bash 评估字符串中的变量
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Evaluating variables in a string
提问by Robert Kubrick
The last line in this script won't work as I expect:
此脚本中的最后一行不会按我的预期工作：
myfile="afile.txt"
mycmd='cat $myfile'
eval $mycmd
echo eval $mycmd
Here echo prints 'eval cat $myfile'. How can I print 'eval cat afile.txt'?
这里 echo 打印“eval cat $myfile”。如何打印“eval cat afile.txt”？
回答by David W.
Let's take things step by step:
让我们一步一步来：
When you do this:
当你这样做时：
mycmd='cat $myfile'
You prevent the shell from interpolating $myfile. Thus:
您可以防止 shell 进行插值$myfile。因此：
$ echo $mycmd
cat $myfile
If you want to allow the interpolation, you can use double quotes:
如果要允许插值，可以使用双引号：
$ mycmd="echo $myfile" #Double quotes!
$ echo "$mycmd"
cat afile.txt
This, of course, freezesthe interpretation of $mycmdwhen you do an eval.
当然，这会冻结对$mycmd执行eval.
$ myfile="afile.txt"
$ mycmd="echo $myfile"
$ echo $mycmd
cat afile.txt
$ eval $mycmd #Prints out afile.txt
$ myfile=bfile.txt
$ eval $mycmd #Still prints out afile.txt and not bfile.txt
Compare this to:
将此与：
$ myfile="afile.txt"
$ mycmd='cat $myfile' #Single quotes hide $myfile from the shell
echo $mycmd
cat $myfile #Shell didn't change "$myfile", so it prints as a literal
$ eval $mycmd #Prints out afile.txt
$ myfile=bfile.txt
$ eval $mycmd #Now prints out bfile.txt
What you probably want to do is to evaluate the $mycmdin an echo statement when you echo it:
您可能想要做的是$mycmd在回显时评估echo 语句中的 ：
$ echo $(eval "echo $mycmd")
$ cat afile.txt
$ myfile=bfile.txt
$ echo $(eval "echo $mycmd")
cat bfile.txt
回答by ruakh
You can write:
你可以写：
eval echo eval $mycmd
or a bit more robust:
或者更健壮一点：
eval echo eval "$mycmd"
That said, I'd recommend avoiding evalwhenever possible; it tends to be very fragile, because there are many complicated steps in Bash command-line processing, and use of evaltypically means you will perform those steps more than once. It's hard to keep track of what's really going on.
也就是说，我建议eval尽可能避免；它往往非常脆弱，因为 Bash 命令行处理中有许多复杂的步骤，而使用eval通常意味着您将多次执行这些步骤。很难跟踪到底发生了什么。
回答by seanmcl
You need to eval the echo, rather than the other way around:
您需要评估回声，而不是相反：
eval echo "eval $mycmd"
or
或者
eval echo eval "$mycmd"
I think the former is preferable, but at least quote the variable expansion.
我认为前者更可取，但至少引用变量扩展。
回答by konsolebox
If you're using bash, the better way to do it is to use arrays:
如果您使用 bash，更好的方法是使用数组：
myfile="afile.txt"
mycmd=(cat "$myfile")
echo "${mycmd[@]}"
"${mycmd[@]}"
