Generating 2FA alike 6-digit Passcode with shuf

This is a one-liner to generate 2FA alike 6-digit passcode:

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$ for i in $(seq 6); do seq 0 9; done | shuf -n 6 | tr -d "\n"; echo
719154

Let’s break it down.

To randomize 10 digits with shuf command, we can do either of the following:

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$ seq 0 9 | shuf | tr -d "\n"; echo
0421568793
$ shuf -i 0-9 | tr -d "\n"; echo
8521369407

The shuf command provides an -i or --input-range option to generate numeric random sequence. This reduces the need for another command. The last echo command is used to add the newline.

If we just need a subset of them, such as 6 digits:

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$ shuf -i 0-9 | head -n 6 | tr -d "\n"; echo
867923

Again, shuf has another option -n or --head-count, which is similar to the head command:

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$ shuf -i 0-9 -n 6 | tr -d "\n"; echo
138472

The output looks like a 2FA passcode.

Let’s try a few iterations:

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$ for i in $(seq 10); do shuf -i 0-9 -n 6 | tr -d "\n"; echo; done
360724
945803
381670
654982
957186
852401
759601
968207
106953
753609

However, there is one problem. The digits are not repeatable.

This is easy to resolve. We can do it one digit at a time and repeat for six times:

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$ for i in $(seq 6); do shuf -i 0-9 -n 1; done | tr -d "\n"; echo
013750

To avoid calling shuf multiple times, we can generate our data space first:

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$ for i in $(seq 6); do seq 0 9; done | shuf -n 6 | tr -d "\n"; echo
781895

Let’s test out a few examples:

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$ for i in $(seq 10); do for j in $(seq 6); do seq 0 9; done | shuf -n 6 | tr -d "\n"; echo; done
614808
738581
864319
334667
319894
576062
072202
103342
770161
940559

Now they look more like those 2FA passcodes.

Finally, test the one-liner on two different shells:

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$ docker run --rm bash:4.4.23 bash -c 'for i in $(seq 10); do for j in $(seq 6); do seq 0 9; done | shuf -n 6 | tr -d "\n"; echo; done'
746163
938135
456273
594338
367268
173899
578514
195571
481287
889203

And Ash:

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$ docker run --rm bash:4.4.23 ash -c 'for i in $(seq 10); do for j in $(seq 6); do seq 0 9; don
e | shuf -n 6 | tr -d "\n"; echo; done'
272842
555047
101576
040439
588857
472194
865996
827611
786152
201563

Note that this is just a demonstration of using CLI command to randomize an array, and the outcome looks like a 2FA passcode.