巴什前pression计算顺序的命令行 [英] Bash Expression Evaluation Order on Command Line

问题描述

背景：

我赶紧努力呼吁并行SGE的作业提交方案QSUB内bash命令行前pressions。虽然这样做，我试图提交前pression（作为参数），另一个脚本内将跑，像这样：

I'm working on quickly calling bash command line expressions inside of SGE's job submission program qSub in parallel. While doing so, I was attempting to submit an expression (as an argument) to be ran inside of another script like so:

./ runArguments.sh的grep富跳回到bar.txt> output.txt的

./runArguments.sh grep foo bar.txt > output.txt

runArguments.sh 的是这样的：

runArguments.sh looks like this:

#!/bin/bash  
${1} ${2} ${3} etc....to 12

的想法是，我想要的grep富跳回到bar.txt> output.txt的脚本进行评估......不是在命令行。在上面的例子中，grep的富跳回到bar.txt将runArguments.sh执行期间评估，但输出重定向将在命令行上进行评估。我终于找到使用如下图所示EVAL工作解决方案，但我不明白的为什么的工作原理。

问题（S）

1）为什么

./runArguments.sh eval "grep foo bar.txt > output.txt"

允许的eval和前pression将被视为参数，但

allow the eval and the expression to be taken as arguments, but

./runArguments.sh $(grep foo bar.txt > output.txt)

在命令行上评估脚本调用之前？ （$（grep的...）的输出被作为替代参数）

evaluates on the command line before the script is called? (the output of $(grep...) is taken as the arguments instead)

2）是否有这样做的更好的办法？

2) Is there a better way of doing this?

在此先感谢！

推荐答案

您的第一个问题是有点难以回答，因为你已经回答了你自己。正如你所看到的，命令替换（即 $（...）或``... 符号）替代的命令的输出，然后处理结果。例如，这

Your first question is a bit hard to answer, because you've already answered it yourself. As you've seen, command substitution (the $(...) or `...` notation) substitutes the output of the command, and then processes the result. For example, this:

cat $(echo tmp.sh)

被转换为这样的：

gets converted to this:

cat tmp.sh

所以你的情况，这样的：

So in your case, this:

./runArguments.sh $(grep foo bar.txt > output.txt)

运行的grep富跳回到bar.txt＆GT; output.txt的，抓住它的输出＆MDASH;这将是什么，因为你已经重定向任何输出到 output.txt的＆MDASH;并替换它，产生：

runs grep foo bar.txt > output.txt, grabs its output — which will be nothing, since you've redirected any output to output.txt — and substitutes it, yielding:

./runArguments.sh

（所以你的脚本不带参数运行）。

(so your script is run with no arguments).

相比之下，这样的：

./runArguments.sh eval "grep foo bar.txt > output.txt"

不执行任何命令替换，所以你的脚本有两个参数运行：评估和的grep富跳回到bar.txt＆GT; output.txt的。你的脚本里面这个命令：

does not perform any command substitution, so your script is run with two arguments: eval, and grep foo bar.txt > output.txt. This command inside your script:

${1} ${2} ${3} ${4} ${5} ${6} ${7} ${8} ${9} ${10} ${11} ${12}

因此​​，相同的：

is therefore equivalent to this:

eval grep foo bar.txt '>' output.txt

该调用评估内建有五个参数：的grep ，富，跳回到bar.txt ，＆GT; 和 output.txt的。在评估内置组装它的参数到命令，并运行它们，甚至平移＆GT; 和 output.txt的参数为输出重定向，所以上面是相同的：

which invokes the eval built-in with five arguments: grep, foo, bar.txt, >, and output.txt. The eval built-in assembles its arguments into a command, and runs them, and even translates the > and output.txt arguments into an output-redirection, so the above is equivalent to this:

grep foo bar.txt > output.txt

。 。 。你已经知道做什么。 ： - ）

. . . and you already know what that does. :-)

至于你的第二个问题＆MDASH;没有，有没有一个真正的更好的方式来做到这一点。你需要通过＆GT; 作为参数，这意味着你需要使用评估... 或的bash -c...或以翻译它放回意思输出重定向等。如果你O.K.与修改脚本，那么你可能要改变这一行：

As for your second question — no, there's not really a better way to do this. You need to pass the > in as an argument, and that means that you need to use eval ... or bash -c "..." or the like in order to "translate" it back into meaning output-redirection. If you're O.K. with modifying the script, then you might want to change this line:

${1} ${2} ${3} ${4} ${5} ${6} ${7} ${8} ${9} ${10} ${11} ${12}

这样：

eval ${1} ${2} ${3} ${4} ${5} ${6} ${7} ${8} ${9} ${10} ${11} ${12}

，这样你就不需要在参数来处理这个问题。或者，其实，你不如改成这样：

so that you don't need to handle this in the parameters. Or, actually, you might as well change it to this:

eval ${@}

这将让你在超过十二参数传递;或者，更好的做法是：

which will let you pass in more than twelve parameters; or, better yet, this:

eval "${@}"

这将给你的略的过字分裂和fileglobbing和诸如此类的东西更多的控制。

which will give you slightly more control over word-splitting and fileglobbing and whatnot.

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