Archive directory with tar/gz to multiple locations and omit an additional copy

Question

I have a simple backup script, which also creates a tar/gzip archive of local data to an external USB device and then copies that archive to a second USB device.

For example:

usb1="/mnt/usbone"
usb2="/mnt/usbtwo"
source="/home/user"

tar cfz ${usb1}/source.tar.gz ${source}
cp -ar ${usb1}/source.tar.gz ${usb2}

This seems like it could be optimized to have tar create copies on both drives, instead of creating one archive, which is copied afterwards. The resulting archive is rather small (<1GB). I am aware that this is not a safe approach for a backup.

---

Edit: I've quickly tested the solution from Archemar and comapred the approaches. For good measure, I also tested the initial approach with rsync. See results with time (not bash time, /usr/bin/time) and the script I used to test this.

Source is created with dd if=/dev/urandom bs=1M count=1024 of=/tmp/random.blob. Host is a Raspberry Pi 3B running from a microSD card, the mounted targets are USB 2.0 flash drives (${a} and ${b}).

a.sh (tar and cp):

tar cfz ${a}/r1.tar.gz ${s}
cp -ar ${a}/r1.tar.gz ${b}/r1.tar.gz

b.sh (tar and tee):

tar cfz - ${s} | tee ${a}/r2.tar.gz > ${b}/r2.tar.gz

c.sh (tar and rsync):

tar cfz ${a}/r3.tar.gz ${s}
rsync -aW ${a}/r3.tar.gz ${b}/r3.tar.gz

Results:

# /usr/bin/time -v bash a.sh 
        Command being timed: "bash a.sh"
        User time (seconds): 218.71
        System time (seconds): 28.33
        Percent of CPU this job got: 68%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 6:03.13
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 2480
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 41
        Minor (reclaiming a frame) page faults: 1250
        Voluntary context switches: 45519
        Involuntary context switches: 25576
        Swaps: 0
        File system inputs: 3668157
        File system outputs: 4197336
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0

        
# /usr/bin/time -v bash b.sh 
        Command being timed: "bash b.sh"
        User time (seconds): 221.64
        System time (seconds): 28.62
        Percent of CPU this job got: 85%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 4:53.98
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 2536
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 31
        Minor (reclaiming a frame) page faults: 1162
        Voluntary context switches: 68310
        Involuntary context switches: 35582
        Swaps: 0
        File system inputs: 2101321
        File system outputs: 4197832
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0

        
# /usr/bin/time -v bash c.sh 
        Command being timed: "bash c.sh"
        User time (seconds): 235.24
        System time (seconds): 35.01
        Percent of CPU this job got: 74%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 6:04.03
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 2652
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 40
        Minor (reclaiming a frame) page faults: 2310
        Voluntary context switches: 65402
        Involuntary context switches: 45179
        Swaps: 0
        File system inputs: 4200957
        File system outputs: 4197496
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0

To my surprise, the results are not that distinct. File system inputs: 2101321 is quite a bit lower with the tar/tee approach though, which I hope is good for the SD card's life.

Answer 1

you can use tee command

usb1="/mnt/usbone"
usb2="/mnt/usbtwo"
source="/home/user"

tar -cz -f - "${source}" | tee "${usb2}/source.tar.gz" > "${usb1}/source.tar.gz"

where

• -f - tells tar to use stdout as backup file.
• tee "${usb2}/source.tar.gz" will copy stdin to specified file and stdout.
• > "${usb1}/source.tar.gz" will redirect stdout from tee to file.