw,sync 0 0

So it's synced automatically allready. Needed a push into the right direction, though.
Thanks again.

jefro · 09-08-2014, 05:15 PM

Thanks for the update and solution.