如何使任何 shell 命令的输出不受缓冲？

有没有一种不用输出缓冲就可以运行 shell 命令的方法？

例如，hexdump file | ./my_script只会以缓冲块的形式将输入从 hexdump 传递给 my _ script，而不会逐行传递。

实际上，我想知道一个通用的解决方案，如何使任何命令无缓冲？

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最佳答案

AFAIK, you can't do it without ugly hacks. Writing to a pipe (or reading from it) automatically turns on full buffering and there is nothing you can do about it :-(. "Line buffering" (which is what you want) is only used when reading/writing a terminal. The ugly hacks exactly do this: They connect a program to a pseudo-terminal, so that the other tools in the pipe read/write from that terminal in line buffering mode. The whole problem is described here:

The page has also some suggestions (the aforementioned "ugly hacks") what to do, i.e. using unbuffer or pulling some tricks with LD_PRELOAD.

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The command unbuffer from the expect package disables the output buffering:
Ubuntu Manpage: unbuffer - unbuffer output

Example usage:

unbuffer hexdump file | ./my_script
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You could also use the script command to make the output of hexdump line-buffered (hexdump will be run in a pseudo terminal which tricks hexdump into thinking its writing its stdout to a terminal, and not to a pipe).

# cf. http://unix.stackexchange.com/questions/25372/turn-off-buffering-in-pipe/
stty -echo -onlcr
script -q /dev/null hexdump file | ./my_script         # FreeBSD, Mac OS X
script -q -c "hexdump file" /dev/null | ./my_script    # Linux
stty echo onlcr
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Try stdbuf, included in GNU coreutils and thus virtually any Linux distro. This sets the buffer length for input, output and error to zero:

stdbuf -i0 -o0 -e0 command
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One should use grep or egrep "--line-buffered" options to solve this. no other tools needed.


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