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Accepted
wjwrpoyob
Asked: 2023-06-01 07:10:02 +0800 CST

为什么 tar 对 stdout 和 - 的处理不同？

  • 772

在OP 发表评论后，我发现/dev/stdout即使在禁用缓冲后也会提供 10 KiB 的块，但-不会。为什么是这样？man tar我在nor中找不到与此相关的任何信息man stdout。

请注意，/dev/stdout转到00002800，而不是000000a1。输出是正确的，除了用空字节填充。

> mkdir -p /tmp/747613
> cd /tmp/747613
> echo 747613 > file.txt

> tar czf /tmp/archive_tgz .
> hd /tmp/archive_tgz
00000000  1f 8b 08 00 00 00 00 00  00 03 ed d1 41 0a c2 30  |............A..0|
00000010  10 85 e1 ac 3d 45 4e 90  66 9a 49 72 9e 2e 22 08  |....=EN.f.Ir..".|
00000020  ea a2 46 f0 f8 a6 8b a2  9b 76 21 04 11 ff 6f f3  |..F......v!...o.|
00000030  20 33 90 81 e7 06 d3 9d  6f 72 8e 4b 4a 8e fe 3d  | 3......or.KJ..=|
00000040  57 46 54 43 6c 8f 69 6c  7b 22 da c6 36 f6 3f cd  |WFTCl.il{"..6.?.|
00000050  98 fb ad 4e b3 b5 2d cb  7c 9d 2e 65 7b 6f 7f fe  |...N..-.|..e{o..|
00000060  a3 dc 70 3c 9d 8b ab 8f  da ef 8f a5 e0 94 74 a7  |..p<..........t.|
00000070  ff bc f6 2f 41 a5 f5 1f  64 14 63 7d bf 93 5e fe  |.../A...d.c}..^.|
00000080  bc ff ac 39 49 38 7c fb  0c 00 00 00 00 00 00 00  |...9I8|.........|
00000090  00 00 00 00 00 00 1f 78  02 88 2a 27 ac 00 28 00  |.......x..*'..(.|
*
000000a1

> tar czf /dev/stdout . | hd
00000000  1f 8b 08 00 00 00 00 00  00 03 ed d1 41 0a c2 30  |............A..0|
00000010  10 85 e1 ac 3d 45 4e 90  66 9a 49 72 9e 2e 22 08  |....=EN.f.Ir..".|
00000020  ea a2 46 f0 f8 a6 8b a2  9b 76 21 04 11 ff 6f f3  |..F......v!...o.|
00000030  20 33 90 81 e7 06 d3 9d  6f 72 8e 4b 4a 8e fe 3d  | 3......or.KJ..=|
00000040  57 46 54 43 6c 8f 69 6c  7b 22 da c6 36 f6 3f cd  |WFTCl.il{"..6.?.|
00000050  98 fb ad 4e b3 b5 2d cb  7c 9d 2e 65 7b 6f 7f fe  |...N..-.|..e{o..|
00000060  a3 dc 70 3c 9d 8b ab 8f  da ef 8f a5 e0 94 74 a7  |..p<..........t.|
00000070  ff bc f6 2f 41 a5 f5 1f  64 14 63 7d bf 93 5e fe  |.../A...d.c}..^.|
00000080  bc ff ac 39 49 38 7c fb  0c 00 00 00 00 00 00 00  |...9I8|.........|
00000090  00 00 00 00 00 00 1f 78  02 88 2a 27 ac 00 28 00  |.......x..*'..(.|
000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002800

