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Add doc about max int64_t values
Signed-off-by: Alexey Gladkov <legion@kernel.org>
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‎Documentation/int64_max.md‎

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# Maximum Int64 Number
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Many shell implementations support 64-bit numbers, but there is a problem with
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parsing their maximum values. Some shell implementations handle this
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differently.
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It is notable that the behavior differs between dash and ash from busybox.
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## Agenda
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* zsh - zsh (5.9) - UNIX Shell similar to the Korn shell (https://www.zsh.org/)
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* dash - dash (0.5.12) - Debian Almquist Shell (https://git.kernel.org/pub/scm/utils/dash/dash.git)
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* bash - bash (5.3_p3) - GNU Bourne-Again Shell (https://git.savannah.gnu.org/cgit/bash.git)
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* ksh - loksh (7.7) - Linux port of OpenBSD's ksh (https://github.com/dimkr/loksh/)
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* lksh - mksh (59c) - MirBSD Korn Shell (https://mbsd.evolvis.org/mksh.htm)
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* ash - busybox (1.36.1) - Tiny utilities for small and embedded systems (https://busybox.net/)
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## Results
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```sh
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %d\n" "$0" $(( 1 << 62 ))'; done
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OUT: zsh: 4611686018427387904
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OUT: dash: 4611686018427387904
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OUT: bash: 4611686018427387904
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OUT: ksh: 4611686018427387904
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OUT: lksh: 4611686018427387904
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OUT: ash: 4611686018427387904
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %s\n" "$0" $(( 1 << 63 ))'; done
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OUT: zsh: -9223372036854775808
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OUT: dash: -9223372036854775808
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OUT: bash: -9223372036854775808
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OUT: ksh: -9223372036854775808
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OUT: lksh: -9223372036854775808
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OUT: ash: -9223372036854775808
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 1 << 63 ))'; done
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zsh:1: number truncated after 18 digits: 9223372036854775808
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OUT: zsh: f333333333333334
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OUT: dash: 8000000000000000
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OUT: bash: 8000000000000000
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OUT: ksh: 8000000000000000
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OUT: lksh: 8000000000000000
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OUT: ash: 8000000000000000
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```
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```sh
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %s\n" "$0" $(( 0x7fffffffffffffff + 1 ))'; done
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OUT: zsh: -9223372036854775808
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OUT: dash: -9223372036854775808
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OUT: bash: -9223372036854775808
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OUT: ksh: -9223372036854775808
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OUT: lksh: -9223372036854775808
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OUT: ash: -9223372036854775808
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 0x7fffffffffffffff + 1 ))'; done
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zsh:1: number truncated after 18 digits: 9223372036854775808
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OUT: zsh: f333333333333334
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OUT: dash: 8000000000000000
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OUT: bash: 8000000000000000
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OUT: ksh: 8000000000000000
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OUT: lksh: 8000000000000000
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OUT: ash: 8000000000000000
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 0x7fffffffffffffff + 2 ))'; done
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OUT: zsh: 8000000000000001
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OUT: dash: 8000000000000001
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OUT: bash: 8000000000000001
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OUT: ksh: 8000000000000001
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OUT: lksh: 8000000000000001
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OUT: ash: 8000000000000001
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 0x7fffffffffffffff + 3 ))'; done
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OUT: zsh: 8000000000000002
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OUT: dash: 8000000000000002
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OUT: bash: 8000000000000002
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OUT: ksh: 8000000000000002
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OUT: lksh: 8000000000000002
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OUT: ash: 8000000000000002
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 0x7fffffffffffffff + 0x7fffffffffffffff + 1 ))'; done
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OUT: zsh: ffffffffffffffff
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OUT: dash: ffffffffffffffff
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OUT: bash: ffffffffffffffff
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OUT: ksh: ffffffffffffffff
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OUT: lksh: ffffffffffffffff
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OUT: ash: ffffffffffffffff
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```
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There are anomalies in parsing large numbers:
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```sh
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %x\n" "$0" $(( 0xffffffffffffffff ))'; done
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zsh:1: number truncated after 15 digits: ffffffffffffffff
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OUT: zsh: fffffffffffffff
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OUT: dash: 7fffffffffffffff
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OUT: bash: ffffffffffffffff
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OUT: ksh: ffffffffffffffff
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OUT: lksh: ffffffffffffffff
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OUT: ash: ffffffffffffffff
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %s\n" "$0" $(( 0x8000000000000000 ))'; done
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zsh:1: number truncated after 15 digits: 8000000000000000
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OUT: zsh: 576460752303423488
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OUT: dash: 9223372036854775807
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OUT: bash: -9223372036854775808
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OUT: ksh: -9223372036854775808
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OUT: lksh: -9223372036854775808
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OUT: ash: -9223372036854775808
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$ for sh in zsh dash bash ksh lksh "busybox ash"; do $sh -c 'printf "OUT: %4s: %d\n" "$0" $(( -9223372036854775808 ))'; done
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zsh:1: number truncated after 18 digits: 9223372036854775808
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OUT: zsh: -922337203685477580
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OUT: dash: -9223372036854775807
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OUT: bash: -9223372036854775808
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OUT: ksh: -9223372036854775808
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OUT: lksh: -9223372036854775808
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OUT: ash: -9223372036854775808
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```

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