> # Even with buffering disabled
> stdbuf -i0 -o0 -e0 tar czf /dev/stdout . | stdbuf -i0 -o0 -e0 hd
00000000  1f 8b 08 00 00 00 00 00  00 03 ed d1 41 0a c2 30  |............A..0|
00000010  10 85 e1 ac 3d 45 4e 90  66 9a 49 72 9e 2e 22 08  |....=EN.f.Ir..".|
00000020  ea a2 46 f0 f8 a6 8b a2  9b 76 21 04 11 ff 6f f3  |..F......v!...o.|
00000030  20 33 90 81 e7 06 d3 9d  6f 72 8e 4b 4a 8e fe 3d  | 3......or.KJ..=|
00000040  57 46 54 43 6c 8f 69 6c  7b 22 da c6 36 f6 3f cd  |WFTCl.il{"..6.?.|
00000050  98 fb ad 4e b3 b5 2d cb  7c 9d 2e 65 7b 6f 7f fe  |...N..-.|..e{o..|
00000060  a3 dc 70 3c 9d 8b ab 8f  da ef 8f a5 e0 94 74 a7  |..p<..........t.|
00000070  ff bc f6 2f 41 a5 f5 1f  64 14 63 7d bf 93 5e fe  |.../A...d.c}..^.|
00000080  bc ff ac 39 49 38 7c fb  0c 00 00 00 00 00 00 00  |...9I8|.........|
00000090  00 00 00 00 00 00 1f 78  02 88 2a 27 ac 00 28 00  |.......x..*'..(.|
000000a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002800

> # Works fine (- means stdout)
> tar czf - . | hd
00000000  1f 8b 08 00 00 00 00 00  00 03 ed d1 41 0a c2 30  |............A..0|
00000010  10 85 e1 ac 3d 45 4e 90  66 9a 49 72 9e 2e 22 08  |....=EN.f.Ir..".|
00000020  ea a2 46 f0 f8 a6 8b a2  9b 76 21 04 11 ff 6f f3  |..F......v!...o.|
00000030  20 33 90 81 e7 06 d3 9d  6f 72 8e 4b 4a 8e fe 3d  | 3......or.KJ..=|
00000040  57 46 54 43 6c 8f 69 6c  7b 22 da c6 36 f6 3f cd  |WFTCl.il{"..6.?.|
00000050  98 fb ad 4e b3 b5 2d cb  7c 9d 2e 65 7b 6f 7f fe  |...N..-.|..e{o..|
00000060  a3 dc 70 3c 9d 8b ab 8f  da ef 8f a5 e0 94 74 a7  |..p<..........t.|
00000070  ff bc f6 2f 41 a5 f5 1f  64 14 63 7d bf 93 5e fe  |.../A...d.c}..^.|
00000080  bc ff ac 39 49 38 7c fb  0c 00 00 00 00 00 00 00  |...9I8|.........|
00000090  00 00 00 00 00 00 1f 78  02 88 2a 27 ac 00 28 00  |.......x..*'..(.|
*
000000a1
linux

1 个回答

  1. Best Answer

    行为上的差异来自tar：写入时，它应用“阻塞因子”，默认情况下使用 10240 字节的记录（十六进制为 2800）。gzip即使在压缩时也会发生这种情况，这就是为什么某些 tarball 在提取时会产生错误消息的原因。当将存档写入常规文件或标准输出时，该行为在存档末尾被禁用（尽管手册说不是）。

    写入时/dev/stdout，tar相信它正在写入设备，并应用其阻塞因子。您可以通过更改阻塞因子来验证这一点：

    $ tar czfb /dev/stdout 1 . | hd
00000000  1f 8b 08 00 00 00 00 00  00 03 ed d1 31 0e 83 30  |............1..0|
00000010  0c 05 50 cf 9c 22 27 08  76 b0 e3 f3 74 48 25 24  |..P.."'.v...tH%$|
00000020  26 08 88 e3 03 43 55 c4  00 53 8a 2a fc 16 0f b6  |&....CU..S.*....|
00000030  e4 2f 7d 5f 43 71 b8 52  91 6d 92 0a ee e7 07 10  |./}_Cq.R.m......|
00000040  73 13 31 10 62 04 24 0c  2c e0 a4 7c 34 80 71 c8  |s.1.b.$.,..|4.q.|
00000050  af de 39 18 72 9a d2 c9  dd d5 fe 4f f9 fa dd 76  |..9.r......O...v|
00000060  c9 e7 39 97 fb b1 15 1c  99 4f fa d7 43 ff a4 21  |..9......O..C..!|
00000070  80 c3 72 91 be 1e de bf  b2 46 6a aa bb 63 18 63  |..r......Fj..c.c|
00000080  8c f9 b1 05 02 0c 89 df  00 0a 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200
    • 8

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